Fix BRIN summarization concurrent with extension

If a process is extending a table concurrently with some BRIN
summarization process, it is possible for the latter to miss pages added
by the former because the number of pages is computed ahead of time.

Fix by determining a fresh relation size after inserting the placeholder
tuple: any process that further extends the table concurrently will
update the placeholder tuple, while previous pages will be processed by
the heap scan.

Reported-by: Tomas Vondra
Reviewed-by: Tom Lane
Author: Álvaro Herrera
Discussion: https://postgr.es/m/083d996a-4a8a-0e13-800a-851dd09ad8cc@2ndquadrant.com
Backpatch-to: 9.5

src/backend/access/brin/brin.c

@@ -67,7 +67,7 @@ static BrinBuildState *initialize_brin_buildstate(Relation idxRel,
 						   BrinRevmap *revmap, BlockNumber pagesPerRange);
 static void terminate_brin_buildstate(BrinBuildState *state);
 static void brinsummarize(Relation index, Relation heapRel, BlockNumber pageRange,
-			  double *numSummarized, double *numExisting);
+			  bool include_partial, double *numSummarized, double *numExisting);
 static void form_and_insert_tuple(BrinBuildState *state);
 static void union_tuples(BrinDesc *bdesc, BrinMemTuple *a,
 			 BrinTuple *b);
@@ -791,7 +791,7 @@ brinvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 
 	brin_vacuum_scan(info->index, info->strategy);
 
-	brinsummarize(info->index, heapRel, BRIN_ALL_BLOCKRANGES,
+	brinsummarize(info->index, heapRel, BRIN_ALL_BLOCKRANGES, false,
 				  &stats->num_index_tuples, &stats->num_index_tuples);
 
 	heap_close(heapRel, AccessShareLock);
@@ -909,7 +909,7 @@ brin_summarize_range(PG_FUNCTION_ARGS)
 						RelationGetRelationName(indexRel))));
 
 	/* OK, do it */
-	brinsummarize(indexRel, heapRel, heapBlk, &numSummarized, NULL);
+	brinsummarize(indexRel, heapRel, heapBlk, true, &numSummarized, NULL);
 
 	relation_close(indexRel, ShareUpdateExclusiveLock);
 	relation_close(heapRel, ShareUpdateExclusiveLock);
@@ -1129,7 +1129,8 @@ terminate_brin_buildstate(BrinBuildState *state)
 }
 
 /*
- * Summarize the given page range of the given index.
+ * On the given BRIN index, summarize the heap page range that corresponds
+ * to the heap block number given.
  *
  * This routine can run in parallel with insertions into the heap.  To avoid
  * missing those values from the summary tuple, we first insert a placeholder
@@ -1139,6 +1140,12 @@ terminate_brin_buildstate(BrinBuildState *state)
  * update of the index value happens in a loop, so that if somebody updates
  * the placeholder tuple after we read it, we detect the case and try again.
  * This ensures that the concurrently inserted tuples are not lost.
+ *
+ * A further corner case is this routine being asked to summarize the partial
+ * range at the end of the table.  heapNumBlocks is the (possibly outdated)
+ * table size; if we notice that the requested range lies beyond that size,
+ * we re-compute the table size after inserting the placeholder tuple, to
+ * avoid missing pages that were appended recently.
  */
 static void
 summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
@@ -1159,6 +1166,33 @@ summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
 						   state->bs_rmAccess, &phbuf,
 						   heapBlk, phtup, phsz);
 
