Date: Wed, 12 Aug 2020 15:58:19 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: [BUG] Error in BRIN summarization
Message-ID: <20200812195819.GA20725@alvherre.pgsql>
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On 2020-Aug-12, Alvaro Herrera wrote:

> 'anyvisible' mode is not required AFAICS; reading the code, I think this
> could also hit REINDEX CONCURRENTLY and CREATE INDEX CONCURRENTLY, which
> do not use that flag.  I didn't try to reproduce it there, though.
> Anyway, I'm going to remove that Assert() I added.

So this is what I propose as the final form of the fix.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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 filename="v4-0001-fix-HOT-tuples-while-scanning-for-index-builds.patch"

From e0abe5d957155285980a40fb33c192100699e8c0 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Wed, 12 Aug 2020 14:02:58 -0400
Subject: [PATCH v4] fix HOT tuples while scanning for index builds

---
 src/backend/access/heap/heapam_handler.c | 20 ++++++++++++++++++++
 src/backend/access/heap/pruneheap.c      |  5 +++--
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 267a6ee25a..ba44e30035 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -1324,6 +1324,12 @@ heapam_index_build_range_scan(Relation heapRelation,
 		 * buffer continuously while visiting the page, so no pruning
 		 * operation can occur either.
 		 *
+		 * In cases with only ShareUpdateExclusiveLock on the table, it's
+		 * possible for some HOT tuples to appear that we didn't know about
+		 * when we first read the page.  To handle that case, we re-obtain the
+		 * list of root offsets when a HOT tuple points to a root item that we
+		 * don't know about.
+		 *
 		 * Also, although our opinions about tuple liveness could change while
 		 * we scan the page (due to concurrent transaction commits/aborts),
 		 * the chain root locations won't, so this info doesn't need to be
@@ -1625,6 +1631,20 @@ heapam_index_build_range_scan(Relation heapRelation,
 
 			offnum = ItemPointerGetOffsetNumber(&heapTuple->t_self);
 
+			/*
+			 * If a HOT tuple points to a root that we don't know
+			 * about, obtain root items afresh.  If that still fails,
+			 * report it as corruption.
+			 */
+			if (root_offsets[offnum - 1] == InvalidOffsetNumber)
+			{
+				Page	page = BufferGetPage(hscan->rs_cbuf);
+
+				LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_SHARE);
+				heap_get_root_tuples(page, root_offsets);
+				LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_UNLOCK);
+			}
+
 			if (!OffsetNumberIsValid(root_offsets[offnum - 1]))
 				ereport(ERROR,
 						(errcode(ERRCODE_DATA_CORRUPTED),
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index 256df4de10..7e3d44dfd6 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -732,7 +732,7 @@ heap_page_prune_execute(Buffer buffer,
  * root_offsets[k - 1] = j.
  *
  * The passed-in root_offsets array must have MaxHeapTuplesPerPage entries.
- * We zero out all unused entries.
+ * Unused entries are filled with InvalidOffsetNumber (zero).
  *
  * The function must be called with at least share lock on the buffer, to
  * prevent concurrent prune operations.
@@ -747,7 +747,8 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
 	OffsetNumber offnum,
 				maxoff;
 
-	MemSet(root_offsets, 0, MaxHeapTuplesPerPage * sizeof(OffsetNumber));
+	MemSet(root_offsets, InvalidOffsetNumber,
+		   MaxHeapTuplesPerPage * sizeof(OffsetNumber));
 
 	maxoff = PageGetMaxOffsetNumber(page);
 	for (offnum = FirstOffsetNumber; offnum <= maxoff; offnum = OffsetNumberNext(offnum))
-- 
2.20.1


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