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 975ad4e602ff 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

diff --git a/src/backend/access/brin/brin_revmap.c b/src/backend/access/brin/brin_revmap.c
index ec84b9ee1d3c2ac7342891ac130c683a02d7bdc8..5232048fda37badb5dbde712768fddff5e63f766 100644 (file)
--- a/src/backend/access/brin/brin_revmap.c
+++ b/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))
            {