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, 6 insertions, 18 deletions

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 4b296a22c45..86ef607ff38 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -589,9 +589,6 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(reln);
-
 	if (RelationUsesLocalBuffers(reln))
 	{
 		/* see comments in ReadBufferExtended */
@@ -601,12 +598,12 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 					 errmsg("cannot access temporary tables of other sessions")));
 
 		/* pass it off to localbuf.c */
-		return PrefetchLocalBuffer(reln->rd_smgr, forkNum, blockNum);
+		return PrefetchLocalBuffer(RelationGetSmgr(reln), forkNum, blockNum);
 	}
 	else
 	{
 		/* pass it to the shared buffer version */
-		return PrefetchSharedBuffer(reln->rd_smgr, forkNum, blockNum);
+		return PrefetchSharedBuffer(RelationGetSmgr(reln), forkNum, blockNum);
 	}
 }
 
@@ -747,9 +744,6 @@ ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
 	bool		hit;
 	Buffer		buf;
 
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(reln);
-
 	/*
 	 * Reject attempts to read non-local temporary relations; we would be
 	 * likely to get wrong data since we have no visibility into the owning
@@ -765,7 +759,7 @@ ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
 	 * miss.
 	 */
 	pgstat_count_buffer_read(reln);
-	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+	buf = ReadBuffer_common(RelationGetSmgr(reln), reln->rd_rel->relpersistence,
 							forkNum, blockNum, mode, strategy, &hit);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
@@ -2949,10 +2943,7 @@ RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
 		case RELKIND_SEQUENCE:
 		case RELKIND_INDEX:
 		case RELKIND_PARTITIONED_INDEX:
-			/* Open it at the smgr level if not already done */
-			RelationOpenSmgr(relation);
-
-			return smgrnblocks(relation->rd_smgr, forkNum);
+			return smgrnblocks(RelationGetSmgr(relation), forkNum);
 
 		case RELKIND_RELATION:
 		case RELKIND_TOASTVALUE:
@@ -3527,9 +3518,6 @@ FlushRelationBuffers(Relation rel)
 	int			i;
 	BufferDesc *bufHdr;
 
-	/* Open rel at the smgr level if not already done */
-	RelationOpenSmgr(rel);
-
 	if (RelationUsesLocalBuffers(rel))
 	{
 		for (i = 0; i < NLocBuffer; i++)
@@ -3554,7 +3542,7 @@ FlushRelationBuffers(Relation rel)
 
 				PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
-				smgrwrite(rel->rd_smgr,
+				smgrwrite(RelationGetSmgr(rel),
 						  bufHdr->tag.forkNum,
 						  bufHdr->tag.blockNum,
 						  localpage,
@@ -3595,7 +3583,7 @@ FlushRelationBuffers(Relation rel)
 		{
 			PinBuffer_Locked(bufHdr);
 			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
-			FlushBuffer(bufHdr, rel->rd_smgr);
+			FlushBuffer(bufHdr, RelationGetSmgr(rel));
 			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
 			UnpinBuffer(bufHdr, true);
 		}