relmapper.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * relmapper.c
 *    Catalog-to-filenode mapping
 *
 * For most tables, the physical file underlying the table is specified by
 * pg_class.relfilenode.  However, that obviously won't work for pg_class
 * itself, nor for the other "nailed" catalogs for which we have to be able
 * to set up working Relation entries without access to pg_class.  It also
 * does not work for shared catalogs, since there is no practical way to
 * update other databases' pg_class entries when relocating a shared catalog.
 * Therefore, for these special catalogs (henceforth referred to as "mapped
 * catalogs") we rely on a separately maintained file that shows the mapping
 * from catalog OIDs to filenode numbers.  Each database has a map file for
 * its local mapped catalogs, and there is a separate map file for shared
 * catalogs.  Mapped catalogs have zero in their pg_class.relfilenode entries.
 *
 * Relocation of a normal table is committed (ie, the new physical file becomes
 * authoritative) when the pg_class row update commits.  For mapped catalogs,
 * the act of updating the map file is effectively commit of the relocation.
 * We postpone the file update till just before commit of the transaction
 * doing the rewrite, but there is necessarily a window between.  Therefore
 * mapped catalogs can only be relocated by operations such as VACUUM FULL
 * and CLUSTER, which make no transactionally-significant changes: it must be
 * safe for the new file to replace the old, even if the transaction itself
 * aborts.  An important factor here is that the indexes and toast table of
 * a mapped catalog must also be mapped, so that the rewrites/relocations of
 * all these files commit in a single map file update rather than being tied
 * to transaction commit.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/cache/relmapper.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

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

#include "access/xact.h"
#include "catalog/catalog.h"
#include "catalog/pg_tablespace.h"
#include "catalog/storage.h"
#include "miscadmin.h"
#include "storage/fd.h"
#include "storage/lwlock.h"
#include "utils/inval.h"
#include "utils/relmapper.h"


/*
 * The map file is critical data: we have no automatic method for recovering
 * from loss or corruption of it.  We use a CRC so that we can detect
 * corruption.  To minimize the risk of failed updates, the map file should
 * be kept to no more than one standard-size disk sector (ie 512 bytes),
 * and we use overwrite-in-place rather than playing renaming games.
 * The struct layout below is designed to occupy exactly 512 bytes, which
 * might make filesystem updates a bit more efficient.
 *
 * Entries in the mappings[] array are in no particular order.  We could
 * speed searching by insisting on OID order, but it really shouldn't be
 * worth the trouble given the intended size of the mapping sets.
 */
#define RELMAPPER_FILENAME      "pg_filenode.map"

#define RELMAPPER_FILEMAGIC     0x592717        /* version ID value */

#define MAX_MAPPINGS            62      /* 62 * 8 + 16 = 512 */

typedef struct RelMapping
{
    Oid         mapoid;         /* OID of a catalog */
    Oid         mapfilenode;    /* its filenode number */
} RelMapping;

typedef struct RelMapFile
{
    int32       magic;          /* always RELMAPPER_FILEMAGIC */
    int32       num_mappings;   /* number of valid RelMapping entries */
    RelMapping  mappings[MAX_MAPPINGS];
    int32       crc;            /* CRC of all above */
    int32       pad;            /* to make the struct size be 512 exactly */
} RelMapFile;

/*
 * The currently known contents of the shared map file and our database's
 * local map file are stored here.  These can be reloaded from disk
 * immediately whenever we receive an update sinval message.
 */
static RelMapFile shared_map;
static RelMapFile local_map;

