connection.c

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/*-------------------------------------------------------------------------
 *
 * connection.c
 *        Connection management functions for postgres_fdw
 *
 * Portions Copyright (c) 2012-2023, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        contrib/postgres_fdw/connection.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "access/xact.h"
#include "catalog/pg_user_mapping.h"
#include "commands/defrem.h"
#include "funcapi.h"
#include "libpq/libpq-be.h"
#include "libpq/libpq-be-fe-helpers.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postgres_fdw.h"
#include "storage/fd.h"
#include "storage/latch.h"
#include "utils/builtins.h"
#include "utils/datetime.h"
#include "utils/hsearch.h"
#include "utils/inval.h"
#include "utils/memutils.h"
#include "utils/syscache.h"
 
/*
 * Connection cache hash table entry
 *
 * The lookup key in this hash table is the user mapping OID. We use just one
 * connection per user mapping ID, which ensures that all the scans use the
 * same snapshot during a query.  Using the user mapping OID rather than
 * the foreign server OID + user OID avoids creating multiple connections when
 * the public user mapping applies to all user OIDs.
 *
 * The "conn" pointer can be NULL if we don't currently have a live connection.
 * When we do have a connection, xact_depth tracks the current depth of
 * transactions and subtransactions open on the remote side.  We need to issue
 * commands at the same nesting depth on the remote as we're executing at
 * ourselves, so that rolling back a subtransaction will kill the right
 * queries and not the wrong ones.
 */
typedef Oid ConnCacheKey;
 
typedef struct ConnCacheEntry
{
    ConnCacheKey key;           /* hash key (must be first) */
    PGconn     *conn;           /* connection to foreign server, or NULL */
    /* Remaining fields are invalid when conn is NULL: */
    int         xact_depth;     /* 0 = no xact open, 1 = main xact open, 2 =
                                 * one level of subxact open, etc */
    bool        have_prep_stmt; /* have we prepared any stmts in this xact? */
    bool        have_error;     /* have any subxacts aborted in this xact? */
    bool        changing_xact_state;    /* xact state change in process */
    bool        parallel_commit;    /* do we commit (sub)xacts in parallel? */
    bool        parallel_abort; /* do we abort (sub)xacts in parallel? */
    bool        invalidated;    /* true if reconnect is pending */
    bool        keep_connections;   /* setting value of keep_connections
                                     * server option */
    Oid         serverid;       /* foreign server OID used to get server name */
    uint32      server_hashvalue;   /* hash value of foreign server OID */
    uint32      mapping_hashvalue;  /* hash value of user mapping OID */
    PgFdwConnState state;       /* extra per-connection state */
} ConnCacheEntry;
 
/*
 * Connection cache (initialized on first use)
 */
static HTAB *ConnectionHash = NULL;
 
/* for assigning cursor numbers and prepared statement numbers */
static unsigned int cursor_number = 0;
static unsigned int prep_stmt_number = 0;
 
/* tracks whether any work is needed in callback functions */
static bool xact_got_connection = false;
 
/*
 * Milliseconds to wait to cancel an in-progress query or execute a cleanup
 * query; if it takes longer than 30 seconds to do these, we assume the
 * connection is dead.
 */
#define CONNECTION_CLEANUP_TIMEOUT  30000
 
/* Macro for constructing abort command to be sent */
#define CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel) \
    do { \
        if (toplevel) \
            snprintf((sql), sizeof(sql), \
                     "ABORT TRANSACTION"); \
        else \
            snprintf((sql), sizeof(sql), \
                     "ROLLBACK TO SAVEPOINT s%d; RELEASE SAVEPOINT s%d", \
                     (entry)->xact_depth, (entry)->xact_depth); \
    } while(0)
 
/*
 * SQL functions
 */
PG_FUNCTION_INFO_V1(postgres_fdw_get_connections);
PG_FUNCTION_INFO_V1(postgres_fdw_disconnect);
PG_FUNCTION_INFO_V1(postgres_fdw_disconnect_all);
 
/* prototypes of private functions */
static void make_new_connection(ConnCacheEntry *entry, UserMapping *user);
static PGconn *connect_pg_server(ForeignServer *server, UserMapping *user);
static void disconnect_pg_server(ConnCacheEntry *entry);
static void check_conn_params(const char **keywords, const char **values, UserMapping *user);
static void configure_remote_session(PGconn *conn);
static void do_sql_command_begin(PGconn *conn, const char *sql);
static void do_sql_command_end(PGconn *conn, const char *sql,
                               bool consume_input);
static void begin_remote_xact(ConnCacheEntry *entry);
static void pgfdw_xact_callback(XactEvent event, void *arg);
static void pgfdw_subxact_callback(SubXactEvent event,
                                   SubTransactionId mySubid,
                                   SubTransactionId parentSubid,
                                   void *arg);
static void pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue);
static void pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry);
static void pgfdw_reset_xact_state(ConnCacheEntry *entry, bool toplevel);
static bool pgfdw_cancel_query(PGconn *conn);
static bool pgfdw_cancel_query_begin(PGconn *conn);
static bool pgfdw_cancel_query_end(PGconn *conn, TimestampTz endtime,
                                   bool consume_input);
static bool pgfdw_exec_cleanup_query(PGconn *conn, const char *query,
                                     bool ignore_errors);
static bool pgfdw_exec_cleanup_query_begin(PGconn *conn, const char *query);
static bool pgfdw_exec_cleanup_query_end(PGconn *conn, const char *query,
                                         TimestampTz endtime,
                                         bool consume_input,
                                         bool ignore_errors);
static bool pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime,
                                     PGresult **result, bool *timed_out);
static void pgfdw_abort_cleanup(ConnCacheEntry *entry, bool toplevel);
static bool pgfdw_abort_cleanup_begin(ConnCacheEntry *entry, bool toplevel,
                                      List **pending_entries,
                                      List **cancel_requested);
static void pgfdw_finish_pre_commit_cleanup(List *pending_entries);
static void pgfdw_finish_pre_subcommit_cleanup(List *pending_entries,
                                               int curlevel);
static void pgfdw_finish_abort_cleanup(List *pending_entries,
                                       List *cancel_requested,
                                       bool toplevel);
static void pgfdw_security_check(const char **keywords, const char **values,
                                 UserMapping *user, PGconn *conn);
static bool UserMappingPasswordRequired(UserMapping *user);
static bool disconnect_cached_connections(Oid serverid);
 
/*
 * Get a PGconn which can be used to execute queries on the remote PostgreSQL
 * server with the user's authorization.  A new connection is established
 * if we don't already have a suitable one, and a transaction is opened at
 * the right subtransaction nesting depth if we didn't do that already.
 *
 * will_prep_stmt must be true if caller intends to create any prepared
 * statements.  Since those don't go away automatically at transaction end
 * (not even on error), we need this flag to cue manual cleanup.
 *
 * If state is not NULL, *state receives the per-connection state associated
 * with the PGconn.
 */
PGconn *
GetConnection(UserMapping *user, bool will_prep_stmt, PgFdwConnState **state)
{
    bool        found;
    bool        retry = false;
    ConnCacheEntry *entry;
    ConnCacheKey key;
    MemoryContext ccxt = CurrentMemoryContext;
 
    /* First time through, initialize connection cache hashtable */
    if (ConnectionHash == NULL)
    {
        HASHCTL     ctl;
 
        ctl.keysize = sizeof(ConnCacheKey);
        ctl.entrysize = sizeof(ConnCacheEntry);
        ConnectionHash = hash_create("postgres_fdw connections", 8,
                                     &ctl,
                                     HASH_ELEM | HASH_BLOBS);
 
        /*
         * Register some callback functions that manage connection cleanup.
         * This should be done just once in each backend.
         */
        RegisterXactCallback(pgfdw_xact_callback, NULL);
        RegisterSubXactCallback(pgfdw_subxact_callback, NULL);
        CacheRegisterSyscacheCallback(FOREIGNSERVEROID,
                                      pgfdw_inval_callback, (Datum) 0);
        CacheRegisterSyscacheCallback(USERMAPPINGOID,
                                      pgfdw_inval_callback, (Datum) 0);
    }
 
    /* Set flag that we did GetConnection during the current transaction */
    xact_got_connection = true;
 
    /* Create hash key for the entry.  Assume no pad bytes in key struct */
    key = user->umid;
 
    /*
     * Find or create cached entry for requested connection.
     */
    entry = hash_search(ConnectionHash, &key, HASH_ENTER, &found);
    if (!found)
    {
        /*
         * We need only clear "conn" here; remaining fields will be filled
         * later when "conn" is set.
         */
        entry->conn = NULL;
    }
 
