Rearrange lazy-vacuum code a little bit to reduce the window between

truncating the table and transaction commit.  This isn't really making
it safe, but at least there is no good reason to do free space map
cleanup within the risk window.  Don't lock out cancel interrupts
until we have to, either.

src/backend/commands/vacuumlazy.c

@@ -29,7 +29,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.129 2010/02/08 04:33:54 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.130 2010/02/09 00:28:30 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -123,7 +123,7 @@ static void lazy_cleanup_index(Relation indrel,
 				   LVRelStats *vacrelstats);
 static int lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
 				 int tupindex, LVRelStats *vacrelstats);
-static void lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats);
+static bool lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats);
 static BlockNumber count_nondeletable_pages(Relation onerel,
 						 LVRelStats *vacrelstats);
 static void lazy_space_alloc(LVRelStats *vacrelstats, BlockNumber relblocks);
@@ -143,7 +143,7 @@ static int	vac_cmp_itemptr(const void *left, const void *right);
  *		and locked the relation.
  *
  *		The return value indicates whether this function has held off
- *		interrupts -- caller must RESUME_INTERRUPTS() after commit if true.
+ *		interrupts -- if true, caller must RESUME_INTERRUPTS() after commit.
  */
 bool
 lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
@@ -194,30 +194,24 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 	/* Done with indexes */
 	vac_close_indexes(nindexes, Irel, NoLock);
 
+	/* Vacuum the Free Space Map */
+	FreeSpaceMapVacuum(onerel);
+
 	/*
 	 * Optionally truncate the relation.
 	 *
+	 * NB: there should be as little code as possible after this point,
+	 * to minimize the chance of failure as well as the time spent ignoring
+	 * cancel/die interrupts.
+	 *
 	 * Don't even think about it unless we have a shot at releasing a goodly
 	 * number of pages.  Otherwise, the time taken isn't worth it.
-	 *
-	 * Note that after we've truncated the heap, it's too late to abort the
-	 * transaction; doing so would lose the sinval messages needed to tell
-	 * the other backends about the table being shrunk.  We prevent interrupts
-	 * in that case; caller is responsible for re-enabling them after
-	 * committing the transaction.
 	 */
 	possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
 	if (possibly_freeable > 0 &&
 		(possibly_freeable >= REL_TRUNCATE_MINIMUM ||
 		 possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION))
-	{
+		heldoff = lazy_truncate_heap(onerel, vacrelstats);
-		HOLD_INTERRUPTS();
-		heldoff = true;
-		lazy_truncate_heap(onerel, vacrelstats);
-	}
-
-	/* Vacuum the Free Space Map */
-	FreeSpaceMapVacuum(onerel);
 
 	/*
 	 * Update statistics in pg_class.  But only if we didn't skip any pages;
@@ -1002,8 +996,11 @@ lazy_cleanup_index(Relation indrel,
 
 /*
  * lazy_truncate_heap - try to truncate off any empty pages at the end
+ *
+ *		The return value indicates whether this function has held off
+ *		interrupts -- if true, caller must RESUME_INTERRUPTS() after commit.
  */
-static void
+static bool
 lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 {
 	BlockNumber old_rel_pages = vacrelstats->rel_pages;
@@ -1019,7 +1016,7 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 	 * possible considering we already hold a lower-grade lock).
 	 */
 	if (!ConditionalLockRelation(onerel, AccessExclusiveLock))
-		return;
+		return false;
 
 	/*
 	 * Now that we have exclusive lock, look to see if the rel has grown
@@ -1032,7 +1029,7 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 		/* might as well use the latest news when we update pg_class stats */
 		vacrelstats->rel_pages = new_rel_pages;
 		UnlockRelation(onerel, AccessExclusiveLock);
-		return;
+		return false;
 	}
 
 	/*
@@ -1047,23 +1044,34 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 	{
 		/* can't do anything after all */
 		UnlockRelation(onerel, AccessExclusiveLock);
-		return;
+		return false;
 	}
 
 	/*
-	 * Okay to truncate.
+	 * Prevent cancel/die interrupts from now till commit.  Once we have
+	 * truncated, it is essential that we send the sinval message before
+	 * releasing exclusive lock on the relation; both of which will
+	 * happen during commit.  Other backends must receive the sinval
+	 * message to reset their rd_targblock values before they can safely
+	 * write to the table again.  While we can't positively guarantee
+	 * no error before commit, we can at least prevent cancel interrupts.
+	 *
+	 * XXX it would be better if we had a way to send the inval message
+	 * nontransactionally; an error after the truncate will mean that the
+	 * message is lost.  Note however that turning this all into a critical
+	 * section would not be an improvement.  Making it critical would mean
+	 * that an error forces PANIC, whereas losing the sinval will at worst
+	 * cause unexpected nonfatal errors in other sessions.
 	 */
-	RelationTruncate(onerel, new_rel_pages);
+	HOLD_INTERRUPTS();
 
 	/* force relcache inval so all backends reset their rd_targblock */
 	CacheInvalidateRelcache(onerel);
 
 	/*
-	 * Note: once we have truncated, we *must* keep the exclusive lock until
+	 * Okay to truncate.  Do as little as possible between here and commit.
-	 * commit.	The sinval message won't be sent until commit, and other
-	 * backends must see it and reset their rd_targblock values before they
-	 * can safely access the table again.
 	 */
+	RelationTruncate(onerel, new_rel_pages);
 
 	/* update statistics */
 	vacrelstats->rel_pages = new_rel_pages;
@@ -1075,6 +1083,8 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
 					old_rel_pages, new_rel_pages),
 			 errdetail("%s.",
 					   pg_rusage_show(&ru0))));
+
+	return true;				/* interrupts are held off */
 }
 
 /*