Prevent race condition while reading relmapper file.

Contrary to the comment here, POSIX does not guarantee atomicity of a
read(), if another process calls write() concurrently. Or at least Linux
does not. Add locking to load_relmap_file() to avoid the race condition.

Fixes bug #17064. Thanks to Alexander Lakhin for the report and test case.

Backpatch-through: 9.6, all supported versions.
Discussion: https://www.postgresql.org/message-id/17064-bb0d7904ef72add3@postgresql.org

diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index 9ba11a73cfad0dfee97d5dd235471dea6de527c7..5ccb6653ae24e706739c0fb91c3411ee74fb48e8 100644 (file)
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -123,7 +123,7 @@ 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 load_relmap_file(bool shared, bool lock_held);
 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);
@@ -393,12 +393,12 @@ RelationMapInvalidate(bool shared)
    if (shared)
    {
        if (shared_map.magic == RELMAPPER_FILEMAGIC)
-           load_relmap_file(true);
+           load_relmap_file(true, false);
    }
    else
    {
        if (local_map.magic == RELMAPPER_FILEMAGIC)
-           load_relmap_file(false);
+           load_relmap_file(false, false);
    }
 }
 
@@ -413,9 +413,9 @@ void
 RelationMapInvalidateAll(void)
 {
    if (shared_map.magic == RELMAPPER_FILEMAGIC)
-       load_relmap_file(true);
+       load_relmap_file(true, false);
    if (local_map.magic == RELMAPPER_FILEMAGIC)
-       load_relmap_file(false);
+       load_relmap_file(false, false);
 }
 
 /*
@@ -594,7 +594,7 @@ RelationMapInitializePhase2(void)
    /*
     * Load the shared map file, die on error.
     */
-   load_relmap_file(true);
+   load_relmap_file(true, false);
 }
 
 /*
@@ -615,7 +615,7 @@ RelationMapInitializePhase3(void)
    /*
     * Load the local map file, die on error.
     */
-   load_relmap_file(false);
+   load_relmap_file(false, false);
 }
 
 /*
@@ -627,7 +627,7 @@ RelationMapInitializePhase3(void)
  * Note that the local case requires DatabasePath to be set up.
  */
 static void
-load_relmap_file(bool shared)
+load_relmap_file(bool shared, bool lock_held)
 {
    RelMapFile *map;
    char        mapfilename[MAXPGPATH];
@@ -656,12 +656,15 @@ load_relmap_file(bool shared)
                        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.
+    * Grab the lock to prevent the file from being updated while we read it,
+    * unless the caller is already holding the lock.  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 (!lock_held)
+       LWLockAcquire(RelationMappingLock, LW_SHARED);
+
    pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_READ);
    if (read(fd, map, sizeof(RelMapFile)) != sizeof(RelMapFile))
        ereport(FATAL,
@@ -670,6 +673,9 @@ load_relmap_file(bool shared)
                        mapfilename)));
    pgstat_report_wait_end();
 
+   if (!lock_held)
+       LWLockRelease(RelationMappingLock);
+
    CloseTransientFile(fd);
 
    /* check for correct magic number, etc */
@@ -888,7 +894,7 @@ perform_relmap_update(bool shared, const RelMapFile *updates)
    LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);
 
    /* Be certain we see any other updates just made */
-   load_relmap_file(shared);
+   load_relmap_file(shared, true);
 
    /* Prepare updated data in a local variable */
    if (shared)