    /* Reject further use of connections which failed abort cleanup. */
    pgfdw_reject_incomplete_xact_state_change(entry);
 
    /*
     * If the connection needs to be remade due to invalidation, disconnect as
     * soon as we're out of all transactions.
     */
    if (entry->conn != NULL && entry->invalidated && entry->xact_depth == 0)
    {
        elog(DEBUG3, "closing connection %p for option changes to take effect",
             entry->conn);
        disconnect_pg_server(entry);
    }
 
    /*
     * If cache entry doesn't have a connection, we have to establish a new
     * connection.  (If connect_pg_server throws an error, the cache entry
     * will remain in a valid empty state, ie conn == NULL.)
     */
    if (entry->conn == NULL)
        make_new_connection(entry, user);
 
    /*
     * We check the health of the cached connection here when using it.  In
     * cases where we're out of all transactions, if a broken connection is
     * detected, we try to reestablish a new connection later.
     */
    PG_TRY();
    {
        /* Process a pending asynchronous request if any. */
        if (entry->state.pendingAreq)
            process_pending_request(entry->state.pendingAreq);
        /* Start a new transaction or subtransaction if needed. */
        begin_remote_xact(entry);
    }
    PG_CATCH();
    {
        MemoryContext ecxt = MemoryContextSwitchTo(ccxt);
        ErrorData  *errdata = CopyErrorData();
 
        /*
         * Determine whether to try to reestablish the connection.
         *
         * After a broken connection is detected in libpq, any error other
         * than connection failure (e.g., out-of-memory) can be thrown
         * somewhere between return from libpq and the expected ereport() call
         * in pgfdw_report_error(). In this case, since PQstatus() indicates
         * CONNECTION_BAD, checking only PQstatus() causes the false detection
         * of connection failure. To avoid this, we also verify that the
         * error's sqlstate is ERRCODE_CONNECTION_FAILURE. Note that also
         * checking only the sqlstate can cause another false detection
         * because pgfdw_report_error() may report ERRCODE_CONNECTION_FAILURE
         * for any libpq-originated error condition.
         */
        if (errdata->sqlerrcode != ERRCODE_CONNECTION_FAILURE ||
            PQstatus(entry->conn) != CONNECTION_BAD ||
            entry->xact_depth > 0)
        {
            MemoryContextSwitchTo(ecxt);
            PG_RE_THROW();
        }
 
        /* Clean up the error state */
        FlushErrorState();
        FreeErrorData(errdata);
        errdata = NULL;
 
        retry = true;
    }
    PG_END_TRY();
 
    /*
     * If a broken connection is detected, disconnect it, reestablish a new
     * connection and retry a new remote transaction. If connection failure is
     * reported again, we give up getting a connection.
     */
    if (retry)
    {
        Assert(entry->xact_depth == 0);
 
        ereport(DEBUG3,
                (errmsg_internal("could not start remote transaction on connection %p",
                                 entry->conn)),
                errdetail_internal("%s", pchomp(PQerrorMessage(entry->conn))));
 
        elog(DEBUG3, "closing connection %p to reestablish a new one",
             entry->conn);
        disconnect_pg_server(entry);
 
        make_new_connection(entry, user);
 
        begin_remote_xact(entry);
    }
 
    /* Remember if caller will prepare statements */
    entry->have_prep_stmt |= will_prep_stmt;
 
    /* If caller needs access to the per-connection state, return it. */
    if (state)
        *state = &entry->state;
 
    return entry->conn;
}
 
/*
 * Reset all transient state fields in the cached connection entry and
 * establish new connection to the remote server.
 */
static void
make_new_connection(ConnCacheEntry *entry, UserMapping *user)
{
    ForeignServer *server = GetForeignServer(user->serverid);
    ListCell   *lc;
 
    Assert(entry->conn == NULL);
 
    /* Reset all transient state fields, to be sure all are clean */
    entry->xact_depth = 0;
    entry->have_prep_stmt = false;
    entry->have_error = false;
    entry->changing_xact_state = false;
    entry->invalidated = false;
    entry->serverid = server->serverid;
    entry->server_hashvalue =
        GetSysCacheHashValue1(FOREIGNSERVEROID,
                              ObjectIdGetDatum(server->serverid));
    entry->mapping_hashvalue =
        GetSysCacheHashValue1(USERMAPPINGOID,
                              ObjectIdGetDatum(user->umid));
    memset(&entry->state, 0, sizeof(entry->state));
 
    /*
     * Determine whether to keep the connection that we're about to make here
     * open even after the transaction using it ends, so that the subsequent
     * transactions can re-use it.
     *
     * By default, all the connections to any foreign servers are kept open.
     *
     * Also determine whether to commit/abort (sub)transactions opened on the
     * remote server in parallel at (sub)transaction end, which is disabled by
     * default.
     *
     * Note: it's enough to determine these only when making a new connection
     * because if these settings for it are changed, it will be closed and
     * re-made later.
     */
    entry->keep_connections = true;
    entry->parallel_commit = false;
    entry->parallel_abort = false;
    foreach(lc, server->options)
    {
        DefElem    *def = (DefElem *) lfirst(lc);
 
        if (strcmp(def->defname, "keep_connections") == 0)
            entry->keep_connections = defGetBoolean(def);
        else if (strcmp(def->defname, "parallel_commit") == 0)
            entry->parallel_commit = defGetBoolean(def);
        else if (strcmp(def->defname, "parallel_abort") == 0)
            entry->parallel_abort = defGetBoolean(def);
    }
 
    /* Now try to make the connection */
    entry->conn = connect_pg_server(server, user);
 
    elog(DEBUG3, "new postgres_fdw connection %p for server \"%s\" (user mapping oid %u, userid %u)",
         entry->conn, server->servername, user->umid, user->userid);
}
 
/*
 * Check that non-superuser has used password or delegated credentials
 * to establish connection; otherwise, he's piggybacking on the
 * postgres server's user identity. See also dblink_security_check()
 * in contrib/dblink and check_conn_params.
 */
static void
pgfdw_security_check(const char **keywords, const char **values, UserMapping *user, PGconn *conn)
{
    /* Superusers bypass the check */
    if (superuser_arg(user->userid))
        return;
 
#ifdef ENABLE_GSS
    /* Connected via GSSAPI with delegated credentials- all good. */
    if (PQconnectionUsedGSSAPI(conn) && be_gssapi_get_delegation(MyProcPort))
        return;
#endif
 
    /* Ok if superuser set PW required false. */
    if (!UserMappingPasswordRequired(user))
        return;
 
    /* Connected via PW, with PW required true, and provided non-empty PW. */
    if (PQconnectionUsedPassword(conn))
    {
        /* ok if params contain a non-empty password */
        for (int i = 0; keywords[i] != NULL; i++)
        {
            if (strcmp(keywords[i], "password") == 0 && values[i][0] != '\0')
                return;
        }
    }
 
    ereport(ERROR,
            (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
             errmsg("password or GSSAPI delegated credentials required"),
             errdetail("Non-superuser cannot connect if the server does not request a password or use GSSAPI with delegated credentials."),
             errhint("Target server's authentication method must be changed or password_required=false set in the user mapping attributes.")));
}
 
/*
 * Connect to remote server using specified server and user mapping properties.
 */
static PGconn *
connect_pg_server(ForeignServer *server, UserMapping *user)
{
    PGconn     *volatile conn = NULL;
 
    /*
     * Use PG_TRY block to ensure closing connection on error.
     */
    PG_TRY();
    {
        const char **keywords;
        const char **values;
        char       *appname = NULL;
        int         n;
 
        /*
         * Construct connection params from generic options of ForeignServer
         * and UserMapping.  (Some of them might not be libpq options, in
         * which case we'll just waste a few array slots.)  Add 4 extra slots
         * for application_name, fallback_application_name, client_encoding,
         * end marker.
         */
        n = list_length(server->options) + list_length(user->options) + 4;
        keywords = (const char **) palloc(n * sizeof(char *));
        values = (const char **) palloc(n * sizeof(char *));
 
        n = 0;
        n += ExtractConnectionOptions(server->options,
                                      keywords + n, values + n);
        n += ExtractConnectionOptions(user->options,
                                      keywords + n, values + n);
 
        /*
         * Use pgfdw_application_name as application_name if set.
         *
         * PQconnectdbParams() processes the parameter arrays from start to
         * end. If any key word is repeated, the last value is used. Therefore
         * note that pgfdw_application_name must be added to the arrays after
         * options of ForeignServer are, so that it can override
         * application_name set in ForeignServer.
         */
        if (pgfdw_application_name && *pgfdw_application_name != '\0')
        {
            keywords[n] = "application_name";
            values[n] = pgfdw_application_name;
            n++;
        }
 
