portalcmds.c

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/*-------------------------------------------------------------------------
 *
 * portalcmds.c
 *    Utility commands affecting portals (that is, SQL cursor commands)
 *
 * Note: see also tcop/pquery.c, which implements portal operations for
 * the FE/BE protocol.  This module uses pquery.c for some operations.
 * And both modules depend on utils/mmgr/portalmem.c, which controls
 * storage management for portals (but doesn't run any queries in them).
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/portalcmds.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "access/xact.h"
#include "commands/portalcmds.h"
#include "executor/executor.h"
#include "executor/tstoreReceiver.h"
#include "miscadmin.h"
#include "rewrite/rewriteHandler.h"
#include "tcop/pquery.h"
#include "tcop/tcopprot.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"


/*
 * PerformCursorOpen
 *      Execute SQL DECLARE CURSOR command.
 */
void
PerformCursorOpen(ParseState *pstate, DeclareCursorStmt *cstmt, ParamListInfo params,
                  bool isTopLevel)
{
    Query      *query = castNode(Query, cstmt->query);
    List       *rewritten;
    PlannedStmt *plan;
    Portal      portal;
    MemoryContext oldContext;
    char       *queryString;

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (!cstmt->portalname || cstmt->portalname[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /*
     * If this is a non-holdable cursor, we require that this statement has
     * been executed inside a transaction block (or else, it would have no
     * user-visible effect).
     */
    if (!(cstmt->options & CURSOR_OPT_HOLD))
        RequireTransactionBlock(isTopLevel, "DECLARE CURSOR");
    else if (InSecurityRestrictedOperation())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("cannot create a cursor WITH HOLD within security-restricted operation")));

    /*
     * Parse analysis was done already, but we still have to run the rule
     * rewriter.  We do not do AcquireRewriteLocks: we assume the query either
     * came straight from the parser, or suitable locks were acquired by
     * plancache.c.
     */
    rewritten = QueryRewrite(query);

    /* SELECT should never rewrite to more or less than one query */
    if (list_length(rewritten) != 1)
        elog(ERROR, "non-SELECT statement in DECLARE CURSOR");

    query = linitial_node(Query, rewritten);

    if (query->commandType != CMD_SELECT)
        elog(ERROR, "non-SELECT statement in DECLARE CURSOR");

    /* Plan the query, applying the specified options */
    plan = pg_plan_query(query, pstate->p_sourcetext, cstmt->options, params);

    /*
     * Create a portal and copy the plan and query string into its memory.
     */
    portal = CreatePortal(cstmt->portalname, false, false);

    oldContext = MemoryContextSwitchTo(portal->portalContext);

    plan = copyObject(plan);

    queryString = pstrdup(pstate->p_sourcetext);

    PortalDefineQuery(portal,
                      NULL,
                      queryString,
                      CMDTAG_SELECT,    /* cursor's query is always a SELECT */
                      list_make1(plan),
                      NULL);

    /*----------
     * Also copy the outer portal's parameter list into the inner portal's
     * memory context.  We want to pass down the parameter values in case we
     * had a command like
     *      DECLARE c CURSOR FOR SELECT ... WHERE foo = $1
     * This will have been parsed using the outer parameter set and the
     * parameter value needs to be preserved for use when the cursor is
     * executed.
     *----------
     */
    params = copyParamList(params);

    MemoryContextSwitchTo(oldContext);

    /*
     * Set up options for portal.
     *
     * If the user didn't specify a SCROLL type, allow or disallow scrolling
     * based on whether it would require any additional runtime overhead to do
     * so.  Also, we disallow scrolling for FOR UPDATE cursors.
     */
    portal->cursorOptions = cstmt->options;
    if (!(portal->cursorOptions & (CURSOR_OPT_SCROLL | CURSOR_OPT_NO_SCROLL)))
    {
        if (plan->rowMarks == NIL &&
            ExecSupportsBackwardScan(plan->planTree))
            portal->cursorOptions |= CURSOR_OPT_SCROLL;
        else
            portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;
    }

    /*
     * Start execution, inserting parameters if any.
     */
    PortalStart(portal, params, 0, GetActiveSnapshot());

    Assert(portal->strategy == PORTAL_ONE_SELECT);

    /*
     * We're done; the query won't actually be run until PerformPortalFetch is
     * called.
     */
}

