dbcommands.c

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/*-------------------------------------------------------------------------
 *
 * dbcommands.c
 *      Database management commands (create/drop database).
 *
 * Note: database creation/destruction commands use exclusive locks on
 * the database objects (as expressed by LockSharedObject()) to avoid
 * stepping on each others' toes.  Formerly we used table-level locks
 * on pg_database, but that's too coarse-grained.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/dbcommands.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <fcntl.h>
#include <locale.h>
#include <unistd.h>
#include <sys/stat.h>

#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/xact.h"
#include "access/xlogutils.h"
#include "catalog/catalog.h"
#include "catalog/dependency.h"
#include "catalog/indexing.h"
#include "catalog/objectaccess.h"
#include "catalog/pg_authid.h"
#include "catalog/pg_database.h"
#include "catalog/pg_db_role_setting.h"
#include "catalog/pg_tablespace.h"
#include "commands/comment.h"
#include "commands/dbcommands.h"
#include "commands/seclabel.h"
#include "commands/tablespace.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/copydir.h"
#include "storage/fd.h"
#include "storage/lmgr.h"
#include "storage/ipc.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/pg_locale.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


typedef struct
{
    Oid         src_dboid;      /* source (template) DB */
    Oid         dest_dboid;     /* DB we are trying to create */
} createdb_failure_params;

typedef struct
{
    Oid         dest_dboid;     /* DB we are trying to move */
    Oid         dest_tsoid;     /* tablespace we are trying to move to */
} movedb_failure_params;

/* non-export function prototypes */
static void createdb_failure_callback(int code, Datum arg);
static void movedb(const char *dbname, const char *tblspcname);
static void movedb_failure_callback(int code, Datum arg);
static bool get_db_info(const char *name, LOCKMODE lockmode,
            Oid *dbIdP, Oid *ownerIdP,
            int *encodingP, bool *dbIsTemplateP, bool *dbAllowConnP,
            Oid *dbLastSysOidP, TransactionId *dbFrozenXidP,
            MultiXactId *dbMinMultiP,
            Oid *dbTablespace, char **dbCollate, char **dbCtype);
static bool have_createdb_privilege(void);
static void remove_dbtablespaces(Oid db_id);
static bool check_db_file_conflict(Oid db_id);
static int  errdetail_busy_db(int notherbackends, int npreparedxacts);


/*
 * CREATE DATABASE
 */
Oid
createdb(const CreatedbStmt *stmt)
{
    HeapScanDesc scan;
    Relation    rel;
    Oid         src_dboid;
    Oid         src_owner;
    int         src_encoding;
    char       *src_collate;
    char       *src_ctype;
    bool        src_istemplate;
    bool        src_allowconn;
    Oid         src_lastsysoid;
    TransactionId src_frozenxid;
    MultiXactId src_minmxid;
    Oid         src_deftablespace;
    volatile Oid dst_deftablespace;
    Relation    pg_database_rel;
    HeapTuple   tuple;
    Datum       new_record[Natts_pg_database];
    bool        new_record_nulls[Natts_pg_database];
    Oid         dboid;
    Oid         datdba;
    ListCell   *option;
    DefElem    *dtablespacename = NULL;
    DefElem    *downer = NULL;
    DefElem    *dtemplate = NULL;
    DefElem    *dencoding = NULL;
    DefElem    *dcollate = NULL;
    DefElem    *dctype = NULL;
    DefElem    *dconnlimit = NULL;
    char       *dbname = stmt->dbname;
    char       *dbowner = NULL;
    const char *dbtemplate = NULL;
    char       *dbcollate = NULL;
    char       *dbctype = NULL;
    char       *canonname;
    int         encoding = -1;
    int         dbconnlimit = -1;
    int         notherbackends;
    int         npreparedxacts;
    createdb_failure_params fparms;
    Snapshot    snapshot;

    /* Extract options from the statement node tree */
    foreach(option, stmt->options)
    {
        DefElem    *defel = (DefElem *) lfirst(option);

        if (strcmp(defel->defname, "tablespace") == 0)
        {
            if (dtablespacename)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dtablespacename = defel;
        }
        else if (strcmp(defel->defname, "owner") == 0)
        {
            if (downer)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            downer = defel;
        }
        else if (strcmp(defel->defname, "template") == 0)
        {
            if (dtemplate)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dtemplate = defel;
        }
        else if (strcmp(defel->defname, "encoding") == 0)
        {
            if (dencoding)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dencoding = defel;
        }
        else if (strcmp(defel->defname, "lc_collate") == 0)
        {
            if (dcollate)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dcollate = defel;
        }
        else if (strcmp(defel->defname, "lc_ctype") == 0)
        {
            if (dctype)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dctype = defel;
        }
        else if (strcmp(defel->defname, "connectionlimit") == 0)
        {
            if (dconnlimit)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dconnlimit = defel;
        }
        else if (strcmp(defel->defname, "location") == 0)
        {
            ereport(WARNING,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("LOCATION is not supported anymore"),
                     errhint("Consider using tablespaces instead.")));
        }
        else
            elog(ERROR, "option \"%s\" not recognized",
                 defel->defname);
    }

    if (downer && downer->arg)
        dbowner = strVal(downer->arg);
    if (dtemplate && dtemplate->arg)
        dbtemplate = strVal(dtemplate->arg);
    if (dencoding && dencoding->arg)
    {
        const char *encoding_name;

        if (IsA(dencoding->arg, Integer))
        {
            encoding = intVal(dencoding->arg);
            encoding_name = pg_encoding_to_char(encoding);
            if (strcmp(encoding_name, "") == 0 ||
                pg_valid_server_encoding(encoding_name) < 0)
                ereport(ERROR,
                        (errcode(ERRCODE_UNDEFINED_OBJECT),
                         errmsg("%d is not a valid encoding code",
                                encoding)));
        }
        else if (IsA(dencoding->arg, String))
        {
            encoding_name = strVal(dencoding->arg);
            encoding = pg_valid_server_encoding(encoding_name);
            if (encoding < 0)
                ereport(ERROR,
                        (errcode(ERRCODE_UNDEFINED_OBJECT),
                         errmsg("%s is not a valid encoding name",
                                encoding_name)));
        }
        else
            elog(ERROR, "unrecognized node type: %d",
                 nodeTag(dencoding->arg));
    }
    if (dcollate && dcollate->arg)
        dbcollate = strVal(dcollate->arg);
    if (dctype && dctype->arg)
        dbctype = strVal(dctype->arg);

    if (dconnlimit && dconnlimit->arg)
    {
        dbconnlimit = intVal(dconnlimit->arg);
        if (dbconnlimit < -1)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid connection limit: %d", dbconnlimit)));
    }

    /* obtain OID of proposed owner */
    if (dbowner)
        datdba = get_role_oid(dbowner, false);
    else
        datdba = GetUserId();

    /*
     * To create a database, must have createdb privilege and must be able to
     * become the target role (this does not imply that the target role itself
     * must have createdb privilege).  The latter provision guards against
     * "giveaway" attacks.  Note that a superuser will always have both of
     * these privileges a fortiori.
     */
    if (!have_createdb_privilege())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to create database")));

    check_is_member_of_role(GetUserId(), datdba);

    /*
     * Lookup database (template) to be cloned, and obtain share lock on it.
     * ShareLock allows two CREATE DATABASEs to work from the same template
     * concurrently, while ensuring no one is busy dropping it in parallel
     * (which would be Very Bad since we'd likely get an incomplete copy
     * without knowing it).  This also prevents any new connections from being
     * made to the source until we finish copying it, so we can be sure it
     * won't change underneath us.
     */
    if (!dbtemplate)
        dbtemplate = "template1";       /* Default template database name */

    if (!get_db_info(dbtemplate, ShareLock,
                     &src_dboid, &src_owner, &src_encoding,
                     &src_istemplate, &src_allowconn, &src_lastsysoid,
                     &src_frozenxid, &src_minmxid, &src_deftablespace,
                     &src_collate, &src_ctype))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("template database \"%s\" does not exist",
                        dbtemplate)));