/*
 * We use the same RelMapFile data structure to track uncommitted local
 * changes in the mappings (but note the magic and crc fields are not made
 * valid in these variables).  Currently, map updates are not allowed within
 * subtransactions, so one set of transaction-level changes is sufficient.
 *
 * The active_xxx variables contain updates that are valid in our transaction
 * and should be honored by RelationMapOidToFilenode.  The pending_xxx
 * variables contain updates we have been told about that aren't active yet;
 * they will become active at the next CommandCounterIncrement.  This setup
 * lets map updates act similarly to updates of pg_class rows, ie, they
 * become visible only at the next CommandCounterIncrement boundary.
 */
static RelMapFile active_shared_updates;
static RelMapFile active_local_updates;
static RelMapFile pending_shared_updates;
static RelMapFile pending_local_updates;


/* non-export function prototypes */
static void apply_map_update(RelMapFile *map, Oid relationId, Oid fileNode,
                 bool add_okay);
static void merge_map_updates(RelMapFile *map, const RelMapFile *updates,
                  bool add_okay);
static void load_relmap_file(bool shared);
static void write_relmap_file(bool shared, RelMapFile *newmap,
                  bool write_wal, bool send_sinval, bool preserve_files,
                  Oid dbid, Oid tsid, const char *dbpath);
static void perform_relmap_update(bool shared, const RelMapFile *updates);


/*
 * RelationMapOidToFilenode
 *
 * The raison d' etre ... given a relation OID, look up its filenode.
 *
 * Although shared and local relation OIDs should never overlap, the caller
 * always knows which we need --- so pass that information to avoid useless
 * searching.
 *
 * Returns InvalidOid if the OID is not known (which should never happen,
 * but the caller is in a better position to report a meaningful error).
 */
Oid
RelationMapOidToFilenode(Oid relationId, bool shared)
{
    const RelMapFile *map;
    int32       i;

    /* If there are active updates, believe those over the main maps */
    if (shared)
    {
        map = &active_shared_updates;
        for (i = 0; i < map->num_mappings; i++)
        {
            if (relationId == map->mappings[i].mapoid)
                return map->mappings[i].mapfilenode;
        }
        map = &shared_map;
        for (i = 0; i < map->num_mappings; i++)
        {
            if (relationId == map->mappings[i].mapoid)
                return map->mappings[i].mapfilenode;
        }
    }
    else
    {
        map = &active_local_updates;
        for (i = 0; i < map->num_mappings; i++)
        {
            if (relationId == map->mappings[i].mapoid)
                return map->mappings[i].mapfilenode;
        }
        map = &local_map;
        for (i = 0; i < map->num_mappings; i++)
        {
            if (relationId == map->mappings[i].mapoid)
                return map->mappings[i].mapfilenode;
        }
    }

    return InvalidOid;
}

/*
 * RelationMapUpdateMap
 *
 * Install a new relfilenode mapping for the specified relation.
 *
 * If immediate is true (or we're bootstrapping), the mapping is activated
 * immediately.  Otherwise it is made pending until CommandCounterIncrement.
 */
void
RelationMapUpdateMap(Oid relationId, Oid fileNode, bool shared,
                     bool immediate)
{
    RelMapFile *map;

    if (IsBootstrapProcessingMode())
    {
        /*
         * In bootstrap mode, the mapping gets installed in permanent map.
         */
        if (shared)
            map = &shared_map;
        else
            map = &local_map;
    }
    else
    {
        /*
         * We don't currently support map changes within subtransactions. This
         * could be done with more bookkeeping infrastructure, but it doesn't
         * presently seem worth it.
         */
        if (GetCurrentTransactionNestLevel() > 1)
            elog(ERROR, "cannot change relation mapping within subtransaction");

        if (immediate)
        {
            /* Make it active, but only locally */
            if (shared)
                map = &active_shared_updates;
            else
                map = &active_local_updates;
        }
        else
        {
            /* Make it pending */
            if (shared)
                map = &pending_shared_updates;
            else
                map = &pending_local_updates;
        }
    }
    apply_map_update(map, relationId, fileNode, true);
}