/*
 * PerformPortalFetch
 *      Execute SQL FETCH or MOVE command.
 *
 *  stmt: parsetree node for command
 *  dest: where to send results
 *  qc: where to store a command completion status data.
 *
 * qc may be NULL if caller doesn't want status data.
 */
void
PerformPortalFetch(FetchStmt *stmt,
                   DestReceiver *dest,
                   QueryCompletion *qc)
{
    Portal      portal;
    uint64      nprocessed;

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (!stmt->portalname || stmt->portalname[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /* get the portal from the portal name */
    portal = GetPortalByName(stmt->portalname);
    if (!PortalIsValid(portal))
    {
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_CURSOR),
                 errmsg("cursor \"%s\" does not exist", stmt->portalname)));
        return;                 /* keep compiler happy */
    }

    /* Adjust dest if needed.  MOVE wants destination DestNone */
    if (stmt->ismove)
        dest = None_Receiver;

    /* Do it */
    nprocessed = PortalRunFetch(portal,
                                stmt->direction,
                                stmt->howMany,
                                dest);

    /* Return command status if wanted */
    if (qc)
        SetQueryCompletion(qc, stmt->ismove ? CMDTAG_MOVE : CMDTAG_FETCH,
                           nprocessed);
}

/*
 * PerformPortalClose
 *      Close a cursor.
 */
void
PerformPortalClose(const char *name)
{
    Portal      portal;

    /* NULL means CLOSE ALL */
    if (name == NULL)
    {
        PortalHashTableDeleteAll();
        return;
    }

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (name[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /*
     * get the portal from the portal name
     */
    portal = GetPortalByName(name);
    if (!PortalIsValid(portal))
    {
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_CURSOR),
                 errmsg("cursor \"%s\" does not exist", name)));
        return;                 /* keep compiler happy */
    }

    /*
     * Note: PortalCleanup is called as a side-effect, if not already done.
     */
    PortalDrop(portal, false);
}

/*
 * PortalCleanup
 *
 * Clean up a portal when it's dropped.  This is the standard cleanup hook
 * for portals.
 *
 * Note: if portal->status is PORTAL_FAILED, we are probably being called
 * during error abort, and must be careful to avoid doing anything that
 * is likely to fail again.
 */
void
PortalCleanup(Portal portal)
{
    QueryDesc  *queryDesc;

    /*
     * sanity checks
     */
    AssertArg(PortalIsValid(portal));
    AssertArg(portal->cleanup == PortalCleanup);

    /*
     * Shut down executor, if still running.  We skip this during error abort,
     * since other mechanisms will take care of releasing executor resources,
     * and we can't be sure that ExecutorEnd itself wouldn't fail.
     */
    queryDesc = portal->queryDesc;
    if (queryDesc)
    {
        /*
         * Reset the queryDesc before anything else.  This prevents us from
         * trying to shut down the executor twice, in case of an error below.
         * The transaction abort mechanisms will take care of resource cleanup
         * in such a case.
         */
        portal->queryDesc = NULL;

        if (portal->status != PORTAL_FAILED)
        {
            ResourceOwner saveResourceOwner;

            /* We must make the portal's resource owner current */
            saveResourceOwner = CurrentResourceOwner;
            if (portal->resowner)
                CurrentResourceOwner = portal->resowner;

            ExecutorFinish(queryDesc);
            ExecutorEnd(queryDesc);
            FreeQueryDesc(queryDesc);

            CurrentResourceOwner = saveResourceOwner;
        }
    }
}

/*
 * PersistHoldablePortal
 *
 * Prepare the specified Portal for access outside of the current
 * transaction. When this function returns, all future accesses to the
 * portal must be done via the Tuplestore (not by invoking the
 * executor).
 */
void
PersistHoldablePortal(Portal portal)
{
    QueryDesc  *queryDesc = portal->queryDesc;
    Portal      saveActivePortal;
    ResourceOwner saveResourceOwner;
    MemoryContext savePortalContext;
    MemoryContext oldcxt;

    /*
     * If we're preserving a holdable portal, we had better be inside the
     * transaction that originally created it.
     */
    Assert(portal->createSubid != InvalidSubTransactionId);
    Assert(queryDesc != NULL);

    /*
     * Caller must have created the tuplestore already ... but not a snapshot.
     */
    Assert(portal->holdContext != NULL);
    Assert(portal->holdStore != NULL);
    Assert(portal->holdSnapshot == NULL);

    /*
     * Before closing down the executor, we must copy the tupdesc into
     * long-term memory, since it was created in executor memory.
     */
    oldcxt = MemoryContextSwitchTo(portal->holdContext);

    portal->tupDesc = CreateTupleDescCopy(portal->tupDesc);