    /*
     * Permission check: to copy a DB that's not marked datistemplate, you
     * must be superuser or the owner thereof.
     */
    if (!src_istemplate)
    {
        if (!pg_database_ownercheck(src_dboid, GetUserId()))
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied to copy database \"%s\"",
                            dbtemplate)));
    }

    /* If encoding or locales are defaulted, use source's setting */
    if (encoding < 0)
        encoding = src_encoding;
    if (dbcollate == NULL)
        dbcollate = src_collate;
    if (dbctype == NULL)
        dbctype = src_ctype;

    /* Some encodings are client only */
    if (!PG_VALID_BE_ENCODING(encoding))
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("invalid server encoding %d", encoding)));

    /* Check that the chosen locales are valid, and get canonical spellings */
    if (!check_locale(LC_COLLATE, dbcollate, &canonname))
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("invalid locale name: \"%s\"", dbcollate)));
    dbcollate = canonname;
    if (!check_locale(LC_CTYPE, dbctype, &canonname))
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("invalid locale name: \"%s\"", dbctype)));
    dbctype = canonname;

    check_encoding_locale_matches(encoding, dbcollate, dbctype);

    /*
     * Check that the new encoding and locale settings match the source
     * database.  We insist on this because we simply copy the source data ---
     * any non-ASCII data would be wrongly encoded, and any indexes sorted
     * according to the source locale would be wrong.
     *
     * However, we assume that template0 doesn't contain any non-ASCII data
     * nor any indexes that depend on collation or ctype, so template0 can be
     * used as template for creating a database with any encoding or locale.
     */
    if (strcmp(dbtemplate, "template0") != 0)
    {
        if (encoding != src_encoding)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("new encoding (%s) is incompatible with the encoding of the template database (%s)",
                            pg_encoding_to_char(encoding),
                            pg_encoding_to_char(src_encoding)),
                     errhint("Use the same encoding as in the template database, or use template0 as template.")));

        if (strcmp(dbcollate, src_collate) != 0)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("new collation (%s) is incompatible with the collation of the template database (%s)",
                            dbcollate, src_collate),
                     errhint("Use the same collation as in the template database, or use template0 as template.")));

        if (strcmp(dbctype, src_ctype) != 0)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("new LC_CTYPE (%s) is incompatible with the LC_CTYPE of the template database (%s)",
                            dbctype, src_ctype),
                     errhint("Use the same LC_CTYPE as in the template database, or use template0 as template.")));
    }

    /* Resolve default tablespace for new database */
    if (dtablespacename && dtablespacename->arg)
    {
        char       *tablespacename;
        AclResult   aclresult;

        tablespacename = strVal(dtablespacename->arg);
        dst_deftablespace = get_tablespace_oid(tablespacename, false);
        /* check permissions */
        aclresult = pg_tablespace_aclcheck(dst_deftablespace, GetUserId(),
                                           ACL_CREATE);
        if (aclresult != ACLCHECK_OK)
            aclcheck_error(aclresult, ACL_KIND_TABLESPACE,
                           tablespacename);

        /* pg_global must never be the default tablespace */
        if (dst_deftablespace == GLOBALTABLESPACE_OID)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                  errmsg("pg_global cannot be used as default tablespace")));

        /*
         * If we are trying to change the default tablespace of the template,
         * we require that the template not have any files in the new default
         * tablespace.  This is necessary because otherwise the copied
         * database would contain pg_class rows that refer to its default
         * tablespace both explicitly (by OID) and implicitly (as zero), which
         * would cause problems.  For example another CREATE DATABASE using
         * the copied database as template, and trying to change its default
         * tablespace again, would yield outright incorrect results (it would
         * improperly move tables to the new default tablespace that should
         * stay in the same tablespace).
         */
        if (dst_deftablespace != src_deftablespace)
        {
            char       *srcpath;
            struct stat st;

            srcpath = GetDatabasePath(src_dboid, dst_deftablespace);

            if (stat(srcpath, &st) == 0 &&
                S_ISDIR(st.st_mode) &&
                !directory_is_empty(srcpath))
                ereport(ERROR,
                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                         errmsg("cannot assign new default tablespace \"%s\"",
                                tablespacename),
                         errdetail("There is a conflict because database \"%s\" already has some tables in this tablespace.",
                                   dbtemplate)));
            pfree(srcpath);
        }
    }
    else
    {
        /* Use template database's default tablespace */
        dst_deftablespace = src_deftablespace;
        /* Note there is no additional permission check in this path */
    }

    /*
     * Check for db name conflict.  This is just to give a more friendly error
     * message than "unique index violation".  There's a race condition but
     * we're willing to accept the less friendly message in that case.
     */
    if (OidIsValid(get_database_oid(dbname, true)))
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_DATABASE),
                 errmsg("database \"%s\" already exists", dbname)));

    /*
     * The source DB can't have any active backends, except this one
     * (exception is to allow CREATE DB while connected to template1).
     * Otherwise we might copy inconsistent data.
     *
     * This should be last among the basic error checks, because it involves
     * potential waiting; we may as well throw an error first if we're gonna
     * throw one.
     */
    if (CountOtherDBBackends(src_dboid, &notherbackends, &npreparedxacts))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
            errmsg("source database \"%s\" is being accessed by other users",
                   dbtemplate),
                 errdetail_busy_db(notherbackends, npreparedxacts)));

    /*
     * Select an OID for the new database, checking that it doesn't have a
     * filename conflict with anything already existing in the tablespace
     * directories.
     */
    pg_database_rel = heap_open(DatabaseRelationId, RowExclusiveLock);

    do
    {
        dboid = GetNewOid(pg_database_rel);
    } while (check_db_file_conflict(dboid));

    /*
     * Insert a new tuple into pg_database.  This establishes our ownership of
     * the new database name (anyone else trying to insert the same name will
     * block on the unique index, and fail after we commit).
     */

    /* Form tuple */
    MemSet(new_record, 0, sizeof(new_record));
    MemSet(new_record_nulls, false, sizeof(new_record_nulls));

    new_record[Anum_pg_database_datname - 1] =
        DirectFunctionCall1(namein, CStringGetDatum(dbname));
    new_record[Anum_pg_database_datdba - 1] = ObjectIdGetDatum(datdba);
    new_record[Anum_pg_database_encoding - 1] = Int32GetDatum(encoding);
    new_record[Anum_pg_database_datcollate - 1] =
        DirectFunctionCall1(namein, CStringGetDatum(dbcollate));
    new_record[Anum_pg_database_datctype - 1] =
        DirectFunctionCall1(namein, CStringGetDatum(dbctype));
    new_record[Anum_pg_database_datistemplate - 1] = BoolGetDatum(false);
    new_record[Anum_pg_database_datallowconn - 1] = BoolGetDatum(true);
    new_record[Anum_pg_database_datconnlimit - 1] = Int32GetDatum(dbconnlimit);
    new_record[Anum_pg_database_datlastsysoid - 1] = ObjectIdGetDatum(src_lastsysoid);
    new_record[Anum_pg_database_datfrozenxid - 1] = TransactionIdGetDatum(src_frozenxid);
    new_record[Anum_pg_database_datminmxid - 1] = TransactionIdGetDatum(src_minmxid);
    new_record[Anum_pg_database_dattablespace - 1] = ObjectIdGetDatum(dst_deftablespace);

    /*
     * We deliberately set datacl to default (NULL), rather than copying it
     * from the template database.  Copying it would be a bad idea when the
     * owner is not the same as the template's owner.
     */
    new_record_nulls[Anum_pg_database_datacl - 1] = true;

    tuple = heap_form_tuple(RelationGetDescr(pg_database_rel),
                            new_record, new_record_nulls);

    HeapTupleSetOid(tuple, dboid);

    simple_heap_insert(pg_database_rel, tuple);

    /* Update indexes */
    CatalogUpdateIndexes(pg_database_rel, tuple);

    /*
     * Now generate additional catalog entries associated with the new DB
     */

    /* Register owner dependency */
    recordDependencyOnOwner(DatabaseRelationId, dboid, datdba);

