Revert "Optimize pg_visibility with read streams."

This reverts commit ed1b1ee59fb3792baa32f669333b75024ef01bcc and its
followup 1c61fd8b527954f0ec522e5e60a11ce82628b681.  They rendered
collect_corrupt_items() unable to detect corruption.

Discussion: https://postgr.es/m/CAN55FZ1_Ru3XpMgTwsU67FTH2fs_FrRROmb7x6zs+F44QBEiww@mail.gmail.com
Discussion: https://postgr.es/m/CAEudQAozv3wTY5TV2t29JcwPydbmKbiWQkZD42S2OgzdixPMDQ@mail.gmail.com

contrib/pg_visibility/pg_visibility.c

@@ -21,7 +21,6 @@
 #include "storage/bufmgr.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
-#include "storage/read_stream.h"
 #include "storage/smgr.h"
 #include "utils/rel.h"
 #include "utils/snapmgr.h"
@@ -42,17 +41,6 @@ typedef struct corrupt_items
 	ItemPointer tids;
 } corrupt_items;
 
-/* for collect_corrupt_items_read_stream_next_block */
-struct collect_corrupt_items_read_stream_private
-{
-	bool		all_frozen;
-	bool		all_visible;
-	BlockNumber current_blocknum;
-	BlockNumber last_exclusive;
-	Relation	rel;
-	Buffer		vmbuffer;
-};
-
 PG_FUNCTION_INFO_V1(pg_visibility_map);
 PG_FUNCTION_INFO_V1(pg_visibility_map_rel);
 PG_FUNCTION_INFO_V1(pg_visibility);
@@ -490,8 +478,6 @@ collect_visibility_data(Oid relid, bool include_pd)
 	BlockNumber blkno;
 	Buffer		vmbuffer = InvalidBuffer;
 	BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
-	BlockRangeReadStreamPrivate p;
-	ReadStream *stream = NULL;
 
 	rel = relation_open(relid, AccessShareLock);
 
@@ -503,20 +489,6 @@ collect_visibility_data(Oid relid, bool include_pd)
 	info->next = 0;
 	info->count = nblocks;
 
-	/* Create a stream if reading main fork. */
-	if (include_pd)
-	{
-		p.current_blocknum = 0;
-		p.last_exclusive = nblocks;
-		stream = read_stream_begin_relation(READ_STREAM_FULL,
-											bstrategy,
-											rel,
-											MAIN_FORKNUM,
-											block_range_read_stream_cb,
-											&p,
-											0);
-	}
-
 	for (blkno = 0; blkno < nblocks; ++blkno)
 	{
 		int32		mapbits;
@@ -541,7 +513,8 @@ collect_visibility_data(Oid relid, bool include_pd)
 			Buffer		buffer;
 			Page		page;
 
-			buffer = read_stream_next_buffer(stream, NULL);
+			buffer = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
+										bstrategy);
 			LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
 			page = BufferGetPage(buffer);
@@ -552,12 +525,6 @@ collect_visibility_data(Oid relid, bool include_pd)
 		}
 	}
 
-	if (include_pd)
-	{
-		Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
-		read_stream_end(stream);
-	}
-
 	/* Clean up. */
 	if (vmbuffer != InvalidBuffer)
 		ReleaseBuffer(vmbuffer);
@@ -643,38 +610,6 @@ GetStrictOldestNonRemovableTransactionId(Relation rel)
 	}
 }
 
