lockcmds.c

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/*-------------------------------------------------------------------------
 *
 * lockcmds.c
 *    LOCK command support code
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/lockcmds.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/table.h"
#include "access/xact.h"
#include "catalog/namespace.h"
#include "catalog/pg_inherits.h"
#include "commands/lockcmds.h"
#include "miscadmin.h"
#include "nodes/nodeFuncs.h"
#include "parser/parse_clause.h"
#include "rewrite/rewriteHandler.h"
#include "storage/lmgr.h"
#include "utils/acl.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"

static void LockTableRecurse(Oid reloid, LOCKMODE lockmode, bool nowait);
static AclResult LockTableAclCheck(Oid relid, LOCKMODE lockmode, Oid userid);
static void RangeVarCallbackForLockTable(const RangeVar *rv, Oid relid,
                                         Oid oldrelid, void *arg);
static void LockViewRecurse(Oid reloid, LOCKMODE lockmode, bool nowait,
                            List *ancestor_views);

/*
 * LOCK TABLE
 */
void
LockTableCommand(LockStmt *lockstmt)
{
    ListCell   *p;

    /*
     * Iterate over the list and process the named relations one at a time
     */
    foreach(p, lockstmt->relations)
    {
        RangeVar   *rv = (RangeVar *) lfirst(p);
        bool        recurse = rv->inh;
        Oid         reloid;

        reloid = RangeVarGetRelidExtended(rv, lockstmt->mode,
                                          lockstmt->nowait ? RVR_NOWAIT : 0,
                                          RangeVarCallbackForLockTable,
                                          (void *) &lockstmt->mode);

        if (get_rel_relkind(reloid) == RELKIND_VIEW)
            LockViewRecurse(reloid, lockstmt->mode, lockstmt->nowait, NIL);
        else if (recurse)
            LockTableRecurse(reloid, lockstmt->mode, lockstmt->nowait);
    }
}

/*
 * Before acquiring a table lock on the named table, check whether we have
 * permission to do so.
 */
static void
RangeVarCallbackForLockTable(const RangeVar *rv, Oid relid, Oid oldrelid,
                             void *arg)
{
    LOCKMODE    lockmode = *(LOCKMODE *) arg;
    char        relkind;
    char        relpersistence;
    AclResult   aclresult;

    if (!OidIsValid(relid))
        return;                 /* doesn't exist, so no permissions check */
    relkind = get_rel_relkind(relid);
    if (!relkind)
        return;                 /* woops, concurrently dropped; no permissions
                                 * check */

    /* Currently, we only allow plain tables or views to be locked */
    if (relkind != RELKIND_RELATION && relkind != RELKIND_PARTITIONED_TABLE &&
        relkind != RELKIND_VIEW)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is not a table or view",
                        rv->relname)));

    /*
     * Make note if a temporary relation has been accessed in this
     * transaction.
     */
    relpersistence = get_rel_persistence(relid);
    if (relpersistence == RELPERSISTENCE_TEMP)
        MyXactFlags |= XACT_FLAGS_ACCESSEDTEMPNAMESPACE;

    /* Check permissions. */
    aclresult = LockTableAclCheck(relid, lockmode, GetUserId());
    if (aclresult != ACLCHECK_OK)
        aclcheck_error(aclresult, get_relkind_objtype(get_rel_relkind(relid)), rv->relname);
}

/*
 * Apply LOCK TABLE recursively over an inheritance tree
 *
 * This doesn't check permission to perform LOCK TABLE on the child tables,
 * because getting here means that the user has permission to lock the
 * parent which is enough.
 */
static void
LockTableRecurse(Oid reloid, LOCKMODE lockmode, bool nowait)
{
    List       *children;
    ListCell   *lc;

    children = find_all_inheritors(reloid, NoLock, NULL);

    foreach(lc, children)
    {
        Oid         childreloid = lfirst_oid(lc);

        /* Parent already locked. */
        if (childreloid == reloid)
            continue;

        if (!nowait)
            LockRelationOid(childreloid, lockmode);
        else if (!ConditionalLockRelationOid(childreloid, lockmode))
        {
            /* try to throw error by name; relation could be deleted... */
            char       *relname = get_rel_name(childreloid);

            if (!relname)
                continue;       /* child concurrently dropped, just skip it */
            ereport(ERROR,
                    (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                     errmsg("could not obtain lock on relation \"%s\"",
                            relname)));
        }