    /* Create pg_shdepend entries for objects within database */
    copyTemplateDependencies(src_dboid, dboid);

    /* Post creation hook for new database */
    InvokeObjectPostCreateHook(DatabaseRelationId, dboid, 0);

    /*
     * Force a checkpoint before starting the copy. This will force dirty
     * buffers out to disk, to ensure source database is up-to-date on disk
     * for the copy. FlushDatabaseBuffers() would suffice for that, but we
     * also want to process any pending unlink requests. Otherwise, if a
     * checkpoint happened while we're copying files, a file might be deleted
     * just when we're about to copy it, causing the lstat() call in copydir()
     * to fail with ENOENT.
     */
    RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

    /*
     * Take an MVCC snapshot to use while scanning through pg_tablespace.  For
     * safety, register the snapshot (this prevents it from changing if
     * something else were to request a snapshot during the loop).
     *
     * Traversing pg_tablespace with an MVCC snapshot is necessary to provide
     * us with a consistent view of the tablespaces that exist.  Using
     * SnapshotNow here would risk seeing the same tablespace multiple times,
     * or worse not seeing a tablespace at all, if its tuple is moved around
     * by a concurrent update (eg an ACL change).
     *
     * Inconsistency of this sort is inherent to all SnapshotNow scans, unless
     * some lock is held to prevent concurrent updates of the rows being
     * sought.  There should be a generic fix for that, but in the meantime
     * it's worth fixing this case in particular because we are doing very
     * heavyweight operations within the scan, so that the elapsed time for
     * the scan is vastly longer than for most other catalog scans.  That
     * means there's a much wider window for concurrent updates to cause
     * trouble here than anywhere else.  XXX this code should be changed
     * whenever a generic fix is implemented.
     */
    snapshot = RegisterSnapshot(GetLatestSnapshot());

    /*
     * Once we start copying subdirectories, we need to be able to clean 'em
     * up if we fail.  Use an ENSURE block to make sure this happens.  (This
     * is not a 100% solution, because of the possibility of failure during
     * transaction commit after we leave this routine, but it should handle
     * most scenarios.)
     */
    fparms.src_dboid = src_dboid;
    fparms.dest_dboid = dboid;
    PG_ENSURE_ERROR_CLEANUP(createdb_failure_callback,
                            PointerGetDatum(&fparms));
    {
        /*
         * Iterate through all tablespaces of the template database, and copy
         * each one to the new database.
         */
        rel = heap_open(TableSpaceRelationId, AccessShareLock);
        scan = heap_beginscan(rel, snapshot, 0, NULL);
        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
            Oid         srctablespace = HeapTupleGetOid(tuple);
            Oid         dsttablespace;
            char       *srcpath;
            char       *dstpath;
            struct stat st;

            /* No need to copy global tablespace */
            if (srctablespace == GLOBALTABLESPACE_OID)
                continue;

            srcpath = GetDatabasePath(src_dboid, srctablespace);

            if (stat(srcpath, &st) < 0 || !S_ISDIR(st.st_mode) ||
                directory_is_empty(srcpath))
            {
                /* Assume we can ignore it */
                pfree(srcpath);
                continue;
            }

            if (srctablespace == src_deftablespace)
                dsttablespace = dst_deftablespace;
            else
                dsttablespace = srctablespace;

            dstpath = GetDatabasePath(dboid, dsttablespace);

            /*
             * Copy this subdirectory to the new location
             *
             * We don't need to copy subdirectories
             */
            copydir(srcpath, dstpath, false);

            /* Record the filesystem change in XLOG */
            {
                xl_dbase_create_rec xlrec;
                XLogRecData rdata[1];

                xlrec.db_id = dboid;
                xlrec.tablespace_id = dsttablespace;
                xlrec.src_db_id = src_dboid;
                xlrec.src_tablespace_id = srctablespace;

                rdata[0].data = (char *) &xlrec;
                rdata[0].len = sizeof(xl_dbase_create_rec);
                rdata[0].buffer = InvalidBuffer;
                rdata[0].next = NULL;

                (void) XLogInsert(RM_DBASE_ID, XLOG_DBASE_CREATE, rdata);
            }
        }
        heap_endscan(scan);
        heap_close(rel, AccessShareLock);

        /*
         * We force a checkpoint before committing.  This effectively means
         * that committed XLOG_DBASE_CREATE operations will never need to be
         * replayed (at least not in ordinary crash recovery; we still have to
         * make the XLOG entry for the benefit of PITR operations). This
         * avoids two nasty scenarios:
         *
         * #1: When PITR is off, we don't XLOG the contents of newly created
         * indexes; therefore the drop-and-recreate-whole-directory behavior
         * of DBASE_CREATE replay would lose such indexes.
         *
         * #2: Since we have to recopy the source database during DBASE_CREATE
         * replay, we run the risk of copying changes in it that were
         * committed after the original CREATE DATABASE command but before the
         * system crash that led to the replay.  This is at least unexpected
         * and at worst could lead to inconsistencies, eg duplicate table
         * names.
         *
         * (Both of these were real bugs in releases 8.0 through 8.0.3.)
         *
         * In PITR replay, the first of these isn't an issue, and the second
         * is only a risk if the CREATE DATABASE and subsequent template
         * database change both occur while a base backup is being taken.
         * There doesn't seem to be much we can do about that except document
         * it as a limitation.
         *
         * Perhaps if we ever implement CREATE DATABASE in a less cheesy way,
         * we can avoid this.
         */
        RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

        /*
         * Close pg_database, but keep lock till commit.
         */
        heap_close(pg_database_rel, NoLock);

        /*
         * Force synchronous commit, thus minimizing the window between
         * creation of the database files and commital of the transaction. If
         * we crash before committing, we'll have a DB that's taking up disk
         * space but is not in pg_database, which is not good.
         */
        ForceSyncCommit();
    }
    PG_END_ENSURE_ERROR_CLEANUP(createdb_failure_callback,
                                PointerGetDatum(&fparms));

    /* Free our snapshot */
    UnregisterSnapshot(snapshot);

    return dboid;
}

/*
 * Check whether chosen encoding matches chosen locale settings.  This
 * restriction is necessary because libc's locale-specific code usually
 * fails when presented with data in an encoding it's not expecting. We
 * allow mismatch in four cases:
 *
 * 1. locale encoding = SQL_ASCII, which means that the locale is C/POSIX
 * which works with any encoding.
 *
 * 2. locale encoding = -1, which means that we couldn't determine the
 * locale's encoding and have to trust the user to get it right.
 *
 * 3. selected encoding is UTF8 and platform is win32. This is because
 * UTF8 is a pseudo codepage that is supported in all locales since it's
 * converted to UTF16 before being used.
 *
 * 4. selected encoding is SQL_ASCII, but only if you're a superuser. This
 * is risky but we have historically allowed it --- notably, the
 * regression tests require it.
 *
 * Note: if you change this policy, fix initdb to match.
 */
void
check_encoding_locale_matches(int encoding, const char *collate, const char *ctype)
{
    int         ctype_encoding = pg_get_encoding_from_locale(ctype, true);
    int         collate_encoding = pg_get_encoding_from_locale(collate, true);

    if (!(ctype_encoding == encoding ||
          ctype_encoding == PG_SQL_ASCII ||
          ctype_encoding == -1 ||
#ifdef WIN32
          encoding == PG_UTF8 ||
#endif
          (encoding == PG_SQL_ASCII && superuser())))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("encoding \"%s\" does not match locale \"%s\"",
                        pg_encoding_to_char(encoding),
                        ctype),
           errdetail("The chosen LC_CTYPE setting requires encoding \"%s\".",
                     pg_encoding_to_char(ctype_encoding))));

    if (!(collate_encoding == encoding ||
          collate_encoding == PG_SQL_ASCII ||
          collate_encoding == -1 ||
#ifdef WIN32
          encoding == PG_UTF8 ||
#endif
          (encoding == PG_SQL_ASCII && superuser())))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("encoding \"%s\" does not match locale \"%s\"",
                        pg_encoding_to_char(encoding),
                        collate),
         errdetail("The chosen LC_COLLATE setting requires encoding \"%s\".",
                   pg_encoding_to_char(collate_encoding))));
}