/*
 * apply_map_update
 *
 * Insert a new mapping into the given map variable, replacing any existing
 * mapping for the same relation.
 *
 * In some cases the caller knows there must be an existing mapping; pass
 * add_okay = false to draw an error if not.
 */
static void
apply_map_update(RelMapFile *map, Oid relationId, Oid fileNode, bool add_okay)
{
    int32       i;

    /* Replace any existing mapping */
    for (i = 0; i < map->num_mappings; i++)
    {
        if (relationId == map->mappings[i].mapoid)
        {
            map->mappings[i].mapfilenode = fileNode;
            return;
        }
    }

    /* Nope, need to add a new mapping */
    if (!add_okay)
        elog(ERROR, "attempt to apply a mapping to unmapped relation %u",
             relationId);
    if (map->num_mappings >= MAX_MAPPINGS)
        elog(ERROR, "ran out of space in relation map");
    map->mappings[map->num_mappings].mapoid = relationId;
    map->mappings[map->num_mappings].mapfilenode = fileNode;
    map->num_mappings++;
}

/*
 * merge_map_updates
 *
 * Merge all the updates in the given pending-update map into the target map.
 * This is just a bulk form of apply_map_update.
 */
static void
merge_map_updates(RelMapFile *map, const RelMapFile *updates, bool add_okay)
{
    int32       i;

    for (i = 0; i < updates->num_mappings; i++)
    {
        apply_map_update(map,
                         updates->mappings[i].mapoid,
                         updates->mappings[i].mapfilenode,
                         add_okay);
    }
}

/*
 * RelationMapRemoveMapping
 *
 * Remove a relation's entry in the map.  This is only allowed for "active"
 * (but not committed) local mappings.  We need it so we can back out the
 * entry for the transient target file when doing VACUUM FULL/CLUSTER on
 * a mapped relation.
 */
void
RelationMapRemoveMapping(Oid relationId)
{
    RelMapFile *map = &active_local_updates;
    int32       i;

    for (i = 0; i < map->num_mappings; i++)
    {
        if (relationId == map->mappings[i].mapoid)
        {
            /* Found it, collapse it out */
            map->mappings[i] = map->mappings[map->num_mappings - 1];
            map->num_mappings--;
            return;
        }
    }
    elog(ERROR, "could not find temporary mapping for relation %u",
         relationId);
}

/*
 * RelationMapInvalidate
 *
 * This routine is invoked for SI cache flush messages.  We must re-read
 * the indicated map file.  However, we might receive a SI message in a
 * process that hasn't yet, and might never, load the mapping files;
 * for example the autovacuum launcher, which *must not* try to read
 * a local map since it is attached to no particular database.
 * So, re-read only if the map is valid now.
 */
void
RelationMapInvalidate(bool shared)
{
    if (shared)
    {
        if (shared_map.magic == RELMAPPER_FILEMAGIC)
            load_relmap_file(true);
    }
    else
    {
        if (local_map.magic == RELMAPPER_FILEMAGIC)
            load_relmap_file(false);
    }
}

/*
 * RelationMapInvalidateAll
 *
 * Reload all map files.  This is used to recover from SI message buffer
 * overflow: we can't be sure if we missed an inval message.
 * Again, reload only currently-valid maps.
 */
void
RelationMapInvalidateAll(void)
{
    if (shared_map.magic == RELMAPPER_FILEMAGIC)
        load_relmap_file(true);
    if (local_map.magic == RELMAPPER_FILEMAGIC)
        load_relmap_file(false);
}

/*
 * AtCCI_RelationMap
 *
 * Activate any "pending" relation map updates at CommandCounterIncrement time.
 */
void
AtCCI_RelationMap(void)
{
    if (pending_shared_updates.num_mappings != 0)
    {
        merge_map_updates(&active_shared_updates,
                          &pending_shared_updates,
                          true);
        pending_shared_updates.num_mappings = 0;
    }
    if (pending_local_updates.num_mappings != 0)
    {
        merge_map_updates(&active_local_updates,
                          &pending_local_updates,
                          true);
        pending_local_updates.num_mappings = 0;
    }
}