-/*
- * Callback function to get next block for read stream object used in
- * collect_corrupt_items() function.
- */
-static BlockNumber
-collect_corrupt_items_read_stream_next_block(ReadStream *stream,
-											 void *callback_private_data,
-											 void *per_buffer_data)
-{
-	struct collect_corrupt_items_read_stream_private *p = callback_private_data;
-
-	for (; p->current_blocknum < p->last_exclusive; p->current_blocknum++)
-	{
-		bool		check_frozen = false;
-		bool		check_visible = false;
-
-		/* Make sure we are interruptible. */
-		CHECK_FOR_INTERRUPTS();
-
-		if (p->all_frozen && VM_ALL_FROZEN(p->rel, p->current_blocknum, &p->vmbuffer))
-			check_frozen = true;
-		if (p->all_visible && VM_ALL_VISIBLE(p->rel, p->current_blocknum, &p->vmbuffer))
-			check_visible = true;
-		if (!check_visible && !check_frozen)
-			continue;
-
-		return p->current_blocknum++;
-	}
-
-	return InvalidBlockNumber;
-}
-
 /*
  * Returns a list of items whose visibility map information does not match
  * the status of the tuples on the page.
@@ -693,13 +628,12 @@ static corrupt_items *
 collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 {
 	Relation	rel;
+	BlockNumber nblocks;
 	corrupt_items *items;
+	BlockNumber blkno;
 	Buffer		vmbuffer = InvalidBuffer;
 	BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
 	TransactionId OldestXmin = InvalidTransactionId;
-	struct collect_corrupt_items_read_stream_private p;
-	ReadStream *stream;
-	Buffer		buffer;
 
 	rel = relation_open(relid, AccessShareLock);
 
@@ -709,6 +643,8 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 	if (all_visible)
 		OldestXmin = GetStrictOldestNonRemovableTransactionId(rel);
 
+	nblocks = RelationGetNumberOfBlocks(rel);
+
 	/*
 	 * Guess an initial array size. We don't expect many corrupted tuples, so
 	 * start with a small array.  This function uses the "next" field to track
@@ -722,46 +658,42 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 	items->count = 64;
 	items->tids = palloc(items->count * sizeof(ItemPointerData));
 
-	p.current_blocknum = 0;
-	p.last_exclusive = RelationGetNumberOfBlocks(rel);
-	p.rel = rel;
-	p.vmbuffer = InvalidBuffer;
-	p.all_frozen = all_frozen;
-	p.all_visible = all_visible;
-	stream = read_stream_begin_relation(READ_STREAM_FULL,
-										bstrategy,
-										rel,
-										MAIN_FORKNUM,
-										collect_corrupt_items_read_stream_next_block,
-										&p,
-										0);
-
 	/* Loop over every block in the relation. */
-	while ((buffer = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
+	for (blkno = 0; blkno < nblocks; ++blkno)
 	{
 		bool		check_frozen = false;
 		bool		check_visible = false;
+		Buffer		buffer;
 		Page		page;
 		OffsetNumber offnum,
 					maxoff;
-		BlockNumber blkno;
 
 		/* Make sure we are interruptible. */
 		CHECK_FOR_INTERRUPTS();
 
+		/* Use the visibility map to decide whether to check this page. */
+		if (all_frozen && VM_ALL_FROZEN(rel, blkno, &vmbuffer))
+			check_frozen = true;
+		if (all_visible && VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
+			check_visible = true;
+		if (!check_visible && !check_frozen)
+			continue;
+
+		/* Read and lock the page. */
+		buffer = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
+									bstrategy);
 		LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
 		page = BufferGetPage(buffer);
 		maxoff = PageGetMaxOffsetNumber(page);
-		blkno = BufferGetBlockNumber(buffer);
 
 		/*
 		 * The visibility map bits might have changed while we were acquiring
 		 * the page lock.  Recheck to avoid returning spurious results.
 		 */
-		if (all_frozen && !VM_ALL_FROZEN(rel, blkno, &vmbuffer))
+		if (check_frozen && !VM_ALL_FROZEN(rel, blkno, &vmbuffer))
 			check_frozen = false;
-		if (all_visible && !VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
+		if (check_visible && !VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
 			check_visible = false;
 		if (!check_visible && !check_frozen)
 		{
@@ -846,13 +778,10 @@ collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
 
 		UnlockReleaseBuffer(buffer);
 	}
-	read_stream_end(stream);
 
 	/* Clean up. */
 	if (vmbuffer != InvalidBuffer)
 		ReleaseBuffer(vmbuffer);
-	if (p.vmbuffer != InvalidBuffer)
-		ReleaseBuffer(p.vmbuffer);
 	relation_close(rel, AccessShareLock);
 
 	/*