+	/*
+	 * Compute range end.  We hold ShareUpdateExclusive lock on table, so it
+	 * cannot shrink concurrently (but it can grow).
+	 */
+	Assert(heapBlk % state->bs_pagesPerRange == 0);
+	if (heapBlk + state->bs_pagesPerRange > heapNumBlks)
+	{
+		/*
+		 * If we're asked to scan what we believe to be the final range on the
+		 * table (i.e. a range that might be partial) we need to recompute our
+		 * idea of what the latest page is after inserting the placeholder
+		 * tuple.  Anyone that grows the table later will update the
+		 * placeholder tuple, so it doesn't matter that we won't scan these
+		 * pages ourselves.  Careful: the table might have been extended
+		 * beyond the current range, so clamp our result.
+		 *
+		 * Fortunately, this should occur infrequently.
+		 */
+		scanNumBlks = Min(RelationGetNumberOfBlocks(heapRel) - heapBlk,
+						  state->bs_pagesPerRange);
+	}
+	else
+	{
+		/* Easy case: range is known to be complete */
+		scanNumBlks = state->bs_pagesPerRange;
+	}
+
 	/*
 	 * Execute the partial heap scan covering the heap blocks in the specified
 	 * page range, summarizing the heap tuples in it.  This scan stops just
@@ -1169,8 +1203,6 @@ summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
 	 * by transactions that are still in progress, among other corner cases.
 	 */
 	state->bs_currRangeStart = heapBlk;
-	scanNumBlks = heapBlk + state->bs_pagesPerRange <= heapNumBlks ?
-		state->bs_pagesPerRange : heapNumBlks - heapBlk;
 	IndexBuildHeapRangeScan(heapRel, state->bs_irel, indexInfo, false, true,
 							heapBlk, scanNumBlks,
 							brinbuildCallback, (void *) state);
@@ -1234,6 +1266,8 @@ summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
  * Summarize page ranges that are not already summarized.  If pageRange is
  * BRIN_ALL_BLOCKRANGES then the whole table is scanned; otherwise, only the
  * page range containing the given heap page number is scanned.
+ * If include_partial is true, then the partial range at the end of the table
+ * is summarized, otherwise not.
  *
  * For each new index tuple inserted, *numSummarized (if not NULL) is
  * incremented; for each existing tuple, *numExisting (if not NULL) is
@@ -1241,56 +1275,54 @@ summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
  */
 static void
 brinsummarize(Relation index, Relation heapRel, BlockNumber pageRange,
-			  double *numSummarized, double *numExisting)
+			  bool include_partial, double *numSummarized, double *numExisting)
 {
 	BrinRevmap *revmap;
 	BrinBuildState *state = NULL;
 	IndexInfo  *indexInfo = NULL;
 	BlockNumber heapNumBlocks;
-	BlockNumber heapBlk;
 	BlockNumber pagesPerRange;
 	Buffer		buf;
 	BlockNumber startBlk;
-	BlockNumber endBlk;
-
-	/* determine range of pages to process; nothing to do for an empty table */
-	heapNumBlocks = RelationGetNumberOfBlocks(heapRel);
-	if (heapNumBlocks == 0)
-		return;
 
 	revmap = brinRevmapInitialize(index, &pagesPerRange, NULL);
 
+	/* determine range of pages to process */
+	heapNumBlocks = RelationGetNumberOfBlocks(heapRel);
 	if (pageRange == BRIN_ALL_BLOCKRANGES)
-	{
 		startBlk = 0;
-		endBlk = heapNumBlocks;
-	}
 	else
-	{
 		startBlk = (pageRange / pagesPerRange) * pagesPerRange;
+	if (startBlk >= heapNumBlocks)
+	{
 		/* Nothing to do if start point is beyond end of table */
-		if (startBlk > heapNumBlocks)
-		{
-			brinRevmapTerminate(revmap);
-			return;
-		}
-		endBlk = startBlk + pagesPerRange;
-		if (endBlk > heapNumBlocks)
-			endBlk = heapNumBlocks;
+		brinRevmapTerminate(revmap);
+		return;
 	}
 
 	/*
 	 * Scan the revmap to find unsummarized items.
 	 */
 	buf = InvalidBuffer;
-	for (heapBlk = startBlk; heapBlk < endBlk; heapBlk += pagesPerRange)
+	for (; startBlk < heapNumBlocks; startBlk += pagesPerRange)
 	{
 		BrinTuple  *tup;
 		OffsetNumber off;
 
+		/*
+		 * Unless requested to summarize even a partial range, go away now if
+		 * we think the next range is partial.  Caller would pass true when
+		 * it is typically run once bulk data loading is done
+		 * (brin_summarize_new_values), and false when it is typically the
+		 * result of arbitrarily-scheduled maintenance command (vacuuming).
+		 */
+		if (!include_partial &&
+			(startBlk + pagesPerRange > heapNumBlocks))
+			break;
+
 		CHECK_FOR_INTERRUPTS();
 
-		tup = brinGetTupleForHeapBlock(revmap, heapBlk, &buf, &off, NULL,
+		tup = brinGetTupleForHeapBlock(revmap, startBlk, &buf, &off, NULL,
 									   BUFFER_LOCK_SHARE, NULL);
 		if (tup == NULL)
 		{
@@ -1303,7 +1335,7 @@ brinsummarize(Relation index, Relation heapRel, BlockNumber pageRange,
 												   pagesPerRange);
 				indexInfo = BuildIndexInfo(index);
 			}
-			summarize_range(indexInfo, state, heapRel, heapBlk, heapNumBlocks);
+			summarize_range(indexInfo, state, heapRel, startBlk, heapNumBlocks);
 
 			/* and re-initialize state for the next range */
 			brin_memtuple_initialize(state->bs_dtuple, state->bs_bdesc);