        /*
         * Search the parameter arrays to find application_name setting, and
         * replace escape sequences in it with status information if found.
         * The arrays are searched backwards because the last value is used if
         * application_name is repeatedly set.
         */
        for (int i = n - 1; i >= 0; i--)
        {
            if (strcmp(keywords[i], "application_name") == 0 &&
                *(values[i]) != '\0')
            {
                /*
                 * Use this application_name setting if it's not empty string
                 * even after any escape sequences in it are replaced.
                 */
                appname = process_pgfdw_appname(values[i]);
                if (appname[0] != '\0')
                {
                    values[i] = appname;
                    break;
                }
 
                /*
                 * This empty application_name is not used, so we set
                 * values[i] to NULL and keep searching the array to find the
                 * next one.
                 */
                values[i] = NULL;
                pfree(appname);
                appname = NULL;
            }
        }
 
        /* Use "postgres_fdw" as fallback_application_name */
        keywords[n] = "fallback_application_name";
        values[n] = "postgres_fdw";
        n++;
 
        /* Set client_encoding so that libpq can convert encoding properly. */
        keywords[n] = "client_encoding";
        values[n] = GetDatabaseEncodingName();
        n++;
 
        keywords[n] = values[n] = NULL;
 
        /* verify the set of connection parameters */
        check_conn_params(keywords, values, user);
 
        /* OK to make connection */
        conn = libpqsrv_connect_params(keywords, values,
                                       false,   /* expand_dbname */
                                       WAIT_EVENT_EXTENSION);
 
        if (!conn || PQstatus(conn) != CONNECTION_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                     errmsg("could not connect to server \"%s\"",
                            server->servername),
                     errdetail_internal("%s", pchomp(PQerrorMessage(conn)))));
 
        /* Perform post-connection security checks */
        pgfdw_security_check(keywords, values, user, conn);
 
        /* Prepare new session for use */
        configure_remote_session(conn);
 
        if (appname != NULL)
            pfree(appname);
        pfree(keywords);
        pfree(values);
    }
    PG_CATCH();
    {
        libpqsrv_disconnect(conn);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    return conn;
}
 
/*
 * Disconnect any open connection for a connection cache entry.
 */
static void
disconnect_pg_server(ConnCacheEntry *entry)
{
    if (entry->conn != NULL)
    {
        libpqsrv_disconnect(entry->conn);
        entry->conn = NULL;
    }
}
 
/*
 * Return true if the password_required is defined and false for this user
 * mapping, otherwise false. The mapping has been pre-validated.
 */
static bool
UserMappingPasswordRequired(UserMapping *user)
{
    ListCell   *cell;
 
    foreach(cell, user->options)
    {
        DefElem    *def = (DefElem *) lfirst(cell);
 
        if (strcmp(def->defname, "password_required") == 0)
            return defGetBoolean(def);
    }
 
    return true;
}
 
/*
 * For non-superusers, insist that the connstr specify a password or that the
 * user provided their own GSSAPI delegated credentials.  This
 * prevents a password from being picked up from .pgpass, a service file, the
 * environment, etc.  We don't want the postgres user's passwords,
 * certificates, etc to be accessible to non-superusers.  (See also
 * dblink_connstr_check in contrib/dblink.)
 */
static void
check_conn_params(const char **keywords, const char **values, UserMapping *user)
{
    int         i;
 
    /* no check required if superuser */
    if (superuser_arg(user->userid))
        return;
 
#ifdef ENABLE_GSS
    /* ok if the user provided their own delegated credentials */
    if (be_gssapi_get_delegation(MyProcPort))
        return;
#endif
 
    /* ok if params contain a non-empty password */
    for (i = 0; keywords[i] != NULL; i++)
    {
        if (strcmp(keywords[i], "password") == 0 && values[i][0] != '\0')
            return;
    }
 
    /* ok if the superuser explicitly said so at user mapping creation time */
    if (!UserMappingPasswordRequired(user))
        return;
 
    ereport(ERROR,
            (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
             errmsg("password or GSSAPI delegated credentials required"),
             errdetail("Non-superusers must delegate GSSAPI credentials or provide a password in the user mapping.")));
}
 
/*
 * Issue SET commands to make sure remote session is configured properly.
 *
 * We do this just once at connection, assuming nothing will change the
 * values later.  Since we'll never send volatile function calls to the
 * remote, there shouldn't be any way to break this assumption from our end.
 * It's possible to think of ways to break it at the remote end, eg making
 * a foreign table point to a view that includes a set_config call ---
 * but once you admit the possibility of a malicious view definition,
 * there are any number of ways to break things.
 */
static void
configure_remote_session(PGconn *conn)
{
    int         remoteversion = PQserverVersion(conn);
 
    /* Force the search path to contain only pg_catalog (see deparse.c) */
    do_sql_command(conn, "SET search_path = pg_catalog");
 
    /*
     * Set remote timezone; this is basically just cosmetic, since all
     * transmitted and returned timestamptzs should specify a zone explicitly
     * anyway.  However it makes the regression test outputs more predictable.
     *
     * We don't risk setting remote zone equal to ours, since the remote
     * server might use a different timezone database.  Instead, use UTC
     * (quoted, because very old servers are picky about case).
     */
    do_sql_command(conn, "SET timezone = 'UTC'");
 
    /*
     * Set values needed to ensure unambiguous data output from remote.  (This
     * logic should match what pg_dump does.  See also set_transmission_modes
     * in postgres_fdw.c.)
     */
    do_sql_command(conn, "SET datestyle = ISO");
    if (remoteversion >= 80400)
        do_sql_command(conn, "SET intervalstyle = postgres");
    if (remoteversion >= 90000)
        do_sql_command(conn, "SET extra_float_digits = 3");
    else
        do_sql_command(conn, "SET extra_float_digits = 2");
}
 
/*
 * Convenience subroutine to issue a non-data-returning SQL command to remote
 */
void
do_sql_command(PGconn *conn, const char *sql)
{
    do_sql_command_begin(conn, sql);
    do_sql_command_end(conn, sql, false);
}
 
static void
do_sql_command_begin(PGconn *conn, const char *sql)
{
    if (!PQsendQuery(conn, sql))
        pgfdw_report_error(ERROR, NULL, conn, false, sql);
}
 
static void
do_sql_command_end(PGconn *conn, const char *sql, bool consume_input)
{
    PGresult   *res;
 
    /*
     * If requested, consume whatever data is available from the socket. (Note
     * that if all data is available, this allows pgfdw_get_result to call
     * PQgetResult without forcing the overhead of WaitLatchOrSocket, which
     * would be large compared to the overhead of PQconsumeInput.)
     */
    if (consume_input && !PQconsumeInput(conn))
        pgfdw_report_error(ERROR, NULL, conn, false, sql);
    res = pgfdw_get_result(conn, sql);
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
        pgfdw_report_error(ERROR, res, conn, true, sql);
    PQclear(res);
}
 
/*
 * Start remote transaction or subtransaction, if needed.
 *
 * Note that we always use at least REPEATABLE READ in the remote session.
 * This is so that, if a query initiates multiple scans of the same or
 * different foreign tables, we will get snapshot-consistent results from
 * those scans.  A disadvantage is that we can't provide sane emulation of
 * READ COMMITTED behavior --- it would be nice if we had some other way to
 * control which remote queries share a snapshot.
 */
static void
begin_remote_xact(ConnCacheEntry *entry)
{
    int         curlevel = GetCurrentTransactionNestLevel();
 
    /* Start main transaction if we haven't yet */
    if (entry->xact_depth <= 0)
    {
        const char *sql;
 
        elog(DEBUG3, "starting remote transaction on connection %p",
             entry->conn);
 
        if (IsolationIsSerializable())
            sql = "START TRANSACTION ISOLATION LEVEL SERIALIZABLE";
        else
            sql = "START TRANSACTION ISOLATION LEVEL REPEATABLE READ";
        entry->changing_xact_state = true;
        do_sql_command(entry->conn, sql);
        entry->xact_depth = 1;
        entry->changing_xact_state = false;
    }
 
    /*
     * If we're in a subtransaction, stack up savepoints to match our level.
     * This ensures we can rollback just the desired effects when a
     * subtransaction aborts.
     */
    while (entry->xact_depth < curlevel)
    {
        char        sql[64];
 
        snprintf(sql, sizeof(sql), "SAVEPOINT s%d", entry->xact_depth + 1);
        entry->changing_xact_state = true;
        do_sql_command(entry->conn, sql);
        entry->xact_depth++;
        entry->changing_xact_state = false;
    }
}
 
/*
 * Release connection reference count created by calling GetConnection.
 */
void
ReleaseConnection(PGconn *conn)
{
    /*
     * Currently, we don't actually track connection references because all
     * cleanup is managed on a transaction or subtransaction basis instead. So
     * there's nothing to do here.
     */
}
 