/*
 * AtEOXact_RelationMap
 *
 * Handle relation mapping at main-transaction commit or abort.
 *
 * During commit, this must be called as late as possible before the actual
 * transaction commit, so as to minimize the window where the transaction
 * could still roll back after committing map changes.  Although nothing
 * critically bad happens in such a case, we still would prefer that it
 * not happen, since we'd possibly be losing useful updates to the relations'
 * pg_class row(s).
 *
 * During abort, we just have to throw away any pending map changes.
 * Normal post-abort cleanup will take care of fixing relcache entries.
 */
void
AtEOXact_RelationMap(bool isCommit)
{
    if (isCommit)
    {
        /*
         * We should not get here with any "pending" updates.  (We could
         * logically choose to treat such as committed, but in the current
         * code this should never happen.)
         */
        Assert(pending_shared_updates.num_mappings == 0);
        Assert(pending_local_updates.num_mappings == 0);

        /*
         * Write any active updates to the actual map files, then reset them.
         */
        if (active_shared_updates.num_mappings != 0)
        {
            perform_relmap_update(true, &active_shared_updates);
            active_shared_updates.num_mappings = 0;
        }
        if (active_local_updates.num_mappings != 0)
        {
            perform_relmap_update(false, &active_local_updates);
            active_local_updates.num_mappings = 0;
        }
    }
    else
    {
        /* Abort --- drop all local and pending updates */
        active_shared_updates.num_mappings = 0;
        active_local_updates.num_mappings = 0;
        pending_shared_updates.num_mappings = 0;
        pending_local_updates.num_mappings = 0;
    }
}

/*
 * AtPrepare_RelationMap
 *
 * Handle relation mapping at PREPARE.
 *
 * Currently, we don't support preparing any transaction that changes the map.
 */
void
AtPrepare_RelationMap(void)
{
    if (active_shared_updates.num_mappings != 0 ||
        active_local_updates.num_mappings != 0 ||
        pending_shared_updates.num_mappings != 0 ||
        pending_local_updates.num_mappings != 0)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot PREPARE a transaction that modified relation mapping")));
}

/*
 * CheckPointRelationMap
 *
 * This is called during a checkpoint.  It must ensure that any relation map
 * updates that were WAL-logged before the start of the checkpoint are
 * securely flushed to disk and will not need to be replayed later.  This
 * seems unlikely to be a performance-critical issue, so we use a simple
 * method: we just take and release the RelationMappingLock.  This ensures
 * that any already-logged map update is complete, because write_relmap_file
 * will fsync the map file before the lock is released.
 */
void
CheckPointRelationMap(void)
{
    LWLockAcquire(RelationMappingLock, LW_SHARED);
    LWLockRelease(RelationMappingLock);
}

/*
 * RelationMapFinishBootstrap
 *
 * Write out the initial relation mapping files at the completion of
 * bootstrap.  All the mapped files should have been made known to us
 * via RelationMapUpdateMap calls.
 */
void
RelationMapFinishBootstrap(void)
{
    Assert(IsBootstrapProcessingMode());

    /* Shouldn't be anything "pending" ... */
    Assert(active_shared_updates.num_mappings == 0);
    Assert(active_local_updates.num_mappings == 0);
    Assert(pending_shared_updates.num_mappings == 0);
    Assert(pending_local_updates.num_mappings == 0);

    /* Write the files; no WAL or sinval needed */
    write_relmap_file(true, &shared_map, false, false, false,
                      InvalidOid, GLOBALTABLESPACE_OID, NULL);
    write_relmap_file(false, &local_map, false, false, false,
                      MyDatabaseId, MyDatabaseTableSpace, DatabasePath);
}

