lockcmds.c

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/*-------------------------------------------------------------------------
 *
 * lockcmds.c
 *    LOCK command support code
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/lockcmds.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "catalog/namespace.h"
#include "catalog/pg_inherits_fn.h"
#include "commands/lockcmds.h"
#include "miscadmin.h"
#include "parser/parse_clause.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);
static void RangeVarCallbackForLockTable(const RangeVar *rv, Oid relid,
                             Oid oldrelid, void *arg);

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

    /*---------
     * During recovery we only accept these variations:
     * LOCK TABLE foo IN ACCESS SHARE MODE
     * LOCK TABLE foo IN ROW SHARE MODE
     * LOCK TABLE foo IN ROW EXCLUSIVE MODE
     * This test must match the restrictions defined in LockAcquireExtended()
     *---------
     */
    if (lockstmt->mode > RowExclusiveLock)
        PreventCommandDuringRecovery("LOCK TABLE");

    /*
     * 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, false,
                                          lockstmt->nowait,
                                          RangeVarCallbackForLockTable,
                                          (void *) &lockstmt->mode);

        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;
    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 to be locked */
    if (relkind != RELKIND_RELATION && relkind != RELKIND_PARTITIONED_TABLE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is not a table",
                        rv->relname)));

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

/*
 * Apply LOCK TABLE recursively over an inheritance tree
 *
 * We use find_inheritance_children not find_all_inheritors to avoid taking
 * locks far in advance of checking privileges.  This means we'll visit
 * multiply-inheriting children more than once, but that's no problem.
 */
static void
LockTableRecurse(Oid reloid, LOCKMODE lockmode, bool nowait)
{
    List       *children;
    ListCell   *lc;

    children = find_inheritance_children(reloid, NoLock);

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

        /* Check permissions before acquiring the lock. */
        aclresult = LockTableAclCheck(childreloid, lockmode);
        if (aclresult != ACLCHECK_OK)
        {
            char       *relname = get_rel_name(childreloid);

            if (!relname)
                continue;       /* child concurrently dropped, just skip it */
            aclcheck_error(aclresult, ACL_KIND_CLASS, relname);
        }

        /* We have enough rights to lock the relation; do so. */
        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;
        }

        LockTableRecurse(childreloid, lockmode, nowait);
    }
}

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

    return aclresult;
}