/*
 * Assign a "unique" number for a cursor.
 *
 * These really only need to be unique per connection within a transaction.
 * For the moment we ignore the per-connection point and assign them across
 * all connections in the transaction, but we ask for the connection to be
 * supplied in case we want to refine that.
 *
 * Note that even if wraparound happens in a very long transaction, actual
 * collisions are highly improbable; just be sure to use %u not %d to print.
 */
unsigned int
GetCursorNumber(PGconn *conn)
{
    return ++cursor_number;
}
 
/*
 * Assign a "unique" number for a prepared statement.
 *
 * This works much like GetCursorNumber, except that we never reset the counter
 * within a session.  That's because we can't be 100% sure we've gotten rid
 * of all prepared statements on all connections, and it's not really worth
 * increasing the risk of prepared-statement name collisions by resetting.
 */
unsigned int
GetPrepStmtNumber(PGconn *conn)
{
    return ++prep_stmt_number;
}
 
/*
 * Submit a query and wait for the result.
 *
 * This function is interruptible by signals.
 *
 * Caller is responsible for the error handling on the result.
 */
PGresult *
pgfdw_exec_query(PGconn *conn, const char *query, PgFdwConnState *state)
{
    /* First, process a pending asynchronous request, if any. */
    if (state && state->pendingAreq)
        process_pending_request(state->pendingAreq);
 
    /*
     * Submit a query.  Since we don't use non-blocking mode, this also can
     * block.  But its risk is relatively small, so we ignore that for now.
     */
    if (!PQsendQuery(conn, query))
        pgfdw_report_error(ERROR, NULL, conn, false, query);
 
    /* Wait for the result. */
    return pgfdw_get_result(conn, query);
}
 
/*
 * Wait for the result from a prior asynchronous execution function call.
 *
 * This function offers quick responsiveness by checking for any interruptions.
 *
 * This function emulates PQexec()'s behavior of returning the last result
 * when there are many.
 *
 * Caller is responsible for the error handling on the result.
 */
PGresult *
pgfdw_get_result(PGconn *conn, const char *query)
{
    PGresult   *volatile last_res = NULL;
 
    /* In what follows, do not leak any PGresults on an error. */
    PG_TRY();
    {
        for (;;)
        {
            PGresult   *res;
 
            while (PQisBusy(conn))
            {
                int         wc;
 
                /* Sleep until there's something to do */
                wc = WaitLatchOrSocket(MyLatch,
                                       WL_LATCH_SET | WL_SOCKET_READABLE |
                                       WL_EXIT_ON_PM_DEATH,
                                       PQsocket(conn),
                                       -1L, WAIT_EVENT_EXTENSION);
                ResetLatch(MyLatch);
 
                CHECK_FOR_INTERRUPTS();
 
                /* Data available in socket? */
                if (wc & WL_SOCKET_READABLE)
                {
                    if (!PQconsumeInput(conn))
                        pgfdw_report_error(ERROR, NULL, conn, false, query);
                }
            }
 
            res = PQgetResult(conn);
            if (res == NULL)
                break;          /* query is complete */
 
            PQclear(last_res);
            last_res = res;
        }
    }
    PG_CATCH();
    {
        PQclear(last_res);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    return last_res;
}
 
/*
 * Report an error we got from the remote server.
 *
 * elevel: error level to use (typically ERROR, but might be less)
 * res: PGresult containing the error
 * conn: connection we did the query on
 * clear: if true, PQclear the result (otherwise caller will handle it)
 * sql: NULL, or text of remote command we tried to execute
 *
 * Note: callers that choose not to throw ERROR for a remote error are
 * responsible for making sure that the associated ConnCacheEntry gets
 * marked with have_error = true.
 */
void
pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
                   bool clear, const char *sql)
{
    /* If requested, PGresult must be released before leaving this function. */
    PG_TRY();
    {
        char       *diag_sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
        char       *message_primary = PQresultErrorField(res, PG_DIAG_MESSAGE_PRIMARY);
        char       *message_detail = PQresultErrorField(res, PG_DIAG_MESSAGE_DETAIL);
        char       *message_hint = PQresultErrorField(res, PG_DIAG_MESSAGE_HINT);
        char       *message_context = PQresultErrorField(res, PG_DIAG_CONTEXT);
        int         sqlstate;
 
        if (diag_sqlstate)
            sqlstate = MAKE_SQLSTATE(diag_sqlstate[0],
                                     diag_sqlstate[1],
                                     diag_sqlstate[2],
                                     diag_sqlstate[3],
                                     diag_sqlstate[4]);
        else
            sqlstate = ERRCODE_CONNECTION_FAILURE;
 
        /*
         * If we don't get a message from the PGresult, try the PGconn.  This
         * is needed because for connection-level failures, PQexec may just
         * return NULL, not a PGresult at all.
         */
        if (message_primary == NULL)
            message_primary = pchomp(PQerrorMessage(conn));
 
        ereport(elevel,
                (errcode(sqlstate),
                 (message_primary != NULL && message_primary[0] != '\0') ?
                 errmsg_internal("%s", message_primary) :
                 errmsg("could not obtain message string for remote error"),
                 message_detail ? errdetail_internal("%s", message_detail) : 0,
                 message_hint ? errhint("%s", message_hint) : 0,
                 message_context ? errcontext("%s", message_context) : 0,
                 sql ? errcontext("remote SQL command: %s", sql) : 0));
    }
    PG_FINALLY();
    {
        if (clear)
            PQclear(res);
    }
    PG_END_TRY();
}
 
/*
 * pgfdw_xact_callback --- cleanup at main-transaction end.
 *
 * This runs just late enough that it must not enter user-defined code
 * locally.  (Entering such code on the remote side is fine.  Its remote
 * COMMIT TRANSACTION may run deferred triggers.)
 */
static void
pgfdw_xact_callback(XactEvent event, void *arg)
{
    HASH_SEQ_STATUS scan;
    ConnCacheEntry *entry;
    List       *pending_entries = NIL;
    List       *cancel_requested = NIL;
 
    /* Quick exit if no connections were touched in this transaction. */
    if (!xact_got_connection)
        return;
 
    /*
     * Scan all connection cache entries to find open remote transactions, and
     * close them.
     */
    hash_seq_init(&scan, ConnectionHash);
    while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
    {
        PGresult   *res;
 
        /* Ignore cache entry if no open connection right now */
        if (entry->conn == NULL)
            continue;
 
        /* If it has an open remote transaction, try to close it */
        if (entry->xact_depth > 0)
        {
            elog(DEBUG3, "closing remote transaction on connection %p",
                 entry->conn);
 
            switch (event)
            {
                case XACT_EVENT_PARALLEL_PRE_COMMIT:
                case XACT_EVENT_PRE_COMMIT:
 
                    /*
                     * If abort cleanup previously failed for this connection,
                     * we can't issue any more commands against it.
                     */
                    pgfdw_reject_incomplete_xact_state_change(entry);
 
                    /* Commit all remote transactions during pre-commit */
                    entry->changing_xact_state = true;
                    if (entry->parallel_commit)
                    {
                        do_sql_command_begin(entry->conn, "COMMIT TRANSACTION");
                        pending_entries = lappend(pending_entries, entry);
                        continue;
                    }
                    do_sql_command(entry->conn, "COMMIT TRANSACTION");
                    entry->changing_xact_state = false;
 
                    /*
                     * If there were any errors in subtransactions, and we
                     * made prepared statements, do a DEALLOCATE ALL to make
                     * sure we get rid of all prepared statements. This is
                     * annoying and not terribly bulletproof, but it's
                     * probably not worth trying harder.
                     *
                     * DEALLOCATE ALL only exists in 8.3 and later, so this
                     * constrains how old a server postgres_fdw can
                     * communicate with.  We intentionally ignore errors in
                     * the DEALLOCATE, so that we can hobble along to some
                     * extent with older servers (leaking prepared statements
                     * as we go; but we don't really support update operations
                     * pre-8.3 anyway).
                     */
                    if (entry->have_prep_stmt && entry->have_error)
                    {
                        res = PQexec(entry->conn, "DEALLOCATE ALL");
                        PQclear(res);
                    }
                    entry->have_prep_stmt = false;
                    entry->have_error = false;
                    break;
                case XACT_EVENT_PRE_PREPARE:
 
