It turns out that TablespaceCreateDbspace fails badly if a relcache flush

occurs when it tries to heap_open pg_tablespace.  When control returns to
smgrcreate, that routine will be holding a dangling pointer to a closed
SMgrRelation, resulting in mayhem.  This is of course a consequence of
the violation of proper module layering inherent in having smgr.c call
a tablespace command routine, but the simplest fix seems to be to change
the locking mechanism.  There's no real need for TablespaceCreateDbspace
to touch pg_tablespace at all --- it's only opening it as a way of locking
against a parallel DROP TABLESPACE command.  A much better answer is to
create a special-purpose LWLock to interlock these two operations.
This drops TablespaceCreateDbspace quite a few layers down the food chain
and makes it something reasonably safe for smgr to call.

diff --git a/src/backend/commands/tablespace.c b/src/backend/commands/tablespace.c
index a1cb5fd8078cbc28096b9158c11be8ab7babd6d3..de051399ebc4bffeb303f9b5810392de0c1287a5 100644 (file)
--- a/src/backend/commands/tablespace.c
+++ b/src/backend/commands/tablespace.c
@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/commands/tablespace.c,v 1.15 2004/12/31 21:59:41 pgsql Exp $
+ *   $PostgreSQL: pgsql/src/backend/commands/tablespace.c,v 1.15.4.1 2006/01/19 04:45:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -83,12 +83,9 @@ static void set_short_version(const char *path);
  * If tablespaces are not supported, this is just a no-op; CREATE DATABASE
  * is expected to create the default subdirectory for the database.
  *
- * isRedo indicates that we are creating an object during WAL replay;
- * we can skip doing locking in that case (and should do so to avoid
- * any possible problems with pg_tablespace not being valid).
- *
- * Also, when isRedo is true, we will cope with the possibility of the
- * tablespace not being there either --- this could happen if we are
+ * isRedo indicates that we are creating an object during WAL replay.
+ * In this case we will cope with the possibility of the tablespace
+ * directory not being there either --- this could happen if we are
  * replaying an operation on a table in a subsequently-dropped tablespace.
  * We handle this by making a directory in the place where the tablespace
  * symlink would normally be.  This isn't an exact replay of course, but
@@ -118,16 +115,10 @@ TablespaceCreateDbspace(Oid spcNode, Oid dbNode, bool isRedo)
        if (errno == ENOENT)
        {
            /*
-            * Acquire ExclusiveLock on pg_tablespace to ensure that no
-            * DROP TABLESPACE or TablespaceCreateDbspace is running
-            * concurrently.  Simple reads from pg_tablespace are OK.
+            * Acquire TablespaceCreateLock to ensure that no DROP TABLESPACE
+            * or TablespaceCreateDbspace is running concurrently.
             */
-           Relation    rel;
-
-           if (!isRedo)
-               rel = heap_openr(TableSpaceRelationName, ExclusiveLock);
-           else
-               rel = NULL;
+           LWLockAcquire(TablespaceCreateLock, LW_EXCLUSIVE);
 
            /*
             * Recheck to see if someone created the directory while we
@@ -166,9 +157,7 @@ TablespaceCreateDbspace(Oid spcNode, Oid dbNode, bool isRedo)
                }
            }
 
-           /* OK to drop the exclusive lock */
-           if (!isRedo)
-               heap_close(rel, ExclusiveLock);
+           LWLockRelease(TablespaceCreateLock);
        }
        else
        {
@@ -402,16 +391,11 @@ DropTableSpace(DropTableSpaceStmt *stmt)
    /* don't call this in a transaction block */
    PreventTransactionChain((void *) stmt, "DROP TABLESPACE");
 
-   /*
-    * Acquire ExclusiveLock on pg_tablespace to ensure that no one else
-    * is trying to do DROP TABLESPACE or TablespaceCreateDbspace
-    * concurrently.
-    */
-   rel = heap_openr(TableSpaceRelationName, ExclusiveLock);
-
    /*
     * Find the target tuple
     */
+   rel = heap_openr(TableSpaceRelationName, RowExclusiveLock);
+
    ScanKeyInit(&entry[0],
                Anum_pg_tablespace_spcname,
                BTEqualStrategyNumber, F_NAMEEQ,
@@ -447,6 +431,12 @@ DropTableSpace(DropTableSpaceStmt *stmt)
 
    heap_endscan(scandesc);
 
+   /*
+    * Acquire TablespaceCreateLock to ensure that no TablespaceCreateDbspace
+    * is running concurrently.
+    */
+   LWLockAcquire(TablespaceCreateLock, LW_EXCLUSIVE);
+
    /*
     * Try to remove the physical infrastructure
     */
@@ -470,6 +460,11 @@ DropTableSpace(DropTableSpaceStmt *stmt)
        (void) XLogInsert(RM_TBLSPC_ID, XLOG_TBLSPC_DROP, rdata);
    }
 
+   /*
+    * Allow TablespaceCreateDbspace again.
+    */
+   LWLockRelease(TablespaceCreateLock);
+
    /* We keep the lock on pg_tablespace until commit */
    heap_close(rel, NoLock);
 
@@ -507,10 +502,10 @@ remove_tablespace_directories(Oid tablespaceoid, bool redo)
     * next attempt to use the tablespace from that database will simply
     * recreate the subdirectory via TablespaceCreateDbspace.)
     *
-    * Since we hold exclusive lock, no one else should be creating any fresh
-    * subdirectories in parallel.  It is possible that new files are
-    * being created within subdirectories, though, so the rmdir call
-    * could fail.  Worst consequence is a less friendly error message.
+    * Since we hold TablespaceCreateLock, no one else should be creating any
+    * fresh subdirectories in parallel. It is possible that new files are
+    * being created within subdirectories, though, so the rmdir call could
+    * fail.  Worst consequence is a less friendly error message.
     */
    dirdesc = AllocateDir(location);
    if (dirdesc == NULL)

diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index eb02eb8a2a0a2139fbd3f772b5d69c1f3a03b88a..1a653ff5b84b968c63d9e61975a9f986fa32ff7e 100644 (file)
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/lwlock.h,v 1.16 2004/12/31 22:03:42 pgsql Exp $
+ * $PostgreSQL: pgsql/src/include/storage/lwlock.h,v 1.16.4.1 2006/01/19 04:45:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -41,6 +41,7 @@ typedef enum LWLockId
    SubtransControlLock,
    RelCacheInitLock,
    BgWriterCommLock,
+   TablespaceCreateLock,
 
    NumFixedLWLocks,            /* must be last except for
                                 * MaxDynamicLWLock */