/*
 * RelationMapInitialize
 *
 * This initializes the mapper module at process startup.  We can't access the
 * database yet, so just make sure the maps are empty.
 */
void
RelationMapInitialize(void)
{
    /* The static variables should initialize to zeroes, but let's be sure */
    shared_map.magic = 0;       /* mark it not loaded */
    local_map.magic = 0;
    shared_map.num_mappings = 0;
    local_map.num_mappings = 0;
    active_shared_updates.num_mappings = 0;
    active_local_updates.num_mappings = 0;
    pending_shared_updates.num_mappings = 0;
    pending_local_updates.num_mappings = 0;
}

/*
 * RelationMapInitializePhase2
 *
 * This is called to prepare for access to pg_database during startup.
 * We should be able to read the shared map file now.
 */
void
RelationMapInitializePhase2(void)
{
    /*
     * In bootstrap mode, the map file isn't there yet, so do nothing.
     */
    if (IsBootstrapProcessingMode())
        return;

    /*
     * Load the shared map file, die on error.
     */
    load_relmap_file(true);
}

/*
 * RelationMapInitializePhase3
 *
 * This is called as soon as we have determined MyDatabaseId and set up
 * DatabasePath.  At this point we should be able to read the local map file.
 */
void
RelationMapInitializePhase3(void)
{
    /*
     * In bootstrap mode, the map file isn't there yet, so do nothing.
     */
    if (IsBootstrapProcessingMode())
        return;

    /*
     * Load the local map file, die on error.
     */
    load_relmap_file(false);
}

/*
 * load_relmap_file -- load data from the shared or local map file
 *
 * Because the map file is essential for access to core system catalogs,
 * failure to read it is a fatal error.
 *
 * Note that the local case requires DatabasePath to be set up.
 */
static void
load_relmap_file(bool shared)
{
    RelMapFile *map;
    char        mapfilename[MAXPGPATH];
    pg_crc32    crc;
    int         fd;

    if (shared)
    {
        snprintf(mapfilename, sizeof(mapfilename), "global/%s",
                 RELMAPPER_FILENAME);
        map = &shared_map;
    }
    else
    {
        snprintf(mapfilename, sizeof(mapfilename), "%s/%s",
                 DatabasePath, RELMAPPER_FILENAME);
        map = &local_map;
    }

    /* Read data ... */
    fd = OpenTransientFile(mapfilename,
                           O_RDONLY | PG_BINARY, S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not open relation mapping file \"%s\": %m",
                        mapfilename)));

    /*
     * Note: we could take RelationMappingLock in shared mode here, but it
     * seems unnecessary since our read() should be atomic against any
     * concurrent updater's write().  If the file is updated shortly after we
     * look, the sinval signaling mechanism will make us re-read it before we
     * are able to access any relation that's affected by the change.
     */
    if (read(fd, map, sizeof(RelMapFile)) != sizeof(RelMapFile))
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not read relation mapping file \"%s\": %m",
                        mapfilename)));

    CloseTransientFile(fd);

    /* check for correct magic number, etc */
    if (map->magic != RELMAPPER_FILEMAGIC ||
        map->num_mappings < 0 ||
        map->num_mappings > MAX_MAPPINGS)
        ereport(FATAL,
                (errmsg("relation mapping file \"%s\" contains invalid data",
                        mapfilename)));

    /* verify the CRC */
    INIT_CRC32(crc);
    COMP_CRC32(crc, (char *) map, offsetof(RelMapFile, crc));
    FIN_CRC32(crc);

    if (!EQ_CRC32(crc, map->crc))
        ereport(FATAL,
          (errmsg("relation mapping file \"%s\" contains incorrect checksum",
                  mapfilename)));
}