/* Error cleanup callback for createdb */
static void
createdb_failure_callback(int code, Datum arg)
{
    createdb_failure_params *fparms = (createdb_failure_params *) DatumGetPointer(arg);

    /*
     * Release lock on source database before doing recursive remove. This is
     * not essential but it seems desirable to release the lock as soon as
     * possible.
     */
    UnlockSharedObject(DatabaseRelationId, fparms->src_dboid, 0, ShareLock);

    /* Throw away any successfully copied subdirectories */
    remove_dbtablespaces(fparms->dest_dboid);
}


/*
 * DROP DATABASE
 */
void
dropdb(const char *dbname, bool missing_ok)
{
    Oid         db_id;
    bool        db_istemplate;
    Relation    pgdbrel;
    HeapTuple   tup;
    int         notherbackends;
    int         npreparedxacts;

    /*
     * Look up the target database's OID, and get exclusive lock on it. We
     * need this to ensure that no new backend starts up in the target
     * database while we are deleting it (see postinit.c), and that no one is
     * using it as a CREATE DATABASE template or trying to delete it for
     * themselves.
     */
    pgdbrel = heap_open(DatabaseRelationId, RowExclusiveLock);

    if (!get_db_info(dbname, AccessExclusiveLock, &db_id, NULL, NULL,
                     &db_istemplate, NULL, NULL, NULL, NULL, NULL, NULL, NULL))
    {
        if (!missing_ok)
        {
            ereport(ERROR,
                    (errcode(ERRCODE_UNDEFINED_DATABASE),
                     errmsg("database \"%s\" does not exist", dbname)));
        }
        else
        {
            /* Close pg_database, release the lock, since we changed nothing */
            heap_close(pgdbrel, RowExclusiveLock);
            ereport(NOTICE,
                    (errmsg("database \"%s\" does not exist, skipping",
                            dbname)));
            return;
        }
    }

    /*
     * Permission checks
     */
    if (!pg_database_ownercheck(db_id, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                       dbname);

    /* DROP hook for the database being removed */
    InvokeObjectDropHook(DatabaseRelationId, db_id, 0);

    /*
     * Disallow dropping a DB that is marked istemplate.  This is just to
     * prevent people from accidentally dropping template0 or template1; they
     * can do so if they're really determined ...
     */
    if (db_istemplate)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("cannot drop a template database")));

    /* Obviously can't drop my own database */
    if (db_id == MyDatabaseId)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("cannot drop the currently open database")));

    /*
     * Check for other backends in the target database.  (Because we hold the
     * database lock, no new ones can start after this.)
     *
     * As in CREATE DATABASE, check this after other error conditions.
     */
    if (CountOtherDBBackends(db_id, &notherbackends, &npreparedxacts))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("database \"%s\" is being accessed by other users",
                        dbname),
                 errdetail_busy_db(notherbackends, npreparedxacts)));

    /*
     * Remove the database's tuple from pg_database.
     */
    tup = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(db_id));
    if (!HeapTupleIsValid(tup))
        elog(ERROR, "cache lookup failed for database %u", db_id);

    simple_heap_delete(pgdbrel, &tup->t_self);

    ReleaseSysCache(tup);

    /*
     * Delete any comments or security labels associated with the database.
     */
    DeleteSharedComments(db_id, DatabaseRelationId);
    DeleteSharedSecurityLabel(db_id, DatabaseRelationId);

    /*
     * Remove settings associated with this database
     */
    DropSetting(db_id, InvalidOid);

    /*
     * Remove shared dependency references for the database.
     */
    dropDatabaseDependencies(db_id);

    /*
     * Drop pages for this database that are in the shared buffer cache. This
     * is important to ensure that no remaining backend tries to write out a
     * dirty buffer to the dead database later...
     */
    DropDatabaseBuffers(db_id);

    /*
     * Tell the stats collector to forget it immediately, too.
     */
    pgstat_drop_database(db_id);

    /*
     * Tell checkpointer to forget any pending fsync and unlink requests for
     * files in the database; else the fsyncs will fail at next checkpoint, or
     * worse, it will delete files that belong to a newly created database
     * with the same OID.
     */
    ForgetDatabaseFsyncRequests(db_id);

    /*
     * Force a checkpoint to make sure the checkpointer has received the
     * message sent by ForgetDatabaseFsyncRequests. On Windows, this also
     * ensures that background procs don't hold any open files, which would
     * cause rmdir() to fail.
     */
    RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

    /*
     * Remove all tablespace subdirs belonging to the database.
     */
    remove_dbtablespaces(db_id);

    /*
     * Close pg_database, but keep lock till commit.
     */
    heap_close(pgdbrel, NoLock);

    /*
     * Force synchronous commit, thus minimizing the window between removal of
     * the database files and commital of the transaction. If we crash before
     * committing, we'll have a DB that's gone on disk but still there
     * according to pg_database, which is not good.
     */
    ForceSyncCommit();
}


/*
 * Rename database
 */
Oid
RenameDatabase(const char *oldname, const char *newname)
{
    Oid         db_id;
    HeapTuple   newtup;
    Relation    rel;
    int         notherbackends;
    int         npreparedxacts;

    /*
     * Look up the target database's OID, and get exclusive lock on it. We
     * need this for the same reasons as DROP DATABASE.
     */
    rel = heap_open(DatabaseRelationId, RowExclusiveLock);

    if (!get_db_info(oldname, AccessExclusiveLock, &db_id, NULL, NULL,
                     NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("database \"%s\" does not exist", oldname)));

    /* must be owner */
    if (!pg_database_ownercheck(db_id, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                       oldname);

    /* must have createdb rights */
    if (!have_createdb_privilege())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to rename database")));

    /*
     * Make sure the new name doesn't exist.  See notes for same error in
     * CREATE DATABASE.
     */
    if (OidIsValid(get_database_oid(newname, true)))
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_DATABASE),
                 errmsg("database \"%s\" already exists", newname)));

    /*
     * XXX Client applications probably store the current database somewhere,
     * so renaming it could cause confusion.  On the other hand, there may not
     * be an actual problem besides a little confusion, so think about this
     * and decide.
     */
    if (db_id == MyDatabaseId)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("current database cannot be renamed")));

    /*
     * Make sure the database does not have active sessions.  This is the same
     * concern as above, but applied to other sessions.
     *
     * As in CREATE DATABASE, check this after other error conditions.
     */
    if (CountOtherDBBackends(db_id, &notherbackends, &npreparedxacts))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("database \"%s\" is being accessed by other users",
                        oldname),
                 errdetail_busy_db(notherbackends, npreparedxacts)));

    /* rename */
    newtup = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(db_id));
    if (!HeapTupleIsValid(newtup))
        elog(ERROR, "cache lookup failed for database %u", db_id);
    namestrcpy(&(((Form_pg_database) GETSTRUCT(newtup))->datname), newname);
    simple_heap_update(rel, &newtup->t_self, newtup);
    CatalogUpdateIndexes(rel, newtup);

    InvokeObjectPostAlterHook(DatabaseRelationId, db_id, 0);

    /*
     * Close pg_database, but keep lock till commit.
     */
    heap_close(rel, NoLock);

    return db_id;
}


/*
 * ALTER DATABASE SET TABLESPACE
 */
static void
movedb(const char *dbname, const char *tblspcname)
{
    Oid         db_id;
    Relation    pgdbrel;
    int         notherbackends;
    int         npreparedxacts;
    HeapTuple   oldtuple,
                newtuple;
    Oid         src_tblspcoid,
                dst_tblspcoid;
    Datum       new_record[Natts_pg_database];
    bool        new_record_nulls[Natts_pg_database];
    bool        new_record_repl[Natts_pg_database];
    ScanKeyData scankey;
    SysScanDesc sysscan;
    AclResult   aclresult;
    char       *src_dbpath;
    char       *dst_dbpath;
    DIR        *dstdir;
    struct dirent *xlde;
    movedb_failure_params fparms;

