Repair very-low-probability race condition between relation extension

and VACUUM: in the interval between adding a new page to the relation
and formatting it, it was possible for VACUUM to come along and decide
it should format the page too.  Though not harmful in itself, this would
cause data loss if a third transaction were able to insert tuples into
the vacuumed page before the original extender got control back.

diff --git a/src/backend/access/heap/hio.c b/src/backend/access/heap/hio.c
index e1e32cd26295ae750da74953b02fa146e9f62911..99d41c2ad9038ea484bba98b9b9d8f5d175c68a9 100644 (file)
--- a/src/backend/access/heap/hio.c
+++ b/src/backend/access/heap/hio.c
@@ -245,13 +245,6 @@ RelationGetBufferForTuple(Relation relation, Size len,
         */
        buffer = ReadBuffer(relation, P_NEW);
 
-       /*
-        * Release the file-extension lock; it's now OK for someone else to
-        * extend the relation some more.
-        */
-       if (!relation->rd_myxactonly)
-               UnlockPage(relation, 0, ExclusiveLock);
-
        /*
         * We can be certain that locking the otherBuffer first is OK, since
         * it must have a lower page number.
@@ -260,9 +253,22 @@ RelationGetBufferForTuple(Relation relation, Size len,
                LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
 
        /*
-        * We need to initialize the empty new page.
+        * Now acquire lock on the new page.
         */
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+
+       /*
+        * Release the file-extension lock; it's now OK for someone else to
+        * extend the relation some more.  Note that we cannot release this
+        * lock before we have buffer lock on the new page, or we risk a
+        * race condition against vacuumlazy.c --- see comments therein.
+        */
+       if (!relation->rd_myxactonly)
+               UnlockPage(relation, 0, ExclusiveLock);
+
+       /*
+        * We need to initialize the empty new page.
+        */
        pageHeader = (Page) BufferGetPage(buffer);
        Assert(PageIsNew((PageHeader) pageHeader));
        PageInit(pageHeader, BufferGetPageSize(buffer), 0);

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index c878b92a2fabf6f608b6c12dc77a8da86b530bd4..05685f831ccb338047845d28e2e1ecaccc181417 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -259,8 +259,30 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 
                if (PageIsNew(page))
                {
-                       /* Not sure we still need to handle this case, but... */
+                       /*
+                        * An all-zeroes page could be left over if a backend extends
+                        * the relation but crashes before initializing the page.
+                        * Reclaim such pages for use.
+                        *
+                        * We have to be careful here because we could be looking at
+                        * a page that someone has just added to the relation and not
+                        * yet been able to initialize (see RelationGetBufferForTuple).
+                        * To interlock against that, release the buffer read lock
+                        * (which we must do anyway) and grab the relation extension
+                        * lock before re-locking in exclusive mode.  If the page is
+                        * still uninitialized by then, it must be left over from a
+                        * crashed backend, and we can initialize it.
+                        *
+                        * We don't really need the relation lock when this is a new
+                        * or temp relation, but it's probably not worth the code space
+                        * to check that, since this surely isn't a critical path.
+                        *
+                        * Note: the comparable code in vacuum.c need not do all this
+                        * because it's got exclusive lock on the whole relation.
+                        */
                        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+                       LockPage(onerel, 0, ExclusiveLock);
+                       UnlockPage(onerel, 0, ExclusiveLock);
                        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                        if (PageIsNew(page))
                        {