/*
 * Write out a new shared or local map file with the given contents.
 *
 * The magic number and CRC are automatically updated in *newmap.  On
 * success, we copy the data to the appropriate permanent static variable.
 *
 * If write_wal is TRUE then an appropriate WAL message is emitted.
 * (It will be false for bootstrap and WAL replay cases.)
 *
 * If send_sinval is TRUE then a SI invalidation message is sent.
 * (This should be true except in bootstrap case.)
 *
 * If preserve_files is TRUE then the storage manager is warned not to
 * delete the files listed in the map.
 *
 * Because this may be called during WAL replay when MyDatabaseId,
 * DatabasePath, etc aren't valid, we require the caller to pass in suitable
 * values.  The caller is also responsible for being sure no concurrent
 * map update could be happening.
 */
static void
write_relmap_file(bool shared, RelMapFile *newmap,
                  bool write_wal, bool send_sinval, bool preserve_files,
                  Oid dbid, Oid tsid, const char *dbpath)
{
    int         fd;
    RelMapFile *realmap;
    char        mapfilename[MAXPGPATH];

    /*
     * Fill in the overhead fields and update CRC.
     */
    newmap->magic = RELMAPPER_FILEMAGIC;
    if (newmap->num_mappings < 0 || newmap->num_mappings > MAX_MAPPINGS)
        elog(ERROR, "attempt to write bogus relation mapping");

    INIT_CRC32(newmap->crc);
    COMP_CRC32(newmap->crc, (char *) newmap, offsetof(RelMapFile, crc));
    FIN_CRC32(newmap->crc);

    /*
     * Open the target file.  We prefer to do this before entering the
     * critical section, so that an open() failure need not force PANIC.
     */
    if (shared)
    {
        snprintf(mapfilename, sizeof(mapfilename), "global/%s",
                 RELMAPPER_FILENAME);
        realmap = &shared_map;
    }
    else
    {
        snprintf(mapfilename, sizeof(mapfilename), "%s/%s",
                 dbpath, RELMAPPER_FILENAME);
        realmap = &local_map;
    }

    fd = OpenTransientFile(mapfilename,
                           O_WRONLY | O_CREAT | PG_BINARY,
                           S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open relation mapping file \"%s\": %m",
                        mapfilename)));

    if (write_wal)
    {
        xl_relmap_update xlrec;
        XLogRecData rdata[2];
        XLogRecPtr  lsn;

        /* now errors are fatal ... */
        START_CRIT_SECTION();

        xlrec.dbid = dbid;
        xlrec.tsid = tsid;
        xlrec.nbytes = sizeof(RelMapFile);

        rdata[0].data = (char *) (&xlrec);
        rdata[0].len = MinSizeOfRelmapUpdate;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);
        rdata[1].data = (char *) newmap;
        rdata[1].len = sizeof(RelMapFile);
        rdata[1].buffer = InvalidBuffer;
        rdata[1].next = NULL;

        lsn = XLogInsert(RM_RELMAP_ID, XLOG_RELMAP_UPDATE, rdata);

        /* As always, WAL must hit the disk before the data update does */
        XLogFlush(lsn);
    }

    errno = 0;
    if (write(fd, newmap, sizeof(RelMapFile)) != sizeof(RelMapFile))
    {
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to relation mapping file \"%s\": %m",
                        mapfilename)));
    }

    /*
     * We choose to fsync the data to disk before considering the task done.
     * It would be possible to relax this if it turns out to be a performance
     * issue, but it would complicate checkpointing --- see notes for
     * CheckPointRelationMap.
     */
    if (pg_fsync(fd) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not fsync relation mapping file \"%s\": %m",
                        mapfilename)));

    if (CloseTransientFile(fd))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close relation mapping file \"%s\": %m",
                        mapfilename)));

    /*
     * Now that the file is safely on disk, send sinval message to let other
     * backends know to re-read it.  We must do this inside the critical
     * section: if for some reason we fail to send the message, we have to
     * force a database-wide PANIC.  Otherwise other backends might continue
     * execution with stale mapping information, which would be catastrophic
     * as soon as others began to use the now-committed data.
     */
    if (send_sinval)
        CacheInvalidateRelmap(dbid);

