Avoid nbtree parallel scan currPos confusion.

Commit 1bd4bc8, which refactored nbtree sibling link traversal, made
_bt_parallel_seize reset the scan's currPos so that things were
consistent with the state of a serial backend moving between pages.
This overlooked the fact that _bt_readnextpage relied on the existing
currPos state to decide when to end the scan -- even though it came from
before the scan was seized.  As a result of all this, parallel nbtree
scans could needlessly behave like full index scans.

To fix, teach _bt_readnextpage to explicitly allow the use of an already
read page's so->currPos when deciding whether to end the scan -- even
during parallel index scans (allow it consistently now).  This requires
moving _bt_readnextpage's seizure of the scan to earlier in its loop.
That way _bt_readnextpage either deals with the true so->currPos state,
or an initialized-by-_bt_parallel_seize currPos state set from when the
scan was seized.  Now _bt_steppage (the most important _bt_readnextpage
caller) takes the same uniform approach to setting up its call using
details taken from so->currPos -- regardless of whether the scan happens
to be parallel or serial.

The new loop structure in _bt_readnextpage is prone to getting confused
by P_NONE blknos set when the rightmost or leftmost page was reached.
We could avoid that by adding an explicit check, but that would be ugly.
Avoid this problem by teaching _bt_parallel_seize to end the parallel
scan instead of returning a P_NONE next block/blkno.  Doing things this
way was arguably a missed opportunity for commit 1bd4bc8.  It allows us
to remove a similar "blkno == P_NONE" check from _bt_first.

Oversight in commit 1bd4bc8, which refactored sibling link traversal
(as part of optimizing nbtree backward scan locking).

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Diagnosed-By: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Reviewed-By: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Discussion: https://postgr.es/m/f8efb9c0f8d1a71b44fd7f8e42e49c25@oss.nttdata.com

Author: petergeoghegan

src/backend/access/nbtree/nbtree.c

@@ -596,9 +596,7 @@ btparallelrescan(IndexScanDesc scan)
  * scan, and *last_curr_page returns the page that *next_scan_page came from.
  * An invalid *next_scan_page means the scan hasn't yet started, or that
  * caller needs to start the next primitive index scan (if it's the latter
- * case we'll set so.needPrimScan).  The first time a participating process
- * reaches the last page, it will return true and set *next_scan_page to
- * P_NONE; after that, further attempts to seize the scan will return false.
+ * case we'll set so.needPrimScan).
  *
  * Callers should ignore the value of *next_scan_page and *last_curr_page if
  * the return value is false.
@@ -608,12 +606,13 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
 				   BlockNumber *last_curr_page, bool first)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
-	bool		exit_loop = false;
-	bool		status = true;
+	bool		exit_loop = false,
+				status = true,
+				endscan = false;
 	ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
 	BTParallelScanDesc btscan;
 
-	*next_scan_page = P_NONE;
+	*next_scan_page = InvalidBlockNumber;
 	*last_curr_page = InvalidBlockNumber;
 
 	/*
@@ -657,6 +656,13 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
 			/* We're done with this parallel index scan */
 			status = false;
 		}
+		else if (btscan->btps_pageStatus == BTPARALLEL_IDLE &&
+				 btscan->btps_nextScanPage == P_NONE)
+		{
+			/* End this parallel index scan */
+			status = false;
+			endscan = true;
+		}
 		else if (btscan->btps_pageStatus == BTPARALLEL_NEED_PRIMSCAN)
 		{
 			Assert(so->numArrayKeys);
@@ -673,7 +679,6 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
 					array->cur_elem = btscan->btps_arrElems[i];
 					skey->sk_argument = array->elem_values[array->cur_elem];
 				}
-				*next_scan_page = InvalidBlockNumber;
 				exit_loop = true;
 			}
 			else
@@ -701,6 +706,7 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
 			 * of advancing it to a new page!
 			 */
 			btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
+			Assert(btscan->btps_nextScanPage != P_NONE);
 			*next_scan_page = btscan->btps_nextScanPage;
 			*last_curr_page = btscan->btps_lastCurrPage;
 			exit_loop = true;
@@ -712,6 +718,10 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
 	}
 	ConditionVariableCancelSleep();
 
+	/* When the scan has reached the rightmost (or leftmost) page, end it */
+	if (endscan)
+		_bt_parallel_done(scan);
+
 	return status;
 }
 
@@ -724,6 +734,10 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
  * that it can be passed to _bt_parallel_primscan_schedule, should caller
  * determine that another primitive index scan is required.
  *
+ * If caller's next_scan_page is P_NONE, the scan has reached the index's
+ * rightmost/leftmost page.  This is treated as reaching the end of the scan
+ * within _bt_parallel_seize.
+ *
  * Note: unlike the serial case, parallel scans don't need to remember both
  * sibling links.  next_scan_page is whichever link is next given the scan's
  * direction.  That's all we'll ever need, since the direction of a parallel
@@ -736,6 +750,8 @@ _bt_parallel_release(IndexScanDesc scan, BlockNumber next_scan_page,
 	ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
 	BTParallelScanDesc btscan;
 