                    /*
                     * We disallow any remote transactions, since it's not
                     * very reasonable to hold them open until the prepared
                     * transaction is committed.  For the moment, throw error
                     * unconditionally; later we might allow read-only cases.
                     * Note that the error will cause us to come right back
                     * here with event == XACT_EVENT_ABORT, so we'll clean up
                     * the connection state at that point.
                     */
                    ereport(ERROR,
                            (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                             errmsg("cannot PREPARE a transaction that has operated on postgres_fdw foreign tables")));
                    break;
                case XACT_EVENT_PARALLEL_COMMIT:
                case XACT_EVENT_COMMIT:
                case XACT_EVENT_PREPARE:
                    /* Pre-commit should have closed the open transaction */
                    elog(ERROR, "missed cleaning up connection during pre-commit");
                    break;
                case XACT_EVENT_PARALLEL_ABORT:
                case XACT_EVENT_ABORT:
                    /* Rollback all remote transactions during abort */
                    if (entry->parallel_abort)
                    {
                        if (pgfdw_abort_cleanup_begin(entry, true,
                                                      &pending_entries,
                                                      &cancel_requested))
                            continue;
                    }
                    else
                        pgfdw_abort_cleanup(entry, true);
                    break;
            }
        }
 
        /* Reset state to show we're out of a transaction */
        pgfdw_reset_xact_state(entry, true);
    }
 
    /* If there are any pending connections, finish cleaning them up */
    if (pending_entries || cancel_requested)
    {
        if (event == XACT_EVENT_PARALLEL_PRE_COMMIT ||
            event == XACT_EVENT_PRE_COMMIT)
        {
            Assert(cancel_requested == NIL);
            pgfdw_finish_pre_commit_cleanup(pending_entries);
        }
        else
        {
            Assert(event == XACT_EVENT_PARALLEL_ABORT ||
                   event == XACT_EVENT_ABORT);
            pgfdw_finish_abort_cleanup(pending_entries, cancel_requested,
                                       true);
        }
    }
 
    /*
     * Regardless of the event type, we can now mark ourselves as out of the
     * transaction.  (Note: if we are here during PRE_COMMIT or PRE_PREPARE,
     * this saves a useless scan of the hashtable during COMMIT or PREPARE.)
     */
    xact_got_connection = false;
 
    /* Also reset cursor numbering for next transaction */
    cursor_number = 0;
}
 
/*
 * pgfdw_subxact_callback --- cleanup at subtransaction end.
 */
static void
pgfdw_subxact_callback(SubXactEvent event, SubTransactionId mySubid,
                       SubTransactionId parentSubid, void *arg)
{
    HASH_SEQ_STATUS scan;
    ConnCacheEntry *entry;
    int         curlevel;
    List       *pending_entries = NIL;
    List       *cancel_requested = NIL;
 
    /* Nothing to do at subxact start, nor after commit. */
    if (!(event == SUBXACT_EVENT_PRE_COMMIT_SUB ||
          event == SUBXACT_EVENT_ABORT_SUB))
        return;
 
    /* Quick exit if no connections were touched in this transaction. */
    if (!xact_got_connection)
        return;
 
    /*
     * Scan all connection cache entries to find open remote subtransactions
     * of the current level, and close them.
     */
    curlevel = GetCurrentTransactionNestLevel();
    hash_seq_init(&scan, ConnectionHash);
    while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
    {
        char        sql[100];
 
        /*
         * We only care about connections with open remote subtransactions of
         * the current level.
         */
        if (entry->conn == NULL || entry->xact_depth < curlevel)
            continue;
 
        if (entry->xact_depth > curlevel)
            elog(ERROR, "missed cleaning up remote subtransaction at level %d",
                 entry->xact_depth);
 
        if (event == SUBXACT_EVENT_PRE_COMMIT_SUB)
        {
            /*
             * If abort cleanup previously failed for this connection, we
             * can't issue any more commands against it.
             */
            pgfdw_reject_incomplete_xact_state_change(entry);
 
            /* Commit all remote subtransactions during pre-commit */
            snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
            entry->changing_xact_state = true;
            if (entry->parallel_commit)
            {
                do_sql_command_begin(entry->conn, sql);
                pending_entries = lappend(pending_entries, entry);
                continue;
            }
            do_sql_command(entry->conn, sql);
            entry->changing_xact_state = false;
        }
        else
        {
            /* Rollback all remote subtransactions during abort */
            if (entry->parallel_abort)
            {
                if (pgfdw_abort_cleanup_begin(entry, false,
                                              &pending_entries,
                                              &cancel_requested))
                    continue;
            }
            else
                pgfdw_abort_cleanup(entry, false);
        }
 
        /* OK, we're outta that level of subtransaction */
        pgfdw_reset_xact_state(entry, false);
    }
 
    /* If there are any pending connections, finish cleaning them up */
    if (pending_entries || cancel_requested)
    {
        if (event == SUBXACT_EVENT_PRE_COMMIT_SUB)
        {
            Assert(cancel_requested == NIL);
            pgfdw_finish_pre_subcommit_cleanup(pending_entries, curlevel);
        }
        else
        {
            Assert(event == SUBXACT_EVENT_ABORT_SUB);
            pgfdw_finish_abort_cleanup(pending_entries, cancel_requested,
                                       false);
        }
    }
}
 
/*
 * Connection invalidation callback function
 *
 * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
 * close connections depending on that entry immediately if current transaction
 * has not used those connections yet. Otherwise, mark those connections as
 * invalid and then make pgfdw_xact_callback() close them at the end of current
 * transaction, since they cannot be closed in the midst of the transaction
 * using them. Closed connections will be remade at the next opportunity if
 * necessary.
 *
 * Although most cache invalidation callbacks blow away all the related stuff
 * regardless of the given hashvalue, connections are expensive enough that
 * it's worth trying to avoid that.
 *
 * NB: We could avoid unnecessary disconnection more strictly by examining
 * individual option values, but it seems too much effort for the gain.
 */
static void
pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue)
{
    HASH_SEQ_STATUS scan;
    ConnCacheEntry *entry;
 
    Assert(cacheid == FOREIGNSERVEROID || cacheid == USERMAPPINGOID);
 
    /* ConnectionHash must exist already, if we're registered */
    hash_seq_init(&scan, ConnectionHash);
    while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
    {
        /* Ignore invalid entries */
        if (entry->conn == NULL)
            continue;
 
        /* hashvalue == 0 means a cache reset, must clear all state */
        if (hashvalue == 0 ||
            (cacheid == FOREIGNSERVEROID &&
             entry->server_hashvalue == hashvalue) ||
            (cacheid == USERMAPPINGOID &&
             entry->mapping_hashvalue == hashvalue))
        {
            /*
             * Close the connection immediately if it's not used yet in this
             * transaction. Otherwise mark it as invalid so that
             * pgfdw_xact_callback() can close it at the end of this
             * transaction.
             */
            if (entry->xact_depth == 0)
            {
                elog(DEBUG3, "discarding connection %p", entry->conn);
                disconnect_pg_server(entry);
            }
            else
                entry->invalidated = true;
        }
    }
}
 
/*
 * Raise an error if the given connection cache entry is marked as being
 * in the middle of an xact state change.  This should be called at which no
 * such change is expected to be in progress; if one is found to be in
 * progress, it means that we aborted in the middle of a previous state change
 * and now don't know what the remote transaction state actually is.
 * Such connections can't safely be further used.  Re-establishing the
 * connection would change the snapshot and roll back any writes already
 * performed, so that's not an option, either. Thus, we must abort.
 */
static void
pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
{
    ForeignServer *server;
 
    /* nothing to do for inactive entries and entries of sane state */
    if (entry->conn == NULL || !entry->changing_xact_state)
        return;
 
    /* make sure this entry is inactive */
    disconnect_pg_server(entry);
 
    /* find server name to be shown in the message below */
    server = GetForeignServer(entry->serverid);
 
    ereport(ERROR,
            (errcode(ERRCODE_CONNECTION_EXCEPTION),
             errmsg("connection to server \"%s\" was lost",
                    server->servername)));
}
 
/*
 * Reset state to show we're out of a (sub)transaction.
 */
static void
pgfdw_reset_xact_state(ConnCacheEntry *entry, bool toplevel)
{
    if (toplevel)
    {
        /* Reset state to show we're out of a transaction */
        entry->xact_depth = 0;
 
        /*
         * If the connection isn't in a good idle state, it is marked as
         * invalid or keep_connections option of its server is disabled, then
         * discard it to recover. Next GetConnection will open a new
         * connection.
         */
        if (PQstatus(entry->conn) != CONNECTION_OK ||
            PQtransactionStatus(entry->conn) != PQTRANS_IDLE ||
            entry->changing_xact_state ||
            entry->invalidated ||
            !entry->keep_connections)
        {
            elog(DEBUG3, "discarding connection %p", entry->conn);
            disconnect_pg_server(entry);
        }
    }
    else
    {
        /* Reset state to show we're out of a subtransaction */
        entry->xact_depth--;
    }
}
 