    /*
     * Make sure that the files listed in the map are not deleted if the outer
     * transaction aborts.  This had better be within the critical section
     * too: it's not likely to fail, but if it did, we'd arrive at transaction
     * abort with the files still vulnerable.  PANICing will leave things in a
     * good state on-disk.
     *
     * Note: we're cheating a little bit here by assuming that mapped files
     * are either in pg_global or the database's default tablespace.
     */
    if (preserve_files)
    {
        int32       i;

        for (i = 0; i < newmap->num_mappings; i++)
        {
            RelFileNode rnode;

            rnode.spcNode = tsid;
            rnode.dbNode = dbid;
            rnode.relNode = newmap->mappings[i].mapfilenode;
            RelationPreserveStorage(rnode, false);
        }
    }

    /* Success, update permanent copy */
    memcpy(realmap, newmap, sizeof(RelMapFile));

    /* Critical section done */
    if (write_wal)
        END_CRIT_SECTION();
}

/*
 * Merge the specified updates into the appropriate "real" map,
 * and write out the changes.  This function must be used for committing
 * updates during normal multiuser operation.
 */
static void
perform_relmap_update(bool shared, const RelMapFile *updates)
{
    RelMapFile  newmap;

    /*
     * Anyone updating a relation's mapping info should take exclusive lock on
     * that rel and hold it until commit.  This ensures that there will not be
     * concurrent updates on the same mapping value; but there could easily be
     * concurrent updates on different values in the same file. We cover that
     * by acquiring the RelationMappingLock, re-reading the target file to
     * ensure it's up to date, applying the updates, and writing the data
     * before releasing RelationMappingLock.
     *
     * There is only one RelationMappingLock.  In principle we could try to
     * have one per mapping file, but it seems unlikely to be worth the
     * trouble.
     */
    LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);

    /* Be certain we see any other updates just made */
    load_relmap_file(shared);

    /* Prepare updated data in a local variable */
    if (shared)
        memcpy(&newmap, &shared_map, sizeof(RelMapFile));
    else
        memcpy(&newmap, &local_map, sizeof(RelMapFile));

    /*
     * Apply the updates to newmap.  No new mappings should appear, unless
     * somebody is adding indexes to system catalogs.
     */
    merge_map_updates(&newmap, updates, allowSystemTableMods);

    /* Write out the updated map and do other necessary tasks */
    write_relmap_file(shared, &newmap, true, true, true,
                      (shared ? InvalidOid : MyDatabaseId),
                      (shared ? GLOBALTABLESPACE_OID : MyDatabaseTableSpace),
                      DatabasePath);

    /* Now we can release the lock */
    LWLockRelease(RelationMappingLock);
}

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

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

    if (info == XLOG_RELMAP_UPDATE)
    {
        xl_relmap_update *xlrec = (xl_relmap_update *) XLogRecGetData(record);
        RelMapFile  newmap;
        char       *dbpath;

        if (xlrec->nbytes != sizeof(RelMapFile))
            elog(PANIC, "relmap_redo: wrong size %u in relmap update record",
                 xlrec->nbytes);
        memcpy(&newmap, xlrec->data, sizeof(newmap));

        /* We need to construct the pathname for this database */
        dbpath = GetDatabasePath(xlrec->dbid, xlrec->tsid);

        /*
         * Write out the new map and send sinval, but of course don't write a
         * new WAL entry.  There's no surrounding transaction to tell to
         * preserve files, either.
         *
         * There shouldn't be anyone else updating relmaps during WAL replay,
         * so we don't bother to take the RelationMappingLock.  We would need
         * to do so if load_relmap_file needed to interlock against writers.
         */
        write_relmap_file((xlrec->dbid == InvalidOid), &newmap,
                          false, true, false,
                          xlrec->dbid, xlrec->tsid, dbpath);

        pfree(dbpath);
    }
    else
        elog(PANIC, "relmap_redo: unknown op code %u", info);
}