    /*
     * Look up the target database's OID, and get exclusive lock on it. We
     * need this to ensure that no new backend starts up in the database while
     * we are moving it, and that no one is using it as a CREATE DATABASE
     * template or trying to delete it.
     */
    pgdbrel = heap_open(DatabaseRelationId, RowExclusiveLock);

    if (!get_db_info(dbname, AccessExclusiveLock, &db_id, NULL, NULL,
                     NULL, NULL, NULL, NULL, NULL, &src_tblspcoid, NULL, NULL))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("database \"%s\" does not exist", dbname)));

    /*
     * We actually need a session lock, so that the lock will persist across
     * the commit/restart below.  (We could almost get away with letting the
     * lock be released at commit, except that someone could try to move
     * relations of the DB back into the old directory while we rmtree() it.)
     */
    LockSharedObjectForSession(DatabaseRelationId, db_id, 0,
                               AccessExclusiveLock);

    /*
     * Permission checks
     */
    if (!pg_database_ownercheck(db_id, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                       dbname);

    /*
     * Obviously can't move the tables of my own database
     */
    if (db_id == MyDatabaseId)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("cannot change the tablespace of the currently open database")));

    /*
     * Get tablespace's oid
     */
    dst_tblspcoid = get_tablespace_oid(tblspcname, false);

    /*
     * Permission checks
     */
    aclresult = pg_tablespace_aclcheck(dst_tblspcoid, GetUserId(),
                                       ACL_CREATE);
    if (aclresult != ACLCHECK_OK)
        aclcheck_error(aclresult, ACL_KIND_TABLESPACE,
                       tblspcname);

    /*
     * pg_global must never be the default tablespace
     */
    if (dst_tblspcoid == GLOBALTABLESPACE_OID)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("pg_global cannot be used as default tablespace")));

    /*
     * No-op if same tablespace
     */
    if (src_tblspcoid == dst_tblspcoid)
    {
        heap_close(pgdbrel, NoLock);
        UnlockSharedObjectForSession(DatabaseRelationId, db_id, 0,
                                     AccessExclusiveLock);
        return;
    }

    /*
     * Check for other backends in the target database.  (Because we hold the
     * database lock, no new ones can start after this.)
     *
     * As in CREATE DATABASE, check this after other error conditions.
     */
    if (CountOtherDBBackends(db_id, &notherbackends, &npreparedxacts))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("database \"%s\" is being accessed by other users",
                        dbname),
                 errdetail_busy_db(notherbackends, npreparedxacts)));

    /*
     * Get old and new database paths
     */
    src_dbpath = GetDatabasePath(db_id, src_tblspcoid);
    dst_dbpath = GetDatabasePath(db_id, dst_tblspcoid);

    /*
     * Force a checkpoint before proceeding. This will force dirty buffers out
     * to disk, to ensure source database is up-to-date on disk for the copy.
     * FlushDatabaseBuffers() would suffice for that, but we also want to
     * process any pending unlink requests. Otherwise, the check for existing
     * files in the target directory might fail unnecessarily, not to mention
     * that the copy might fail due to source files getting deleted under it.
     * On Windows, this also ensures that background procs don't hold any open
     * files, which would cause rmdir() to fail.
     */
    RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

    /*
     * Check for existence of files in the target directory, i.e., objects of
     * this database that are already in the target tablespace.  We can't
     * allow the move in such a case, because we would need to change those
     * relations' pg_class.reltablespace entries to zero, and we don't have
     * access to the DB's pg_class to do so.
     */
    dstdir = AllocateDir(dst_dbpath);
    if (dstdir != NULL)
    {
        while ((xlde = ReadDir(dstdir, dst_dbpath)) != NULL)
        {
            if (strcmp(xlde->d_name, ".") == 0 ||
                strcmp(xlde->d_name, "..") == 0)
                continue;

            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("some relations of database \"%s\" are already in tablespace \"%s\"",
                            dbname, tblspcname),
                     errhint("You must move them back to the database's default tablespace before using this command.")));
        }

        FreeDir(dstdir);

        /*
         * The directory exists but is empty. We must remove it before using
         * the copydir function.
         */
        if (rmdir(dst_dbpath) != 0)
            elog(ERROR, "could not remove directory \"%s\": %m",
                 dst_dbpath);
    }

    /*
     * Use an ENSURE block to make sure we remove the debris if the copy fails
     * (eg, due to out-of-disk-space).  This is not a 100% solution, because
     * of the possibility of failure during transaction commit, but it should
     * handle most scenarios.
     */
    fparms.dest_dboid = db_id;
    fparms.dest_tsoid = dst_tblspcoid;
    PG_ENSURE_ERROR_CLEANUP(movedb_failure_callback,
                            PointerGetDatum(&fparms));
    {
        /*
         * Copy files from the old tablespace to the new one
         */
        copydir(src_dbpath, dst_dbpath, false);

        /*
         * Record the filesystem change in XLOG
         */
        {
            xl_dbase_create_rec xlrec;
            XLogRecData rdata[1];

            xlrec.db_id = db_id;
            xlrec.tablespace_id = dst_tblspcoid;
            xlrec.src_db_id = db_id;
            xlrec.src_tablespace_id = src_tblspcoid;

            rdata[0].data = (char *) &xlrec;
            rdata[0].len = sizeof(xl_dbase_create_rec);
            rdata[0].buffer = InvalidBuffer;
            rdata[0].next = NULL;

            (void) XLogInsert(RM_DBASE_ID, XLOG_DBASE_CREATE, rdata);
        }

        /*
         * Update the database's pg_database tuple
         */
        ScanKeyInit(&scankey,
                    Anum_pg_database_datname,
                    BTEqualStrategyNumber, F_NAMEEQ,
                    NameGetDatum(dbname));
        sysscan = systable_beginscan(pgdbrel, DatabaseNameIndexId, true,
                                     SnapshotNow, 1, &scankey);
        oldtuple = systable_getnext(sysscan);
        if (!HeapTupleIsValid(oldtuple))        /* shouldn't happen... */
            ereport(ERROR,
                    (errcode(ERRCODE_UNDEFINED_DATABASE),
                     errmsg("database \"%s\" does not exist", dbname)));

        MemSet(new_record, 0, sizeof(new_record));
        MemSet(new_record_nulls, false, sizeof(new_record_nulls));
        MemSet(new_record_repl, false, sizeof(new_record_repl));

        new_record[Anum_pg_database_dattablespace - 1] = ObjectIdGetDatum(dst_tblspcoid);
        new_record_repl[Anum_pg_database_dattablespace - 1] = true;

        newtuple = heap_modify_tuple(oldtuple, RelationGetDescr(pgdbrel),
                                     new_record,
                                     new_record_nulls, new_record_repl);
        simple_heap_update(pgdbrel, &oldtuple->t_self, newtuple);

        /* Update indexes */
        CatalogUpdateIndexes(pgdbrel, newtuple);

        InvokeObjectPostAlterHook(DatabaseRelationId,
                                  HeapTupleGetOid(newtuple), 0);

        systable_endscan(sysscan);

        /*
         * Force another checkpoint here.  As in CREATE DATABASE, this is to
         * ensure that we don't have to replay a committed XLOG_DBASE_CREATE
         * operation, which would cause us to lose any unlogged operations
         * done in the new DB tablespace before the next checkpoint.
         */
        RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

        /*
         * Force synchronous commit, thus minimizing the window between
         * copying the database files and commital of the transaction. If we
         * crash before committing, we'll leave an orphaned set of files on
         * disk, which is not fatal but not good either.
         */
        ForceSyncCommit();

        /*
         * Close pg_database, but keep lock till commit.
         */
        heap_close(pgdbrel, NoLock);
    }
    PG_END_ENSURE_ERROR_CLEANUP(movedb_failure_callback,
                                PointerGetDatum(&fparms));