+	Assert(BlockNumberIsValid(next_scan_page));
+
 	btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
 												  parallel_scan->ps_offset);
 
@@ -770,7 +786,7 @@ _bt_parallel_done(IndexScanDesc scan)
 
 	/*
 	 * Should not mark parallel scan done when there's still a pending
-	 * primitive index scan (defensive)
+	 * primitive index scan
 	 */
 	if (so->needPrimScan)
 		return;

src/backend/access/nbtree/nbtsearch.c

@@ -46,7 +46,8 @@ static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
 static bool _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum,
 							  ScanDirection dir);
 static bool _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
-							 BlockNumber lastcurrblkno, ScanDirection dir);
+							 BlockNumber lastcurrblkno, ScanDirection dir,
+							 bool seized);
 static Buffer _bt_lock_and_validate_left(Relation rel, BlockNumber *blkno,
 										 BlockNumber lastcurrblkno);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
@@ -888,7 +889,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	ScanKey		startKeys[INDEX_MAX_KEYS];
 	ScanKeyData notnullkeys[INDEX_MAX_KEYS];
 	int			keysz = 0;
-	int			i;
 	StrategyNumber strat_total;
 	BTScanPosItem *currItem;
 
@@ -924,33 +924,31 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	{
 		BlockNumber blkno,
 					lastcurrblkno;
-		bool		status;
 
-		status = _bt_parallel_seize(scan, &blkno, &lastcurrblkno, true);
+		if (!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, true))
+			return false;
 
 		/*
+		 * Successfully seized the scan, which _bt_readfirstpage or possibly
+		 * _bt_readnextpage will release (unless the scan ends right away, in
+		 * which case we'll call _bt_parallel_done directly).
+		 *
 		 * Initialize arrays (when _bt_parallel_seize didn't already set up
-		 * the next primitive index scan)
+		 * the next primitive index scan).
 		 */
 		if (so->numArrayKeys && !so->needPrimScan)
 			_bt_start_array_keys(scan, dir);
 
-		if (!status)
-			return false;
-		else if (blkno == P_NONE)
-		{
-			_bt_parallel_done(scan);
-			return false;
-		}
-		else if (blkno != InvalidBlockNumber)
+		Assert(blkno != P_NONE);
+		if (blkno != InvalidBlockNumber)
 		{
 			Assert(!so->needPrimScan);
 
 			/*
 			 * We anticipated starting another primitive scan, but some other
 			 * worker bet us to it
 			 */
-			if (!_bt_readnextpage(scan, blkno, lastcurrblkno, dir))
+			if (!_bt_readnextpage(scan, blkno, lastcurrblkno, dir, true))
 				return false;
 			goto readcomplete;
 		}
@@ -1043,6 +1041,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		 * We don't cast the decision in stone until we reach keys for the
 		 * next attribute.
 		 */
+		cur = so->keyData;
 		curattr = 1;
 		chosen = NULL;
 		/* Also remember any scankey that implies a NOT NULL constraint */
@@ -1053,7 +1052,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		 * pass to handle after-last-key processing.  Actual exit from the
 		 * loop is at one of the "break" statements below.
 		 */
-		for (cur = so->keyData, i = 0;; cur++, i++)
+		for (int i = 0;; cur++, i++)
 		{
 			if (i >= so->numberOfKeys || cur->sk_attno != curattr)
 			{
@@ -1168,7 +1167,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	 * initialized after initial-positioning scan keys are finalized.)
 	 */
 	Assert(keysz <= INDEX_MAX_KEYS);
-	for (i = 0; i < keysz; i++)
+	for (int i = 0; i < keysz; i++)
 	{
 		ScanKey		cur = startKeys[i];
 
@@ -2006,18 +2005,12 @@ _bt_savepostingitem(BTScanOpaque so, int itemIndex, OffsetNumber offnum,
 /*
  *	_bt_steppage() -- Step to next page containing valid data for scan
  *
+ * Wrapper on _bt_readnextpage that performs final steps for the current page.
+ *
  * On entry, if so->currPos.buf is valid the buffer is pinned but not locked.
  * If there's no pin held, it's because _bt_drop_lock_and_maybe_pin dropped
  * the pin eagerly earlier on.  The scan must have so->currPos.currPage set to
  * a valid block, in any case.
- *
- * This is a wrapper on _bt_readnextpage that performs final steps for the
- * current page.  It sets up the _bt_readnextpage call using either local
- * state saved in so->currPos by the most recent _bt_readpage call, or using
- * shared parallel scan state (obtained by seizing the parallel scan here).
- *
- * Parallel scan callers that have already seized the scan should directly
- * call _bt_readnextpage, rather than calling here.
  */
 static bool
 _bt_steppage(IndexScanDesc scan, ScanDirection dir)
@@ -2081,37 +2074,22 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 	BTScanPosUnpinIfPinned(so->currPos);
 