/*
 * Cancel the currently-in-progress query (whose query text we do not have)
 * and ignore the result.  Returns true if we successfully cancel the query
 * and discard any pending result, and false if not.
 *
 * It's not a huge problem if we throw an ERROR here, but if we get into error
 * recursion trouble, we'll end up slamming the connection shut, which will
 * necessitate failing the entire toplevel transaction even if subtransactions
 * were used.  Try to use WARNING where we can.
 *
 * XXX: if the query was one sent by fetch_more_data_begin(), we could get the
 * query text from the pendingAreq saved in the per-connection state, then
 * report the query using it.
 */
static bool
pgfdw_cancel_query(PGconn *conn)
{
    TimestampTz endtime;
 
    /*
     * If it takes too long to cancel the query and discard the result, assume
     * the connection is dead.
     */
    endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                          CONNECTION_CLEANUP_TIMEOUT);
 
    if (!pgfdw_cancel_query_begin(conn))
        return false;
    return pgfdw_cancel_query_end(conn, endtime, false);
}
 
static bool
pgfdw_cancel_query_begin(PGconn *conn)
{
    PGcancel   *cancel;
    char        errbuf[256];
 
    /*
     * Issue cancel request.  Unfortunately, there's no good way to limit the
     * amount of time that we might block inside PQgetCancel().
     */
    if ((cancel = PQgetCancel(conn)))
    {
        if (!PQcancel(cancel, errbuf, sizeof(errbuf)))
        {
            ereport(WARNING,
                    (errcode(ERRCODE_CONNECTION_FAILURE),
                     errmsg("could not send cancel request: %s",
                            errbuf)));
            PQfreeCancel(cancel);
            return false;
        }
        PQfreeCancel(cancel);
    }
 
    return true;
}
 
static bool
pgfdw_cancel_query_end(PGconn *conn, TimestampTz endtime, bool consume_input)
{
    PGresult   *result = NULL;
    bool        timed_out;
 
    /*
     * If requested, consume whatever data is available from the socket. (Note
     * that if all data is available, this allows pgfdw_get_cleanup_result to
     * call PQgetResult without forcing the overhead of WaitLatchOrSocket,
     * which would be large compared to the overhead of PQconsumeInput.)
     */
    if (consume_input && !PQconsumeInput(conn))
    {
        ereport(WARNING,
                (errcode(ERRCODE_CONNECTION_FAILURE),
                 errmsg("could not get result of cancel request: %s",
                        pchomp(PQerrorMessage(conn)))));
        return false;
    }
 
    /* Get and discard the result of the query. */
    if (pgfdw_get_cleanup_result(conn, endtime, &result, &timed_out))
    {
        if (timed_out)
            ereport(WARNING,
                    (errmsg("could not get result of cancel request due to timeout")));
        else
            ereport(WARNING,
                    (errcode(ERRCODE_CONNECTION_FAILURE),
                     errmsg("could not get result of cancel request: %s",
                            pchomp(PQerrorMessage(conn)))));
 
        return false;
    }
    PQclear(result);
 
    return true;
}
 
/*
 * Submit a query during (sub)abort cleanup and wait up to 30 seconds for the
 * result.  If the query is executed without error, the return value is true.
 * If the query is executed successfully but returns an error, the return
 * value is true if and only if ignore_errors is set.  If the query can't be
 * sent or times out, the return value is false.
 *
 * It's not a huge problem if we throw an ERROR here, but if we get into error
 * recursion trouble, we'll end up slamming the connection shut, which will
 * necessitate failing the entire toplevel transaction even if subtransactions
 * were used.  Try to use WARNING where we can.
 */
static bool
pgfdw_exec_cleanup_query(PGconn *conn, const char *query, bool ignore_errors)
{
    TimestampTz endtime;
 
    /*
     * If it takes too long to execute a cleanup query, assume the connection
     * is dead.  It's fairly likely that this is why we aborted in the first
     * place (e.g. statement timeout, user cancel), so the timeout shouldn't
     * be too long.
     */
    endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                          CONNECTION_CLEANUP_TIMEOUT);
 
    if (!pgfdw_exec_cleanup_query_begin(conn, query))
        return false;
    return pgfdw_exec_cleanup_query_end(conn, query, endtime,
                                        false, ignore_errors);
}
 
static bool
pgfdw_exec_cleanup_query_begin(PGconn *conn, const char *query)
{
    /*
     * Submit a query.  Since we don't use non-blocking mode, this also can
     * block.  But its risk is relatively small, so we ignore that for now.
     */
    if (!PQsendQuery(conn, query))
    {
        pgfdw_report_error(WARNING, NULL, conn, false, query);
        return false;
    }
 
    return true;
}
 
static bool
pgfdw_exec_cleanup_query_end(PGconn *conn, const char *query,
                             TimestampTz endtime, bool consume_input,
                             bool ignore_errors)
{
    PGresult   *result = NULL;
    bool        timed_out;
 
    /*
     * If requested, consume whatever data is available from the socket. (Note
     * that if all data is available, this allows pgfdw_get_cleanup_result to
     * call PQgetResult without forcing the overhead of WaitLatchOrSocket,
     * which would be large compared to the overhead of PQconsumeInput.)
     */
    if (consume_input && !PQconsumeInput(conn))
    {
        pgfdw_report_error(WARNING, NULL, conn, false, query);
        return false;
    }
 
    /* Get the result of the query. */
    if (pgfdw_get_cleanup_result(conn, endtime, &result, &timed_out))
    {
        if (timed_out)
            ereport(WARNING,
                    (errmsg("could not get query result due to timeout"),
                     query ? errcontext("remote SQL command: %s", query) : 0));
        else
            pgfdw_report_error(WARNING, NULL, conn, false, query);
 
        return false;
    }
 
    /* Issue a warning if not successful. */
    if (PQresultStatus(result) != PGRES_COMMAND_OK)
    {
        pgfdw_report_error(WARNING, result, conn, true, query);
        return ignore_errors;
    }
    PQclear(result);
 
    return true;
}
 
/*
 * Get, during abort cleanup, the result of a query that is in progress.  This
 * might be a query that is being interrupted by transaction abort, or it might
 * be a query that was initiated as part of transaction abort to get the remote
 * side back to the appropriate state.
 *
 * endtime is the time at which we should give up and assume the remote
 * side is dead.  Returns true if the timeout expired or connection trouble
 * occurred, false otherwise.  Sets *result except in case of a timeout.
 * Sets timed_out to true only when the timeout expired.
 */
static bool
pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime, PGresult **result,
                         bool *timed_out)
{
    volatile bool failed = false;
    PGresult   *volatile last_res = NULL;
 
    *timed_out = false;
 
    /* In what follows, do not leak any PGresults on an error. */
    PG_TRY();
    {
        for (;;)
        {
            PGresult   *res;
 
            while (PQisBusy(conn))
            {
                int         wc;
                TimestampTz now = GetCurrentTimestamp();
                long        cur_timeout;
 
                /* If timeout has expired, give up, else get sleep time. */
                cur_timeout = TimestampDifferenceMilliseconds(now, endtime);
                if (cur_timeout <= 0)
                {
                    *timed_out = true;
                    failed = true;
                    goto exit;
                }
 
                /* Sleep until there's something to do */
                wc = WaitLatchOrSocket(MyLatch,
                                       WL_LATCH_SET | WL_SOCKET_READABLE |
                                       WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                                       PQsocket(conn),
                                       cur_timeout, WAIT_EVENT_EXTENSION);
                ResetLatch(MyLatch);
 
                CHECK_FOR_INTERRUPTS();
 
                /* Data available in socket? */
                if (wc & WL_SOCKET_READABLE)
                {
                    if (!PQconsumeInput(conn))
                    {
                        /* connection trouble */
                        failed = true;
                        goto exit;
                    }
                }
            }
 
            res = PQgetResult(conn);
            if (res == NULL)
                break;          /* query is complete */
 
            PQclear(last_res);
            last_res = res;
        }
exit:   ;
    }
    PG_CATCH();
    {
        PQclear(last_res);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    if (failed)
        PQclear(last_res);
    else
        *result = last_res;
    return failed;
}
 
/*
 * Abort remote transaction or subtransaction.
 *
 * "toplevel" should be set to true if toplevel (main) transaction is
 * rollbacked, false otherwise.
 *
 * Set entry->changing_xact_state to false on success, true on failure.
 */
static void
pgfdw_abort_cleanup(ConnCacheEntry *entry, bool toplevel)
{
    char        sql[100];
 
    /*
     * Don't try to clean up the connection if we're already in error
     * recursion trouble.
     */
    if (in_error_recursion_trouble())
        entry->changing_xact_state = true;
 