    /*
     * Commit the transaction so that the pg_database update is committed. If
     * we crash while removing files, the database won't be corrupt, we'll
     * just leave some orphaned files in the old directory.
     *
     * (This is OK because we know we aren't inside a transaction block.)
     *
     * XXX would it be safe/better to do this inside the ensure block?  Not
     * convinced it's a good idea; consider elog just after the transaction
     * really commits.
     */
    PopActiveSnapshot();
    CommitTransactionCommand();

    /* Start new transaction for the remaining work; don't need a snapshot */
    StartTransactionCommand();

    /*
     * Remove files from the old tablespace
     */
    if (!rmtree(src_dbpath, true))
        ereport(WARNING,
                (errmsg("some useless files may be left behind in old database directory \"%s\"",
                        src_dbpath)));

    /*
     * Record the filesystem change in XLOG
     */
    {
        xl_dbase_drop_rec xlrec;
        XLogRecData rdata[1];

        xlrec.db_id = db_id;
        xlrec.tablespace_id = src_tblspcoid;

        rdata[0].data = (char *) &xlrec;
        rdata[0].len = sizeof(xl_dbase_drop_rec);
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = NULL;

        (void) XLogInsert(RM_DBASE_ID, XLOG_DBASE_DROP, rdata);
    }

    /* Now it's safe to release the database lock */
    UnlockSharedObjectForSession(DatabaseRelationId, db_id, 0,
                                 AccessExclusiveLock);
}

/* Error cleanup callback for movedb */
static void
movedb_failure_callback(int code, Datum arg)
{
    movedb_failure_params *fparms = (movedb_failure_params *) DatumGetPointer(arg);
    char       *dstpath;

    /* Get rid of anything we managed to copy to the target directory */
    dstpath = GetDatabasePath(fparms->dest_dboid, fparms->dest_tsoid);

    (void) rmtree(dstpath, true);
}


/*
 * ALTER DATABASE name ...
 */
Oid
AlterDatabase(AlterDatabaseStmt *stmt, bool isTopLevel)
{
    Relation    rel;
    Oid         dboid;
    HeapTuple   tuple,
                newtuple;
    ScanKeyData scankey;
    SysScanDesc scan;
    ListCell   *option;
    int         connlimit = -1;
    DefElem    *dconnlimit = NULL;
    DefElem    *dtablespace = NULL;
    Datum       new_record[Natts_pg_database];
    bool        new_record_nulls[Natts_pg_database];
    bool        new_record_repl[Natts_pg_database];

    /* Extract options from the statement node tree */
    foreach(option, stmt->options)
    {
        DefElem    *defel = (DefElem *) lfirst(option);

        if (strcmp(defel->defname, "connectionlimit") == 0)
        {
            if (dconnlimit)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dconnlimit = defel;
        }
        else if (strcmp(defel->defname, "tablespace") == 0)
        {
            if (dtablespace)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            dtablespace = defel;
        }
        else
            elog(ERROR, "option \"%s\" not recognized",
                 defel->defname);
    }

    if (dtablespace)
    {
        /* currently, can't be specified along with any other options */
        Assert(!dconnlimit);
        /* this case isn't allowed within a transaction block */
        PreventTransactionChain(isTopLevel, "ALTER DATABASE SET TABLESPACE");
        movedb(stmt->dbname, strVal(dtablespace->arg));
        return InvalidOid;
    }

    if (dconnlimit)
    {
        connlimit = intVal(dconnlimit->arg);
        if (connlimit < -1)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid connection limit: %d", connlimit)));
    }

    /*
     * Get the old tuple.  We don't need a lock on the database per se,
     * because we're not going to do anything that would mess up incoming
     * connections.
     */
    rel = heap_open(DatabaseRelationId, RowExclusiveLock);
    ScanKeyInit(&scankey,
                Anum_pg_database_datname,
                BTEqualStrategyNumber, F_NAMEEQ,
                NameGetDatum(stmt->dbname));
    scan = systable_beginscan(rel, DatabaseNameIndexId, true,
                              SnapshotNow, 1, &scankey);
    tuple = systable_getnext(scan);
    if (!HeapTupleIsValid(tuple))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("database \"%s\" does not exist", stmt->dbname)));

    dboid = HeapTupleGetOid(tuple);

    if (!pg_database_ownercheck(HeapTupleGetOid(tuple), GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                       stmt->dbname);

    /*
     * Build an updated tuple, perusing the information just obtained
     */
    MemSet(new_record, 0, sizeof(new_record));
    MemSet(new_record_nulls, false, sizeof(new_record_nulls));
    MemSet(new_record_repl, false, sizeof(new_record_repl));

    if (dconnlimit)
    {
        new_record[Anum_pg_database_datconnlimit - 1] = Int32GetDatum(connlimit);
        new_record_repl[Anum_pg_database_datconnlimit - 1] = true;
    }

    newtuple = heap_modify_tuple(tuple, RelationGetDescr(rel), new_record,
                                 new_record_nulls, new_record_repl);
    simple_heap_update(rel, &tuple->t_self, newtuple);

    /* Update indexes */
    CatalogUpdateIndexes(rel, newtuple);

    InvokeObjectPostAlterHook(DatabaseRelationId,
                              HeapTupleGetOid(newtuple), 0);

    systable_endscan(scan);

    /* Close pg_database, but keep lock till commit */
    heap_close(rel, NoLock);

    return dboid;
}


/*
 * ALTER DATABASE name SET ...
 */
Oid
AlterDatabaseSet(AlterDatabaseSetStmt *stmt)
{
    Oid         datid = get_database_oid(stmt->dbname, false);

    /*
     * Obtain a lock on the database and make sure it didn't go away in the
     * meantime.
     */
    shdepLockAndCheckObject(DatabaseRelationId, datid);

    if (!pg_database_ownercheck(datid, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                       stmt->dbname);

    AlterSetting(datid, InvalidOid, stmt->setstmt);

    UnlockSharedObject(DatabaseRelationId, datid, 0, AccessShareLock);

    return datid;
}


/*
 * ALTER DATABASE name OWNER TO newowner
 */
Oid
AlterDatabaseOwner(const char *dbname, Oid newOwnerId)
{
    Oid         db_id;
    HeapTuple   tuple;
    Relation    rel;
    ScanKeyData scankey;
    SysScanDesc scan;
    Form_pg_database datForm;

    /*
     * Get the old tuple.  We don't need a lock on the database per se,
     * because we're not going to do anything that would mess up incoming
     * connections.
     */
    rel = heap_open(DatabaseRelationId, RowExclusiveLock);
    ScanKeyInit(&scankey,
                Anum_pg_database_datname,
                BTEqualStrategyNumber, F_NAMEEQ,
                NameGetDatum(dbname));
    scan = systable_beginscan(rel, DatabaseNameIndexId, true,
                              SnapshotNow, 1, &scankey);
    tuple = systable_getnext(scan);
    if (!HeapTupleIsValid(tuple))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("database \"%s\" does not exist", dbname)));

    db_id = HeapTupleGetOid(tuple);
    datForm = (Form_pg_database) GETSTRUCT(tuple);

    /*
     * If the new owner is the same as the existing owner, consider the
     * command to have succeeded.  This is to be consistent with other
     * objects.
     */
    if (datForm->datdba != newOwnerId)
    {
        Datum       repl_val[Natts_pg_database];
        bool        repl_null[Natts_pg_database];
        bool        repl_repl[Natts_pg_database];
        Acl        *newAcl;
        Datum       aclDatum;
        bool        isNull;
        HeapTuple   newtuple;

        /* Otherwise, must be owner of the existing object */
        if (!pg_database_ownercheck(HeapTupleGetOid(tuple), GetUserId()))
            aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_DATABASE,
                           dbname);

        /* Must be able to become new owner */
        check_is_member_of_role(GetUserId(), newOwnerId);

        /*
         * must have createdb rights
         *
         * NOTE: This is different from other alter-owner checks in that the
         * current user is checked for createdb privileges instead of the
         * destination owner.  This is consistent with the CREATE case for
         * databases.  Because superusers will always have this right, we need
         * no special case for them.
         */
        if (!have_createdb_privilege())
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                   errmsg("permission denied to change owner of database")));

        memset(repl_null, false, sizeof(repl_null));
        memset(repl_repl, false, sizeof(repl_repl));

        repl_repl[Anum_pg_database_datdba - 1] = true;
        repl_val[Anum_pg_database_datdba - 1] = ObjectIdGetDatum(newOwnerId);