 	/* Walk to the next page with data */
-	if (!scan->parallel_scan)
-	{
-		/* Not parallel, so use local state set by the last _bt_readpage */
-		if (ScanDirectionIsForward(dir))
-			blkno = so->currPos.nextPage;
-		else
-			blkno = so->currPos.prevPage;
-		lastcurrblkno = so->currPos.currPage;
-
-		/*
-		 * Cancel primitive index scans that were scheduled when the call to
-		 * _bt_readpage for currPos happened to use the opposite direction to
-		 * the one that we're stepping in now.  (It's okay to leave the scan's
-		 * array keys as-is, since the next _bt_readpage will advance them.)
-		 */
-		if (so->currPos.dir != dir)
-			so->needPrimScan = false;
-	}
+	if (ScanDirectionIsForward(dir))
+		blkno = so->currPos.nextPage;
 	else
-	{
-		/*
-		 * Seize the scan to get the nextPage and currPage from shared
-		 * parallel state (saved from parallel scan's last _bt_readpage)
-		 */
-		if (!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
-			return false;
+		blkno = so->currPos.prevPage;
+	lastcurrblkno = so->currPos.currPage;
 
-		Assert(!so->needPrimScan);
-	}
+	/*
+	 * Cancel primitive index scans that were scheduled when the call to
+	 * _bt_readpage for currPos happened to use the opposite direction to the
+	 * one that we're stepping in now.  (It's okay to leave the scan's array
+	 * keys as-is, since the next _bt_readpage will advance them.)
+	 */
+	if (so->currPos.dir != dir)
+		so->needPrimScan = false;
 
-	return _bt_readnextpage(scan, blkno, lastcurrblkno, dir);
+	return _bt_readnextpage(scan, blkno, lastcurrblkno, dir, false);
 }
 
 /*
@@ -2203,14 +2181,19 @@ _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
  *
  * On entry, caller shouldn't hold any locks or pins on any page (we work
  * directly off of blkno and lastcurrblkno instead).  Parallel scan callers
- * must have seized the scan before calling here (blkno and lastcurrblkno
- * arguments should come from the seized scan).
+ * that seized the scan before calling here should pass seized=true; such a
+ * caller's blkno and lastcurrblkno arguments come from the seized scan.
+ * seized=false callers just pass us the blkno/lastcurrblkno taken from their
+ * so->currPos, which (along with so->currPos itself) can be used to end the
+ * scan.  A seized=false caller's blkno can never be assumed to be the page
+ * that must be read next during a parallel scan, though.  We must figure that
+ * part out for ourselves by seizing the scan (the correct page to read might
+ * already be beyond the seized=false caller's blkno during a parallel scan).
  *
  * On success exit, so->currPos is updated to contain data from the next
- * interesting page, and we return true (parallel scan callers should not use
- * so->currPos to determine which page to scan next, though).  We hold a pin
- * on the buffer on success exit, except when _bt_drop_lock_and_maybe_pin
- * decided it was safe to eagerly drop the pin (to avoid blocking VACUUM).
+ * interesting page, and we return true.  We hold a pin on the buffer on
+ * success exit, except when _bt_drop_lock_and_maybe_pin decided it was safe
+ * to eagerly drop the pin (to avoid blocking VACUUM).
  *
  * If there are no more matching records in the given direction, we drop all
  * locks and pins, invalidate so->currPos, and return false.
@@ -2220,12 +2203,12 @@ _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
  */
 static bool
 _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
-				 BlockNumber lastcurrblkno, ScanDirection dir)
+				 BlockNumber lastcurrblkno, ScanDirection dir, bool seized)
 {
 	Relation	rel = scan->indexRelation;
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
-	Assert(so->currPos.currPage == lastcurrblkno || scan->parallel_scan != NULL);
+	Assert(so->currPos.currPage == lastcurrblkno || seized);
 	Assert(!BTScanPosIsPinned(so->currPos));
 
 	/*
@@ -2254,6 +2237,15 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 
 		Assert(!so->needPrimScan);
 
+		/* parallel scan must never actually visit so->currPos blkno */
+		if (!seized && scan->parallel_scan != NULL &&
+			!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
+		{
+			/* whole scan is now done (or another primitive scan required) */
+			BTScanPosInvalidate(so->currPos);
+			return false;
+		}
+
 		if (ScanDirectionIsForward(dir))
 		{
 			/* read blkno, but check for interrupts first */
@@ -2308,14 +2300,7 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 
 		/* no matching tuples on this page */
 		_bt_relbuf(rel, so->currPos.buf);
-
-		/* parallel scan seizes another page (won't use so->currPos blkno) */
-		if (scan->parallel_scan != NULL &&
-			!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
-		{
-			BTScanPosInvalidate(so->currPos);
-			return false;
-		}
+		seized = false;			/* released by _bt_readpage (or by us) */
 	}
 
 	/*

src/backend/access/nbtree/nbtutils.c

@@ -2402,6 +2402,8 @@ _bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
 
 new_prim_scan:
 
+	Assert(pstate->finaltup);	/* not on rightmost/leftmost page */
+
 	/*
 	 * End this primitive index scan, but schedule another.
 	 *