Make smgr access for a BufferManagerRelation safer in relcache inval

Currently there's no bug, because we have no code path where we
invalidate relcache entries where it'd cause a problem.  But it's more
robust to do it this way in case we introduce such a path later, as some
Postgres forks reportedly already have.

Author: Daniil Davydov <[email protected]>
Reviewed-by: Stepan Neretin <[email protected]>
Discussion: https://postgr.es/m/CAJDiXgj3FNzAhV+jjPqxMs3jz=OgPohsoXFj_fh-L+nS+13CKQ@mail.gmail.com

Author: alvherre

src/backend/storage/buffer/bufmgr.c

@@ -883,14 +883,11 @@ ExtendBufferedRelBy(BufferManagerRelation bmr,
 					uint32 *extended_by)
 {
 	Assert((bmr.rel != NULL) != (bmr.smgr != NULL));
-	Assert(bmr.smgr == NULL || bmr.relpersistence != 0);
+	Assert(bmr.smgr == NULL || bmr.relpersistence != '\0');
 	Assert(extend_by > 0);
 
-	if (bmr.smgr == NULL)
-	{
-		bmr.smgr = RelationGetSmgr(bmr.rel);
+	if (bmr.relpersistence == '\0')
 		bmr.relpersistence = bmr.rel->rd_rel->relpersistence;
-	}
 
 	return ExtendBufferedRelCommon(bmr, fork, strategy, flags,
 								   extend_by, InvalidBlockNumber,
@@ -919,30 +916,27 @@ ExtendBufferedRelTo(BufferManagerRelation bmr,
 	Buffer		buffers[64];
 
 	Assert((bmr.rel != NULL) != (bmr.smgr != NULL));
-	Assert(bmr.smgr == NULL || bmr.relpersistence != 0);
+	Assert(bmr.smgr == NULL || bmr.relpersistence != '\0');
 	Assert(extend_to != InvalidBlockNumber && extend_to > 0);
 
-	if (bmr.smgr == NULL)
-	{
-		bmr.smgr = RelationGetSmgr(bmr.rel);
+	if (bmr.relpersistence == '\0')
 		bmr.relpersistence = bmr.rel->rd_rel->relpersistence;
-	}
 
 	/*
 	 * If desired, create the file if it doesn't exist.  If
 	 * smgr_cached_nblocks[fork] is positive then it must exist, no need for
 	 * an smgrexists call.
 	 */
 	if ((flags & EB_CREATE_FORK_IF_NEEDED) &&
-		(bmr.smgr->smgr_cached_nblocks[fork] == 0 ||
-		 bmr.smgr->smgr_cached_nblocks[fork] == InvalidBlockNumber) &&
-		!smgrexists(bmr.smgr, fork))
+		(BMR_GET_SMGR(bmr)->smgr_cached_nblocks[fork] == 0 ||
+		 BMR_GET_SMGR(bmr)->smgr_cached_nblocks[fork] == InvalidBlockNumber) &&
+		!smgrexists(BMR_GET_SMGR(bmr), fork))
 	{
 		LockRelationForExtension(bmr.rel, ExclusiveLock);
 
 		/* recheck, fork might have been created concurrently */
-		if (!smgrexists(bmr.smgr, fork))
-			smgrcreate(bmr.smgr, fork, flags & EB_PERFORMING_RECOVERY);
+		if (!smgrexists(BMR_GET_SMGR(bmr), fork))
+			smgrcreate(BMR_GET_SMGR(bmr), fork, flags & EB_PERFORMING_RECOVERY);
 
 		UnlockRelationForExtension(bmr.rel, ExclusiveLock);
 	}
@@ -952,13 +946,13 @@ ExtendBufferedRelTo(BufferManagerRelation bmr,
 	 * kernel.
 	 */
 	if (flags & EB_CLEAR_SIZE_CACHE)
-		bmr.smgr->smgr_cached_nblocks[fork] = InvalidBlockNumber;
+		BMR_GET_SMGR(bmr)->smgr_cached_nblocks[fork] = InvalidBlockNumber;
 
