BRIN: Handle concurrent desummarization properly

If a page range is desummarized at just the right time concurrently with
an index walk, BRIN would raise an error indicating index corruption.
This is scary and unhelpful; silently returning that the page range is
not summarized is sufficient reaction.

This bug was introduced by commit 975ad4e602 as additional protection
against a bug whose actual fix was elsewhere.  Backpatch equally.

Reported-By: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Diagnosed-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/2588667e-d07d-7e10-74e2-7e1e46194491@postgrespro.ru
Backpatch: 9.5 - master

src/backend/access/brin/brin_revmap.c

@@ -282,10 +282,17 @@ brinGetTupleForHeapBlock(BrinRevmap *revmap, BlockNumber heapBlk,
 		/* If we land on a revmap page, start over */
 		if (BRIN_IS_REGULAR_PAGE(page))
 		{
+			/*
+			 * If the offset number is greater than what's in the page, it's
+			 * possible that the range was desummarized concurrently. Just
+			 * return NULL to handle that case.
+			 */
 			if (*off > PageGetMaxOffsetNumber(page))
-				ereport(ERROR,
-						(errcode(ERRCODE_INDEX_CORRUPTED),
-						 errmsg_internal("corrupted BRIN index: inconsistent range map")));
+			{
+				LockBuffer(*buf, BUFFER_LOCK_UNLOCK);
+				return NULL;
+			}
+
 			lp = PageGetItemId(page, *off);
 			if (ItemIdIsUsed(lp))
 			{