    /*
     * If connection is already unsalvageable, don't touch it further.
     */
    if (entry->changing_xact_state)
        return;
 
    /*
     * Mark this connection as in the process of changing transaction state.
     */
    entry->changing_xact_state = true;
 
    /* Assume we might have lost track of prepared statements */
    entry->have_error = true;
 
    /*
     * If a command has been submitted to the remote server by using an
     * asynchronous execution function, the command might not have yet
     * completed.  Check to see if a command is still being processed by the
     * remote server, and if so, request cancellation of the command.
     */
    if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE &&
        !pgfdw_cancel_query(entry->conn))
        return;                 /* Unable to cancel running query */
 
    CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
    if (!pgfdw_exec_cleanup_query(entry->conn, sql, false))
        return;                 /* Unable to abort remote (sub)transaction */
 
    if (toplevel)
    {
        if (entry->have_prep_stmt && entry->have_error &&
            !pgfdw_exec_cleanup_query(entry->conn,
                                      "DEALLOCATE ALL",
                                      true))
            return;             /* Trouble clearing prepared statements */
 
        entry->have_prep_stmt = false;
        entry->have_error = false;
    }
 
    /*
     * If pendingAreq of the per-connection state is not NULL, it means that
     * an asynchronous fetch begun by fetch_more_data_begin() was not done
     * successfully and thus the per-connection state was not reset in
     * fetch_more_data(); in that case reset the per-connection state here.
     */
    if (entry->state.pendingAreq)
        memset(&entry->state, 0, sizeof(entry->state));
 
    /* Disarm changing_xact_state if it all worked */
    entry->changing_xact_state = false;
}
 
/*
 * Like pgfdw_abort_cleanup, submit an abort command or cancel request, but
 * don't wait for the result.
 *
 * Returns true if the abort command or cancel request is successfully issued,
 * false otherwise.  If the abort command is successfully issued, the given
 * connection cache entry is appended to *pending_entries.  Otherwise, if the
 * cancel request is successfully issued, it is appended to *cancel_requested.
 */
static bool
pgfdw_abort_cleanup_begin(ConnCacheEntry *entry, bool toplevel,
                          List **pending_entries, List **cancel_requested)
{
    /*
     * Don't try to clean up the connection if we're already in error
     * recursion trouble.
     */
    if (in_error_recursion_trouble())
        entry->changing_xact_state = true;
 
    /*
     * If connection is already unsalvageable, don't touch it further.
     */
    if (entry->changing_xact_state)
        return false;
 
    /*
     * Mark this connection as in the process of changing transaction state.
     */
    entry->changing_xact_state = true;
 
    /* Assume we might have lost track of prepared statements */
    entry->have_error = true;
 
    /*
     * If a command has been submitted to the remote server by using an
     * asynchronous execution function, the command might not have yet
     * completed.  Check to see if a command is still being processed by the
     * remote server, and if so, request cancellation of the command.
     */
    if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE)
    {
        if (!pgfdw_cancel_query_begin(entry->conn))
            return false;       /* Unable to cancel running query */
        *cancel_requested = lappend(*cancel_requested, entry);
    }
    else
    {
        char        sql[100];
 
        CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
        if (!pgfdw_exec_cleanup_query_begin(entry->conn, sql))
            return false;       /* Unable to abort remote transaction */
        *pending_entries = lappend(*pending_entries, entry);
    }
 
    return true;
}
 
/*
 * Finish pre-commit cleanup of connections on each of which we've sent a
 * COMMIT command to the remote server.
 */
static void
pgfdw_finish_pre_commit_cleanup(List *pending_entries)
{
    ConnCacheEntry *entry;
    List       *pending_deallocs = NIL;
    ListCell   *lc;
 
    Assert(pending_entries);
 
    /*
     * Get the result of the COMMIT command for each of the pending entries
     */
    foreach(lc, pending_entries)
    {
        entry = (ConnCacheEntry *) lfirst(lc);
 
        Assert(entry->changing_xact_state);
 
        /*
         * We might already have received the result on the socket, so pass
         * consume_input=true to try to consume it first
         */
        do_sql_command_end(entry->conn, "COMMIT TRANSACTION", true);
        entry->changing_xact_state = false;
 
        /* Do a DEALLOCATE ALL in parallel if needed */
        if (entry->have_prep_stmt && entry->have_error)
        {
            /* Ignore errors (see notes in pgfdw_xact_callback) */
            if (PQsendQuery(entry->conn, "DEALLOCATE ALL"))
            {
                pending_deallocs = lappend(pending_deallocs, entry);
                continue;
            }
        }
        entry->have_prep_stmt = false;
        entry->have_error = false;
 
        pgfdw_reset_xact_state(entry, true);
    }
 
    /* No further work if no pending entries */
    if (!pending_deallocs)
        return;
 
    /*
     * Get the result of the DEALLOCATE command for each of the pending
     * entries
     */
    foreach(lc, pending_deallocs)
    {
        PGresult   *res;
 
        entry = (ConnCacheEntry *) lfirst(lc);
 
        /* Ignore errors (see notes in pgfdw_xact_callback) */
        while ((res = PQgetResult(entry->conn)) != NULL)
        {
            PQclear(res);
            /* Stop if the connection is lost (else we'll loop infinitely) */
            if (PQstatus(entry->conn) == CONNECTION_BAD)
                break;
        }
        entry->have_prep_stmt = false;
        entry->have_error = false;
 
        pgfdw_reset_xact_state(entry, true);
    }
}
 
/*
 * Finish pre-subcommit cleanup of connections on each of which we've sent a
 * RELEASE command to the remote server.
 */
static void
pgfdw_finish_pre_subcommit_cleanup(List *pending_entries, int curlevel)
{
    ConnCacheEntry *entry;
    char        sql[100];
    ListCell   *lc;
 
    Assert(pending_entries);
 
    /*
     * Get the result of the RELEASE command for each of the pending entries
     */
    snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
    foreach(lc, pending_entries)
    {
        entry = (ConnCacheEntry *) lfirst(lc);
 
        Assert(entry->changing_xact_state);
 
        /*
         * We might already have received the result on the socket, so pass
         * consume_input=true to try to consume it first
         */
        do_sql_command_end(entry->conn, sql, true);
        entry->changing_xact_state = false;
 
        pgfdw_reset_xact_state(entry, false);
    }
}
 
/*
 * Finish abort cleanup of connections on each of which we've sent an abort
 * command or cancel request to the remote server.
 */
static void
pgfdw_finish_abort_cleanup(List *pending_entries, List *cancel_requested,
                           bool toplevel)
{
    List       *pending_deallocs = NIL;
    ListCell   *lc;
 
    /*
     * For each of the pending cancel requests (if any), get and discard the
     * result of the query, and submit an abort command to the remote server.
     */
    if (cancel_requested)
    {
        foreach(lc, cancel_requested)
        {
            ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
            TimestampTz endtime;
            char        sql[100];
 
            Assert(entry->changing_xact_state);
 
            /*
             * Set end time.  You might think we should do this before issuing
             * cancel request like in normal mode, but that is problematic,
             * because if, for example, it took longer than 30 seconds to
             * process the first few entries in the cancel_requested list, it
             * would cause a timeout error when processing each of the
             * remaining entries in the list, leading to slamming that entry's
             * connection shut.
             */
            endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                                  CONNECTION_CLEANUP_TIMEOUT);
 
            if (!pgfdw_cancel_query_end(entry->conn, endtime, true))
            {
                /* Unable to cancel running query */
                pgfdw_reset_xact_state(entry, toplevel);
                continue;
            }
 
            /* Send an abort command in parallel if needed */
            CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
            if (!pgfdw_exec_cleanup_query_begin(entry->conn, sql))
            {
                /* Unable to abort remote (sub)transaction */
                pgfdw_reset_xact_state(entry, toplevel);
            }
            else
                pending_entries = lappend(pending_entries, entry);
        }
    }
 
    /* No further work if no pending entries */
    if (!pending_entries)
        return;
 
    /*
     * Get the result of the abort command for each of the pending entries
     */
    foreach(lc, pending_entries)
    {
        ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
        TimestampTz endtime;
        char        sql[100];
 
        Assert(entry->changing_xact_state);
 
        /*
         * Set end time.  We do this now, not before issuing the command like
         * in normal mode, for the same reason as for the cancel_requested
         * entries.
         */
        endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                              CONNECTION_CLEANUP_TIMEOUT);
 
        CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
        if (!pgfdw_exec_cleanup_query_end(entry->conn, sql, endtime,
                                          true, false))
        {
            /* Unable to abort remote (sub)transaction */
            pgfdw_reset_xact_state(entry, toplevel);
            continue;
        }
 
        if (toplevel)
        {
            /* Do a DEALLOCATE ALL in parallel if needed */
            if (entry->have_prep_stmt && entry->have_error)
            {
                if (!pgfdw_exec_cleanup_query_begin(entry->conn,
                                                    "DEALLOCATE ALL"))
                {
                    /* Trouble clearing prepared statements */
                    pgfdw_reset_xact_state(entry, toplevel);
                }
                else
                    pending_deallocs = lappend(pending_deallocs, entry);
                continue;
            }
            entry->have_prep_stmt = false;
            entry->have_error = false;
        }
 
        /* Reset the per-connection state if needed */
        if (entry->state.pendingAreq)
            memset(&entry->state, 0, sizeof(entry->state));
 
        /* We're done with this entry; unset the changing_xact_state flag */
        entry->changing_xact_state = false;
        pgfdw_reset_xact_state(entry, toplevel);
    }
 
    /* No further work if no pending entries */
    if (!pending_deallocs)
        return;
    Assert(toplevel);
 
    /*
     * Get the result of the DEALLOCATE command for each of the pending
     * entries
     */
    foreach(lc, pending_deallocs)
    {
        ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
        TimestampTz endtime;
 
        Assert(entry->changing_xact_state);
        Assert(entry->have_prep_stmt);
        Assert(entry->have_error);
 
        /*
         * Set end time.  We do this now, not before issuing the command like
         * in normal mode, for the same reason as for the cancel_requested
         * entries.
         */
        endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                              CONNECTION_CLEANUP_TIMEOUT);
 
        if (!pgfdw_exec_cleanup_query_end(entry->conn, "DEALLOCATE ALL",
                                          endtime, true, true))
        {
            /* Trouble clearing prepared statements */
            pgfdw_reset_xact_state(entry, toplevel);
            continue;
        }
        entry->have_prep_stmt = false;
        entry->have_error = false;
 
        /* Reset the per-connection state if needed */
        if (entry->state.pendingAreq)
            memset(&entry->state, 0, sizeof(entry->state));
 
        /* We're done with this entry; unset the changing_xact_state flag */
        entry->changing_xact_state = false;
        pgfdw_reset_xact_state(entry, toplevel);
    }
}
 
/*
 * List active foreign server connections.
 *
 * This function takes no input parameter and returns setof record made of
 * following values:
 * - server_name - server name of active connection. In case the foreign server
 *   is dropped but still the connection is active, then the server name will
 *   be NULL in output.
 * - valid - true/false representing whether the connection is valid or not.
 *   Note that the connections can get invalidated in pgfdw_inval_callback.
 *
 * No records are returned when there are no cached connections at all.
 */
Datum
postgres_fdw_get_connections(PG_FUNCTION_ARGS)
{
#define POSTGRES_FDW_GET_CONNECTIONS_COLS   2
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    HASH_SEQ_STATUS scan;
    ConnCacheEntry *entry;
 
    InitMaterializedSRF(fcinfo, 0);
 
    /* If cache doesn't exist, we return no records */
    if (!ConnectionHash)
        PG_RETURN_VOID();
 
    hash_seq_init(&scan, ConnectionHash);
    while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
    {
        ForeignServer *server;
        Datum       values[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
        bool        nulls[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
 
        /* We only look for open remote connections */
        if (!entry->conn)
            continue;
 
        server = GetForeignServerExtended(entry->serverid, FSV_MISSING_OK);
 
        /*
         * The foreign server may have been dropped in current explicit
         * transaction. It is not possible to drop the server from another
         * session when the connection associated with it is in use in the
         * current transaction, if tried so, the drop query in another session
         * blocks until the current transaction finishes.
         *
         * Even though the server is dropped in the current transaction, the
         * cache can still have associated active connection entry, say we
         * call such connections dangling. Since we can not fetch the server
         * name from system catalogs for dangling connections, instead we show
         * NULL value for server name in output.
         *
         * We could have done better by storing the server name in the cache
         * entry instead of server oid so that it could be used in the output.
         * But the server name in each cache entry requires 64 bytes of
         * memory, which is huge, when there are many cached connections and
         * the use case i.e. dropping the foreign server within the explicit
         * current transaction seems rare. So, we chose to show NULL value for
         * server name in output.
         *
         * Such dangling connections get closed either in next use or at the
         * end of current explicit transaction in pgfdw_xact_callback.
         */
        if (!server)
        {
            /*
             * If the server has been dropped in the current explicit
             * transaction, then this entry would have been invalidated in
             * pgfdw_inval_callback at the end of drop server command. Note
             * that this connection would not have been closed in
             * pgfdw_inval_callback because it is still being used in the
             * current explicit transaction. So, assert that here.
             */
            Assert(entry->conn && entry->xact_depth > 0 && entry->invalidated);
 
            /* Show null, if no server name was found */
            nulls[0] = true;
        }
        else
            values[0] = CStringGetTextDatum(server->servername);
 
        values[1] = BoolGetDatum(!entry->invalidated);
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
    }
 
    PG_RETURN_VOID();
}
 
/*
 * Disconnect the specified cached connections.
 *
 * This function discards the open connections that are established by
 * postgres_fdw from the local session to the foreign server with
 * the given name. Note that there can be multiple connections to
 * the given server using different user mappings. If the connections
 * are used in the current local transaction, they are not disconnected
 * and warning messages are reported. This function returns true
 * if it disconnects at least one connection, otherwise false. If no
 * foreign server with the given name is found, an error is reported.
 */
Datum
postgres_fdw_disconnect(PG_FUNCTION_ARGS)
{
    ForeignServer *server;
    char       *servername;
 
    servername = text_to_cstring(PG_GETARG_TEXT_PP(0));
    server = GetForeignServerByName(servername, false);
 
    PG_RETURN_BOOL(disconnect_cached_connections(server->serverid));
}
 
/*
 * Disconnect all the cached connections.
 *
 * This function discards all the open connections that are established by
 * postgres_fdw from the local session to the foreign servers.
 * If the connections are used in the current local transaction, they are
 * not disconnected and warning messages are reported. This function
 * returns true if it disconnects at least one connection, otherwise false.
 */
Datum
postgres_fdw_disconnect_all(PG_FUNCTION_ARGS)
{
    PG_RETURN_BOOL(disconnect_cached_connections(InvalidOid));
}
 
/*
 * Workhorse to disconnect cached connections.
 *
 * This function scans all the connection cache entries and disconnects
 * the open connections whose foreign server OID matches with
 * the specified one. If InvalidOid is specified, it disconnects all
 * the cached connections.
 *
 * This function emits a warning for each connection that's used in
 * the current transaction and doesn't close it. It returns true if
 * it disconnects at least one connection, otherwise false.
 *
 * Note that this function disconnects even the connections that are
 * established by other users in the same local session using different
 * user mappings. This leads even non-superuser to be able to close
 * the connections established by superusers in the same local session.
 *
 * XXX As of now we don't see any security risk doing this. But we should
 * set some restrictions on that, for example, prevent non-superuser
 * from closing the connections established by superusers even
 * in the same session?
 */
static bool
disconnect_cached_connections(Oid serverid)
{
    HASH_SEQ_STATUS scan;
    ConnCacheEntry *entry;
    bool        all = !OidIsValid(serverid);
    bool        result = false;
 
    /*
     * Connection cache hashtable has not been initialized yet in this
     * session, so return false.
     */
    if (!ConnectionHash)
        return false;
 
    hash_seq_init(&scan, ConnectionHash);
    while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
    {
        /* Ignore cache entry if no open connection right now. */
        if (!entry->conn)
            continue;
 
        if (all || entry->serverid == serverid)
        {
            /*
             * Emit a warning because the connection to close is used in the
             * current transaction and cannot be disconnected right now.
             */
            if (entry->xact_depth > 0)
            {
                ForeignServer *server;
 
                server = GetForeignServerExtended(entry->serverid,
                                                  FSV_MISSING_OK);
 
                if (!server)
                {
                    /*
                     * If the foreign server was dropped while its connection
                     * was used in the current transaction, the connection
                     * must have been marked as invalid by
                     * pgfdw_inval_callback at the end of DROP SERVER command.
                     */
                    Assert(entry->invalidated);
 
                    ereport(WARNING,
                            (errmsg("cannot close dropped server connection because it is still in use")));
                }
                else
                    ereport(WARNING,
                            (errmsg("cannot close connection for server \"%s\" because it is still in use",
                                    server->servername)));
            }
            else
            {
                elog(DEBUG3, "discarding connection %p", entry->conn);
                disconnect_pg_server(entry);
                result = true;
            }
        }
    }
 
    return result;
}