 	/*
 	 * Estimate how many pages we'll need to extend by. This avoids acquiring
 	 * unnecessarily many victim buffers.
 	 */
-	current_size = smgrnblocks(bmr.smgr, fork);
+	current_size = smgrnblocks(BMR_GET_SMGR(bmr), fork);
 
 	/*
 	 * Since no-one else can be looking at the page contents yet, there is no
@@ -1002,7 +996,7 @@ ExtendBufferedRelTo(BufferManagerRelation bmr,
 	if (buffer == InvalidBuffer)
 	{
 		Assert(extended_by == 0);
-		buffer = ReadBuffer_common(bmr.rel, bmr.smgr, bmr.relpersistence,
+		buffer = ReadBuffer_common(bmr.rel, BMR_GET_SMGR(bmr), bmr.relpersistence,
 								   fork, extend_to - 1, mode, strategy);
 	}
 
@@ -2540,10 +2534,10 @@ ExtendBufferedRelCommon(BufferManagerRelation bmr,
 	BlockNumber first_block;
 
 	TRACE_POSTGRESQL_BUFFER_EXTEND_START(fork,
-										 bmr.smgr->smgr_rlocator.locator.spcOid,
-										 bmr.smgr->smgr_rlocator.locator.dbOid,
-										 bmr.smgr->smgr_rlocator.locator.relNumber,
-										 bmr.smgr->smgr_rlocator.backend,
+										 BMR_GET_SMGR(bmr)->smgr_rlocator.locator.spcOid,
+										 BMR_GET_SMGR(bmr)->smgr_rlocator.locator.dbOid,
+										 BMR_GET_SMGR(bmr)->smgr_rlocator.locator.relNumber,
+										 BMR_GET_SMGR(bmr)->smgr_rlocator.backend,
 										 extend_by);
 
 	if (bmr.relpersistence == RELPERSISTENCE_TEMP)
@@ -2557,10 +2551,10 @@ ExtendBufferedRelCommon(BufferManagerRelation bmr,
 	*extended_by = extend_by;
 
 	TRACE_POSTGRESQL_BUFFER_EXTEND_DONE(fork,
-										bmr.smgr->smgr_rlocator.locator.spcOid,
-										bmr.smgr->smgr_rlocator.locator.dbOid,
-										bmr.smgr->smgr_rlocator.locator.relNumber,
-										bmr.smgr->smgr_rlocator.backend,
+										BMR_GET_SMGR(bmr)->smgr_rlocator.locator.spcOid,
+										BMR_GET_SMGR(bmr)->smgr_rlocator.locator.dbOid,
+										BMR_GET_SMGR(bmr)->smgr_rlocator.locator.relNumber,
+										BMR_GET_SMGR(bmr)->smgr_rlocator.backend,
 										*extended_by,
 										first_block);
 
@@ -2626,9 +2620,9 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 	 * kernel.
 	 */
 	if (flags & EB_CLEAR_SIZE_CACHE)
-		bmr.smgr->smgr_cached_nblocks[fork] = InvalidBlockNumber;
+		BMR_GET_SMGR(bmr)->smgr_cached_nblocks[fork] = InvalidBlockNumber;
 
-	first_block = smgrnblocks(bmr.smgr, fork);
+	first_block = smgrnblocks(BMR_GET_SMGR(bmr), fork);
 
 	/*
 	 * Now that we have the accurate relation size, check if the caller wants
@@ -2666,7 +2660,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend relation %s beyond %u blocks",
-						relpath(bmr.smgr->smgr_rlocator, fork).str,
+						relpath(BMR_GET_SMGR(bmr)->smgr_rlocator, fork).str,
 						MaxBlockNumber)));
 
 	/*
@@ -2688,7 +2682,8 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 		ResourceOwnerEnlarge(CurrentResourceOwner);
 		ReservePrivateRefCountEntry();
 
-		InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
+		InitBufferTag(&tag, &BMR_GET_SMGR(bmr)->smgr_rlocator.locator, fork,
+					  first_block + i);
 		hash = BufTableHashCode(&tag);
 		partition_lock = BufMappingPartitionLock(hash);
 
