Replace RelationOpenSmgr() with RelationGetSmgr().

The idea behind this patch is to design out bugs like the one fixed
by commit 9d523119f.  Previously, once one did RelationOpenSmgr(rel),
it was considered okay to access rel->rd_smgr directly for some
not-very-clear interval.  But since that pointer will be cleared by
relcache flushes, we had bugs arising from overreliance on a previous
RelationOpenSmgr call still being effective.

Now, very little code except that in rel.h and relcache.c should ever
touch the rd_smgr field directly.  The normal coding rule is to use
RelationGetSmgr(rel) and not expect the result to be valid for longer
than one smgr function call.  There are a couple of places where using
the function every single time seemed like overkill, but they are now
annotated with large warning comments.

Amul Sul, after an idea of mine.

Discussion: https://postgr.es/m/CANiYTQsU7yMFpQYnv=BrcRVqK_3U3mtAzAsJCaqtzsDHfsUbdQ@mail.gmail.com

1 files changed, 27 insertions, 26 deletions

diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c
index e198df65d82..4720b35ee5c 100644
--- a/src/backend/access/heap/visibilitymap.c
+++ b/src/backend/access/heap/visibilitymap.c
@@ -455,13 +455,11 @@ visibilitymap_prepare_truncate(Relation rel, BlockNumber nheapblocks)
 	elog(DEBUG1, "vm_truncate %s %d", RelationGetRelationName(rel), nheapblocks);
 #endif
 
-	RelationOpenSmgr(rel);
-
 	/*
 	 * If no visibility map has been created yet for this relation, there's
 	 * nothing to truncate.
 	 */
-	if (!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+	if (!smgrexists(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM))
 		return InvalidBlockNumber;
 
 	/*
@@ -528,7 +526,7 @@ visibilitymap_prepare_truncate(Relation rel, BlockNumber nheapblocks)
 	else
 		newnblocks = truncBlock;
 
-	if (smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM) <= newnblocks)
+	if (smgrnblocks(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM) <= newnblocks)
 	{
 		/* nothing to do, the file was already smaller than requested size */
 		return InvalidBlockNumber;
@@ -547,30 +545,29 @@ static Buffer
 vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
 {
 	Buffer		buf;
+	SMgrRelation reln;
 
 	/*
-	 * We might not have opened the relation at the smgr level yet, or we
-	 * might have been forced to close it by a sinval message.  The code below
-	 * won't necessarily notice relation extension immediately when extend =
-	 * false, so we rely on sinval messages to ensure that our ideas about the
-	 * size of the map aren't too far out of date.
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
 	 */
-	RelationOpenSmgr(rel);
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * If we haven't cached the size of the visibility map fork yet, check it
 	 * first.
 	 */
-	if (rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
+	if (reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
 	{
-		if (smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
-			smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
+		if (smgrexists(reln, VISIBILITYMAP_FORKNUM))
+			smgrnblocks(reln, VISIBILITYMAP_FORKNUM);
 		else
-			rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
+			reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
 	}
 
 	/* Handle requests beyond EOF */
-	if (blkno >= rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
+	if (blkno >= reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
 	{
 		if (extend)
 			vm_extend(rel, blkno + 1);
@@ -618,6 +615,7 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 {
 	BlockNumber vm_nblocks_now;
 	PGAlignedBlock pg;
+	SMgrRelation reln;
 
 	PageInit((Page) pg.data, BLCKSZ, 0);
 
@@ -633,29 +631,32 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 */
 	LockRelationForExtension(rel, ExclusiveLock);
 
-	/* Might have to re-open if a cache flush happened */
-	RelationOpenSmgr(rel);
+	/*
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
+	 */
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * Create the file first if it doesn't exist.  If smgr_vm_nblocks is
 	 * positive then it must exist, no need for an smgrexists call.
 	 */
-	if ((rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == 0 ||
-		 rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber) &&
-		!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
-		smgrcreate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, false);
+	if ((reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == 0 ||
+		 reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber) &&
+		!smgrexists(reln, VISIBILITYMAP_FORKNUM))
+		smgrcreate(reln, VISIBILITYMAP_FORKNUM, false);
 
 	/* Invalidate cache so that smgrnblocks() asks the kernel. */
-	rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
-	vm_nblocks_now = smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
+	reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
+	vm_nblocks_now = smgrnblocks(reln, VISIBILITYMAP_FORKNUM);
 
 	/* Now extend the file */
 	while (vm_nblocks_now < vm_nblocks)
 	{
 		PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
 
-		smgrextend(rel->rd_smgr, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
-				   pg.data, false);
+		smgrextend(reln, VISIBILITYMAP_FORKNUM, vm_nblocks_now, pg.data, false);
 		vm_nblocks_now++;
 	}
 
@@ -666,7 +667,7 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 * to keep checking for creation or extension of the file, which happens
 	 * infrequently.
 	 */
-	CacheInvalidateSmgr(rel->rd_smgr->smgr_rnode);
+	CacheInvalidateSmgr(reln->smgr_rnode);
 
 	UnlockRelationForExtension(rel, ExclusiveLock);
 }