        /*
         * Determine the modified ACL for the new owner.  This is only
         * necessary when the ACL is non-null.
         */
        aclDatum = heap_getattr(tuple,
                                Anum_pg_database_datacl,
                                RelationGetDescr(rel),
                                &isNull);
        if (!isNull)
        {
            newAcl = aclnewowner(DatumGetAclP(aclDatum),
                                 datForm->datdba, newOwnerId);
            repl_repl[Anum_pg_database_datacl - 1] = true;
            repl_val[Anum_pg_database_datacl - 1] = PointerGetDatum(newAcl);
        }

        newtuple = heap_modify_tuple(tuple, RelationGetDescr(rel), repl_val, repl_null, repl_repl);
        simple_heap_update(rel, &newtuple->t_self, newtuple);
        CatalogUpdateIndexes(rel, newtuple);

        heap_freetuple(newtuple);

        /* Update owner dependency reference */
        changeDependencyOnOwner(DatabaseRelationId, HeapTupleGetOid(tuple),
                                newOwnerId);
    }

    InvokeObjectPostAlterHook(DatabaseRelationId, HeapTupleGetOid(tuple), 0);

    systable_endscan(scan);

    /* Close pg_database, but keep lock till commit */
    heap_close(rel, NoLock);

    return db_id;
}


/*
 * Helper functions
 */

/*
 * Look up info about the database named "name".  If the database exists,
 * obtain the specified lock type on it, fill in any of the remaining
 * parameters that aren't NULL, and return TRUE.  If no such database,
 * return FALSE.
 */
static bool
get_db_info(const char *name, LOCKMODE lockmode,
            Oid *dbIdP, Oid *ownerIdP,
            int *encodingP, bool *dbIsTemplateP, bool *dbAllowConnP,
            Oid *dbLastSysOidP, TransactionId *dbFrozenXidP,
            MultiXactId *dbMinMultiP,
            Oid *dbTablespace, char **dbCollate, char **dbCtype)
{
    bool        result = false;
    Relation    relation;

    AssertArg(name);

    /* Caller may wish to grab a better lock on pg_database beforehand... */
    relation = heap_open(DatabaseRelationId, AccessShareLock);

    /*
     * Loop covers the rare case where the database is renamed before we can
     * lock it.  We try again just in case we can find a new one of the same
     * name.
     */
    for (;;)
    {
        ScanKeyData scanKey;
        SysScanDesc scan;
        HeapTuple   tuple;
        Oid         dbOid;

        /*
         * there's no syscache for database-indexed-by-name, so must do it the
         * hard way
         */
        ScanKeyInit(&scanKey,
                    Anum_pg_database_datname,
                    BTEqualStrategyNumber, F_NAMEEQ,
                    NameGetDatum(name));

        scan = systable_beginscan(relation, DatabaseNameIndexId, true,
                                  SnapshotNow, 1, &scanKey);

        tuple = systable_getnext(scan);

        if (!HeapTupleIsValid(tuple))
        {
            /* definitely no database of that name */
            systable_endscan(scan);
            break;
        }

        dbOid = HeapTupleGetOid(tuple);

        systable_endscan(scan);

        /*
         * Now that we have a database OID, we can try to lock the DB.
         */
        if (lockmode != NoLock)
            LockSharedObject(DatabaseRelationId, dbOid, 0, lockmode);

        /*
         * And now, re-fetch the tuple by OID.  If it's still there and still
         * the same name, we win; else, drop the lock and loop back to try
         * again.
         */
        tuple = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(dbOid));
        if (HeapTupleIsValid(tuple))
        {
            Form_pg_database dbform = (Form_pg_database) GETSTRUCT(tuple);

            if (strcmp(name, NameStr(dbform->datname)) == 0)
            {
                /* oid of the database */
                if (dbIdP)
                    *dbIdP = dbOid;
                /* oid of the owner */
                if (ownerIdP)
                    *ownerIdP = dbform->datdba;
                /* character encoding */
                if (encodingP)
                    *encodingP = dbform->encoding;
                /* allowed as template? */
                if (dbIsTemplateP)
                    *dbIsTemplateP = dbform->datistemplate;
                /* allowing connections? */
                if (dbAllowConnP)
                    *dbAllowConnP = dbform->datallowconn;
                /* last system OID used in database */
                if (dbLastSysOidP)
                    *dbLastSysOidP = dbform->datlastsysoid;
                /* limit of frozen XIDs */
                if (dbFrozenXidP)
                    *dbFrozenXidP = dbform->datfrozenxid;
                /* limit of frozen Multixacts */
                if (dbMinMultiP)
                    *dbMinMultiP = dbform->datminmxid;
                /* default tablespace for this database */
                if (dbTablespace)
                    *dbTablespace = dbform->dattablespace;
                /* default locale settings for this database */
                if (dbCollate)
                    *dbCollate = pstrdup(NameStr(dbform->datcollate));
                if (dbCtype)
                    *dbCtype = pstrdup(NameStr(dbform->datctype));
                ReleaseSysCache(tuple);
                result = true;
                break;
            }
            /* can only get here if it was just renamed */
            ReleaseSysCache(tuple);
        }

        if (lockmode != NoLock)
            UnlockSharedObject(DatabaseRelationId, dbOid, 0, lockmode);
    }

    heap_close(relation, AccessShareLock);

    return result;
}

/* Check if current user has createdb privileges */
static bool
have_createdb_privilege(void)
{
    bool        result = false;
    HeapTuple   utup;

    /* Superusers can always do everything */
    if (superuser())
        return true;

    utup = SearchSysCache1(AUTHOID, ObjectIdGetDatum(GetUserId()));
    if (HeapTupleIsValid(utup))
    {
        result = ((Form_pg_authid) GETSTRUCT(utup))->rolcreatedb;
        ReleaseSysCache(utup);
    }
    return result;
}

/*
 * Remove tablespace directories
 *
 * We don't know what tablespaces db_id is using, so iterate through all
 * tablespaces removing <tablespace>/db_id
 */
static void
remove_dbtablespaces(Oid db_id)
{
    Relation    rel;
    HeapScanDesc scan;
    HeapTuple   tuple;
    Snapshot    snapshot;

    /*
     * As in createdb(), we'd better use an MVCC snapshot here, since this
     * scan can run for a long time.  Duplicate visits to tablespaces would be
     * harmless, but missing a tablespace could result in permanently leaked
     * files.
     *
     * XXX change this when a generic fix for SnapshotNow races is implemented
     */
    snapshot = RegisterSnapshot(GetLatestSnapshot());

    rel = heap_open(TableSpaceRelationId, AccessShareLock);
    scan = heap_beginscan(rel, snapshot, 0, NULL);
    while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
    {
        Oid         dsttablespace = HeapTupleGetOid(tuple);
        char       *dstpath;
        struct stat st;

        /* Don't mess with the global tablespace */
        if (dsttablespace == GLOBALTABLESPACE_OID)
            continue;

        dstpath = GetDatabasePath(db_id, dsttablespace);

        if (lstat(dstpath, &st) < 0 || !S_ISDIR(st.st_mode))
        {
            /* Assume we can ignore it */
            pfree(dstpath);
            continue;
        }

        if (!rmtree(dstpath, true))
            ereport(WARNING,
                    (errmsg("some useless files may be left behind in old database directory \"%s\"",
                            dstpath)));

        /* Record the filesystem change in XLOG */
        {
            xl_dbase_drop_rec xlrec;
            XLogRecData rdata[1];

            xlrec.db_id = db_id;
            xlrec.tablespace_id = dsttablespace;

            rdata[0].data = (char *) &xlrec;
            rdata[0].len = sizeof(xl_dbase_drop_rec);
            rdata[0].buffer = InvalidBuffer;
            rdata[0].next = NULL;

