Handle new HOT chains in index-build table scans

When a table is scanned by heapam_index_build_range_scan (née
IndexBuildHeapScan) and the table lock being held allows concurrent data
changes, it is possible for new HOT chains to sprout in a page that were
unknown when the scan of a page happened.  This leads to an error such
as
  ERROR:  failed to find parent tuple for heap-only tuple at (X,Y) in table "tbl"
because the root tuple was not present when we first obtained the list
of the page's root tuples.  This can be fixed by re-obtaining the list
of root tuples, if we see that a heap-only tuple appears to point to a
non-existing root.

This was reported by Anastasia as occurring for BRIN summarization
(which exists since 9.5), but I think it could theoretically also happen
with CREATE INDEX CONCURRENTLY (much older) or REINDEX CONCURRENTLY
(very recent).  It seems a happy coincidence that BRIN forces us to
backpatch this all the way to 9.5.

Reported-by: Anastasia Lubennikova 
Diagnosed-by: Anastasia Lubennikova 
Co-authored-by: Anastasia Lubennikova 
Co-authored-by: Álvaro Herrera 
Discussion: https://postgr.es/m/602d8487-f0b2-5486-0088-0f372b2549fa@postgrespro.ru
Backpatch: 9.5 - master

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index fc19f40a2e395a4f81fc3e27f26d5ad164f448b3..ae16fc2bc7c0f39f049a44699e42ec935fd9e473 100644 (file)
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -1341,6 +1341,12 @@ 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
@@ -1643,6 +1649,20 @@ heapam_index_build_range_scan(Relation heapRelation,
            rootTuple = *heapTuple;
            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 0efe3ce999540e9e442d1bfba7471e74014e7a57..349b82d60936b0ec0566dae2e0434e8e4ab31014 100644 (file)
--- 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))