    MemoryContextSwitchTo(oldcxt);

    /*
     * Check for improper portal use, and mark portal active.
     */
    MarkPortalActive(portal);

    /*
     * Set up global portal context pointers.
     */
    saveActivePortal = ActivePortal;
    saveResourceOwner = CurrentResourceOwner;
    savePortalContext = PortalContext;
    PG_TRY();
    {
        ScanDirection direction = ForwardScanDirection;

        ActivePortal = portal;
        if (portal->resowner)
            CurrentResourceOwner = portal->resowner;
        PortalContext = portal->portalContext;

        MemoryContextSwitchTo(PortalContext);

        PushActiveSnapshot(queryDesc->snapshot);

        /*
         * If the portal is marked scrollable, we need to store the entire
         * result set in the tuplestore, so that subsequent backward FETCHs
         * can be processed.  Otherwise, store only the not-yet-fetched rows.
         * (The latter is not only more efficient, but avoids semantic
         * problems if the query's output isn't stable.)
         *
         * In the no-scroll case, tuple indexes in the tuplestore will not
         * match the cursor's nominal position (portalPos).  Currently this
         * causes no difficulty because we only navigate in the tuplestore by
         * relative position, except for the tuplestore_skiptuples call below
         * and the tuplestore_rescan call in DoPortalRewind, both of which are
         * disabled for no-scroll cursors.  But someday we might need to track
         * the offset between the holdStore and the cursor's nominal position
         * explicitly.
         */
        if (portal->cursorOptions & CURSOR_OPT_SCROLL)
        {
            ExecutorRewind(queryDesc);
        }
        else
        {
            /*
             * If we already reached end-of-query, set the direction to
             * NoMovement to avoid trying to fetch any tuples.  (This check
             * exists because not all plan node types are robust about being
             * called again if they've already returned NULL once.)  We'll
             * still set up an empty tuplestore, though, to keep this from
             * being a special case later.
             */
            if (portal->atEnd)
                direction = NoMovementScanDirection;
        }

        /*
         * Change the destination to output to the tuplestore.  Note we tell
         * the tuplestore receiver to detoast all data passed through it; this
         * makes it safe to not keep a snapshot associated with the data.
         */
        queryDesc->dest = CreateDestReceiver(DestTuplestore);
        SetTuplestoreDestReceiverParams(queryDesc->dest,
                                        portal->holdStore,
                                        portal->holdContext,
                                        true,
                                        NULL,
                                        NULL);

        /* Fetch the result set into the tuplestore */
        ExecutorRun(queryDesc, direction, 0L, false);

        queryDesc->dest->rDestroy(queryDesc->dest);
        queryDesc->dest = NULL;

        /*
         * Now shut down the inner executor.
         */
        portal->queryDesc = NULL;   /* prevent double shutdown */
        ExecutorFinish(queryDesc);
        ExecutorEnd(queryDesc);
        FreeQueryDesc(queryDesc);

        /*
         * Set the position in the result set.
         */
        MemoryContextSwitchTo(portal->holdContext);

        if (portal->atEnd)
        {
            /*
             * Just force the tuplestore forward to its end.  The size of the
             * skip request here is arbitrary.
             */
            while (tuplestore_skiptuples(portal->holdStore, 1000000, true))
                 /* continue */ ;
        }
        else
        {
            tuplestore_rescan(portal->holdStore);

            /*
             * In the no-scroll case, the start of the tuplestore is exactly
             * where we want to be, so no repositioning is wanted.
             */
            if (portal->cursorOptions & CURSOR_OPT_SCROLL)
            {
                if (!tuplestore_skiptuples(portal->holdStore,
                                           portal->portalPos,
                                           true))
                    elog(ERROR, "unexpected end of tuple stream");
            }
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);

        /* Restore global vars and propagate error */
        ActivePortal = saveActivePortal;
        CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;

        PG_RE_THROW();
    }
    PG_END_TRY();

    MemoryContextSwitchTo(oldcxt);

    /* Mark portal not active */
    portal->status = PORTAL_READY;

    ActivePortal = saveActivePortal;
    CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;

    PopActiveSnapshot();

    /*
     * We can now release any subsidiary memory of the portal's context; we'll
     * never use it again.  The executor already dropped its context, but this
     * will clean up anything that glommed onto the portal's context via
     * PortalContext.
     */
    MemoryContextDeleteChildren(portal->portalContext);
}