            (void) XLogInsert(RM_DBASE_ID, XLOG_DBASE_DROP, rdata);
        }

        pfree(dstpath);
    }

    heap_endscan(scan);
    heap_close(rel, AccessShareLock);
    UnregisterSnapshot(snapshot);
}

/*
 * Check for existing files that conflict with a proposed new DB OID;
 * return TRUE if there are any
 *
 * If there were a subdirectory in any tablespace matching the proposed new
 * OID, we'd get a create failure due to the duplicate name ... and then we'd
 * try to remove that already-existing subdirectory during the cleanup in
 * remove_dbtablespaces.  Nuking existing files seems like a bad idea, so
 * instead we make this extra check before settling on the OID of the new
 * database.  This exactly parallels what GetNewRelFileNode() does for table
 * relfilenode values.
 */
static bool
check_db_file_conflict(Oid db_id)
{
    bool        result = false;
    Relation    rel;
    HeapScanDesc scan;
    HeapTuple   tuple;
    Snapshot    snapshot;

    /*
     * As in createdb(), we'd better use an MVCC snapshot here; missing a
     * tablespace could result in falsely reporting the OID is unique, with
     * disastrous future consequences per the comment above.
     *
     * XXX change this when a generic fix for SnapshotNow races is implemented
     */
    snapshot = RegisterSnapshot(GetLatestSnapshot());

    rel = heap_open(TableSpaceRelationId, AccessShareLock);
    scan = heap_beginscan(rel, snapshot, 0, NULL);
    while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
    {
        Oid         dsttablespace = HeapTupleGetOid(tuple);
        char       *dstpath;
        struct stat st;

        /* Don't mess with the global tablespace */
        if (dsttablespace == GLOBALTABLESPACE_OID)
            continue;

        dstpath = GetDatabasePath(db_id, dsttablespace);

        if (lstat(dstpath, &st) == 0)
        {
            /* Found a conflicting file (or directory, whatever) */
            pfree(dstpath);
            result = true;
            break;
        }

        pfree(dstpath);
    }

    heap_endscan(scan);
    heap_close(rel, AccessShareLock);
    UnregisterSnapshot(snapshot);

    return result;
}

/*
 * Issue a suitable errdetail message for a busy database
 */
static int
errdetail_busy_db(int notherbackends, int npreparedxacts)
{
    if (notherbackends > 0 && npreparedxacts > 0)
        /* We don't deal with singular versus plural here, since gettext
         * doesn't support multiple plurals in one string. */
        errdetail("There are %d other session(s) and %d prepared transaction(s) using the database.",
                  notherbackends, npreparedxacts);
    else if (notherbackends > 0)
        errdetail_plural("There is %d other session using the database.",
                         "There are %d other sessions using the database.",
                         notherbackends,
                         notherbackends);
    else
        errdetail_plural("There is %d prepared transaction using the database.",
                         "There are %d prepared transactions using the database.",
                         npreparedxacts,
                         npreparedxacts);
    return 0;                   /* just to keep ereport macro happy */
}

/*
 * get_database_oid - given a database name, look up the OID
 *
 * If missing_ok is false, throw an error if database name not found.  If
 * true, just return InvalidOid.
 */
Oid
get_database_oid(const char *dbname, bool missing_ok)
{
    Relation    pg_database;
    ScanKeyData entry[1];
    SysScanDesc scan;
    HeapTuple   dbtuple;
    Oid         oid;

    /*
     * There's no syscache for pg_database indexed by name, so we must look
     * the hard way.
     */
    pg_database = heap_open(DatabaseRelationId, AccessShareLock);
    ScanKeyInit(&entry[0],
                Anum_pg_database_datname,
                BTEqualStrategyNumber, F_NAMEEQ,
                CStringGetDatum(dbname));
    scan = systable_beginscan(pg_database, DatabaseNameIndexId, true,
                              SnapshotNow, 1, entry);

    dbtuple = systable_getnext(scan);

    /* We assume that there can be at most one matching tuple */
    if (HeapTupleIsValid(dbtuple))
        oid = HeapTupleGetOid(dbtuple);
    else
        oid = InvalidOid;

    systable_endscan(scan);
    heap_close(pg_database, AccessShareLock);

    if (!OidIsValid(oid) && !missing_ok)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_DATABASE),
                 errmsg("database \"%s\" does not exist",
                        dbname)));

    return oid;
}


/*
 * get_database_name - given a database OID, look up the name
 *
 * Returns a palloc'd string, or NULL if no such database.
 */
char *
get_database_name(Oid dbid)
{
    HeapTuple   dbtuple;
    char       *result;

    dbtuple = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(dbid));
    if (HeapTupleIsValid(dbtuple))
    {
        result = pstrdup(NameStr(((Form_pg_database) GETSTRUCT(dbtuple))->datname));
        ReleaseSysCache(dbtuple);
    }
    else
        result = NULL;

    return result;
}

/*
 * DATABASE resource manager's routines
 */
void
dbase_redo(XLogRecPtr lsn, XLogRecord *record)
{
    uint8       info = record->xl_info & ~XLR_INFO_MASK;

    /* Backup blocks are not used in dbase records */
    Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

    if (info == XLOG_DBASE_CREATE)
    {
        xl_dbase_create_rec *xlrec = (xl_dbase_create_rec *) XLogRecGetData(record);
        char       *src_path;
        char       *dst_path;
        struct stat st;

        src_path = GetDatabasePath(xlrec->src_db_id, xlrec->src_tablespace_id);
        dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_id);

        /*
         * Our theory for replaying a CREATE is to forcibly drop the target
         * subdirectory if present, then re-copy the source data. This may be
         * more work than needed, but it is simple to implement.
         */
        if (stat(dst_path, &st) == 0 && S_ISDIR(st.st_mode))
        {
            if (!rmtree(dst_path, true))
                /* If this failed, copydir() below is going to error. */
                ereport(WARNING,
                        (errmsg("some useless files may be left behind in old database directory \"%s\"",
                                dst_path)));
        }

        /*
         * Force dirty buffers out to disk, to ensure source database is
         * up-to-date for the copy.
         */
        FlushDatabaseBuffers(xlrec->src_db_id);

        /*
         * Copy this subdirectory to the new location
         *
         * We don't need to copy subdirectories
         */
        copydir(src_path, dst_path, false);
    }
    else if (info == XLOG_DBASE_DROP)
    {
        xl_dbase_drop_rec *xlrec = (xl_dbase_drop_rec *) XLogRecGetData(record);
        char       *dst_path;

        dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_id);

        if (InHotStandby)
        {
            /*
             * Lock database while we resolve conflicts to ensure that
             * InitPostgres() cannot fully re-execute concurrently. This
             * avoids backends re-connecting automatically to same database,
             * which can happen in some cases.
             */
            LockSharedObjectForSession(DatabaseRelationId, xlrec->db_id, 0, AccessExclusiveLock);
            ResolveRecoveryConflictWithDatabase(xlrec->db_id);
        }

        /* Drop pages for this database that are in the shared buffer cache */
        DropDatabaseBuffers(xlrec->db_id);

        /* Also, clean out any fsync requests that might be pending in md.c */
        ForgetDatabaseFsyncRequests(xlrec->db_id);

        /* Clean out the xlog relcache too */
        XLogDropDatabase(xlrec->db_id);

        /* And remove the physical files */
        if (!rmtree(dst_path, true))
            ereport(WARNING,
                    (errmsg("some useless files may be left behind in old database directory \"%s\"",
                            dst_path)));

        if (InHotStandby)
        {
            /*
             * Release locks prior to commit. XXX There is a race condition
             * here that may allow backends to reconnect, but the window for
             * this is small because the gap between here and commit is mostly
             * fairly small and it is unlikely that people will be dropping
             * databases that we are trying to connect to anyway.
             */
            UnlockSharedObjectForSession(DatabaseRelationId, xlrec->db_id, 0, AccessExclusiveLock);
        }
    }
    else
        elog(PANIC, "dbase_redo: unknown op code %u", info);
}