        /*
         * Even if we got the lock, child might have been concurrently
         * dropped. If so, we can skip it.
         */
        if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(childreloid)))
        {
            /* Release useless lock */
            UnlockRelationOid(childreloid, lockmode);
            continue;
        }
    }
}

/*
 * Apply LOCK TABLE recursively over a view
 *
 * All tables and views appearing in the view definition query are locked
 * recursively with the same lock mode.
 */

typedef struct
{
    LOCKMODE    lockmode;       /* lock mode to use */
    bool        nowait;         /* no wait mode */
    Oid         viewowner;      /* view owner for checking the privilege */
    Oid         viewoid;        /* OID of the view to be locked */
    List       *ancestor_views; /* OIDs of ancestor views */
} LockViewRecurse_context;

static bool
LockViewRecurse_walker(Node *node, LockViewRecurse_context *context)
{
    if (node == NULL)
        return false;

    if (IsA(node, Query))
    {
        Query      *query = (Query *) node;
        ListCell   *rtable;

        foreach(rtable, query->rtable)
        {
            RangeTblEntry *rte = lfirst(rtable);
            AclResult   aclresult;

            Oid         relid = rte->relid;
            char        relkind = rte->relkind;
            char       *relname = get_rel_name(relid);

            /*
             * The OLD and NEW placeholder entries in the view's rtable are
             * skipped.
             */
            if (relid == context->viewoid &&
                (strcmp(rte->eref->aliasname, "old") == 0 ||
                 strcmp(rte->eref->aliasname, "new") == 0))
                continue;

            /* Currently, we only allow plain tables or views to be locked. */
            if (relkind != RELKIND_RELATION && relkind != RELKIND_PARTITIONED_TABLE &&
                relkind != RELKIND_VIEW)
                continue;

            /*
             * We might be dealing with a self-referential view.  If so, we
             * can just stop recursing, since we already locked it.
             */
            if (list_member_oid(context->ancestor_views, relid))
                continue;

            /* Check permissions with the view owner's privilege. */
            aclresult = LockTableAclCheck(relid, context->lockmode, context->viewowner);
            if (aclresult != ACLCHECK_OK)
                aclcheck_error(aclresult, get_relkind_objtype(relkind), relname);

            /* We have enough rights to lock the relation; do so. */
            if (!context->nowait)
                LockRelationOid(relid, context->lockmode);
            else if (!ConditionalLockRelationOid(relid, context->lockmode))
                ereport(ERROR,
                        (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                         errmsg("could not obtain lock on relation \"%s\"",
                                relname)));

            if (relkind == RELKIND_VIEW)
                LockViewRecurse(relid, context->lockmode, context->nowait,
                                context->ancestor_views);
            else if (rte->inh)
                LockTableRecurse(relid, context->lockmode, context->nowait);
        }

        return query_tree_walker(query,
                                 LockViewRecurse_walker,
                                 context,
                                 QTW_IGNORE_JOINALIASES);
    }

    return expression_tree_walker(node,
                                  LockViewRecurse_walker,
                                  context);
}

static void
LockViewRecurse(Oid reloid, LOCKMODE lockmode, bool nowait,
                List *ancestor_views)
{
    LockViewRecurse_context context;
    Relation    view;
    Query      *viewquery;

    /* caller has already locked the view */
    view = table_open(reloid, NoLock);
    viewquery = get_view_query(view);

    context.lockmode = lockmode;
    context.nowait = nowait;
    context.viewowner = view->rd_rel->relowner;
    context.viewoid = reloid;
    context.ancestor_views = lappend_oid(ancestor_views, reloid);

    LockViewRecurse_walker((Node *) viewquery, &context);

    context.ancestor_views = list_delete_last(context.ancestor_views);

    table_close(view, NoLock);
}

/*
 * Check whether the current user is permitted to lock this relation.
 */
static AclResult
LockTableAclCheck(Oid reloid, LOCKMODE lockmode, Oid userid)
{
    AclResult   aclresult;
    AclMode     aclmask;

    /* Verify adequate privilege */
    if (lockmode == AccessShareLock)
        aclmask = ACL_SELECT;
    else if (lockmode == RowExclusiveLock)
        aclmask = ACL_INSERT | ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE;
    else
        aclmask = ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE;

    aclresult = pg_class_aclcheck(reloid, userid, aclmask);

    return aclresult;
}