@@ -2730,7 +2725,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 				ereport(ERROR,
 						(errmsg("unexpected data beyond EOF in block %u of relation \"%s\"",
 								existing_hdr->tag.blockNum,
-								relpath(bmr.smgr->smgr_rlocator, fork).str)));
+								relpath(BMR_GET_SMGR(bmr)->smgr_rlocator, fork).str)));
 
 			/*
 			 * We *must* do smgr[zero]extend before succeeding, else the page
@@ -2787,7 +2782,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 	 *
 	 * We don't need to set checksum for all-zero pages.
 	 */
-	smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);
+	smgrzeroextend(BMR_GET_SMGR(bmr), fork, first_block, extend_by, false);
 
 	/*
 	 * Release the file-extension lock; it's now OK for someone else to extend

src/backend/storage/buffer/localbuf.c

@@ -25,6 +25,7 @@
 #include "utils/guc_hooks.h"
 #include "utils/memdebug.h"
 #include "utils/memutils.h"
+#include "utils/rel.h"
 #include "utils/resowner.h"
 
 
@@ -372,7 +373,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		MemSet(buf_block, 0, BLCKSZ);
 	}
 
-	first_block = smgrnblocks(bmr.smgr, fork);
+	first_block = smgrnblocks(BMR_GET_SMGR(bmr), fork);
 
 	if (extend_upto != InvalidBlockNumber)
 	{
@@ -391,7 +392,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend relation %s beyond %u blocks",
-						relpath(bmr.smgr->smgr_rlocator, fork).str,
+						relpath(BMR_GET_SMGR(bmr)->smgr_rlocator, fork).str,
 						MaxBlockNumber)));
 
 	for (uint32 i = 0; i < extend_by; i++)
@@ -408,7 +409,8 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		/* in case we need to pin an existing buffer below */
 		ResourceOwnerEnlarge(CurrentResourceOwner);
 
-		InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
+		InitBufferTag(&tag, &BMR_GET_SMGR(bmr)->smgr_rlocator.locator, fork,
+					  first_block + i);
 
 		hresult = (LocalBufferLookupEnt *)
 			hash_search(LocalBufHash, &tag, HASH_ENTER, &found);
@@ -456,7 +458,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/* actually extend relation */
-	smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);
+	smgrzeroextend(BMR_GET_SMGR(bmr), fork, first_block, extend_by, false);
 
 	pgstat_count_io_op_time(IOOBJECT_TEMP_RELATION, IOCONTEXT_NORMAL, IOOP_EXTEND,
 							io_start, 1, extend_by * BLCKSZ);

src/include/storage/bufmgr.h

@@ -98,8 +98,11 @@ typedef struct SMgrRelationData *SMgrRelation;
 
 /*
  * Some functions identify relations either by relation or smgr +
- * relpersistence.  Used via the BMR_REL()/BMR_SMGR() macros below.  This
- * allows us to use the same function for both recovery and normal operation.
+ * relpersistence, initialized via the BMR_REL()/BMR_SMGR() macros below.
+ * This allows us to use the same function for both recovery and normal
+ * operation.  When BMR_REL is used, it's not valid to cache its rd_smgr here,
+ * because our pointer would be obsolete in case of relcache invalidation.
+ * For simplicity, use BMR_GET_SMGR to read the smgr.
  */
 typedef struct BufferManagerRelation
 {
@@ -108,8 +111,12 @@ typedef struct BufferManagerRelation
 	char		relpersistence;
 } BufferManagerRelation;
 
-#define BMR_REL(p_rel) ((BufferManagerRelation){.rel = p_rel})
-#define BMR_SMGR(p_smgr, p_relpersistence) ((BufferManagerRelation){.smgr = p_smgr, .relpersistence = p_relpersistence})
+#define BMR_REL(p_rel) \
+	((BufferManagerRelation){.rel = p_rel})
+#define BMR_SMGR(p_smgr, p_relpersistence) \
+	((BufferManagerRelation){.smgr = p_smgr, .relpersistence = p_relpersistence})
+#define BMR_GET_SMGR(bmr) \
+	(RelationIsValid((bmr).rel) ? RelationGetSmgr((bmr).rel) : (bmr).smgr)
 
 /* Zero out page if reading fails. */
 #define READ_BUFFERS_ZERO_ON_ERROR (1 << 0)