Fix race in SSI interaction with empty btrees.

When predicate-locking btrees, we have a special case for completely
empty btrees, since there is no page to lock.  This was racy, because,
without buffer lock held, a matching key could be inserted between the
_bt_search() and the PredicateLockRelation() calls.

Fix, by rechecking _bt_search() after taking the relation-level SIREAD
lock, if using SERIALIZABLE isolation and an empty btree is discovered.

Back-patch to all supported releases.  Fixes one aspect of bug #17949.

Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>
Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org

src/backend/access/nbtree/nbtsearch.c

@@ -17,6 +17,7 @@
 
 #include "access/nbtree.h"
 #include "access/relscan.h"
+#include "access/xact.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/predicate.h"
@@ -1382,22 +1383,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	{
 		/*
 		 * We only get here if the index is completely empty. Lock relation
-		 * because nothing finer to lock exists.
+		 * because nothing finer to lock exists.  Without a buffer lock, it's
+		 * possible for another transaction to insert data between
+		 * _bt_search() and PredicateLockRelation().  We have to try again
+		 * after taking the relation-level predicate lock, to close a narrow
+		 * window where we wouldn't scan concurrently inserted tuples, but the
+		 * writer wouldn't see our predicate lock.
 		 */
-		PredicateLockRelation(rel, scan->xs_snapshot);
+		if (IsolationIsSerializable())
-
+		{
-		/*
+			PredicateLockRelation(rel, scan->xs_snapshot);
-		 * mark parallel scan as done, so that all the workers can finish
+			stack = _bt_search(rel, NULL, &inskey, &buf, BT_READ,
-		 * their scan
+							   scan->xs_snapshot);
-		 */
+			_bt_freestack(stack);
-		_bt_parallel_done(scan);
+		}
-		BTScanPosInvalidate(so->currPos);
 
-		return false;
+		if (!BufferIsValid(buf))
+		{
+			/*
+			 * Mark parallel scan as done, so that all the workers can finish
+			 * their scan.
+			 */
+			_bt_parallel_done(scan);
+			BTScanPosInvalidate(so->currPos);
+			return false;
+		}
 	}
-	else
+
-		PredicateLockPage(rel, BufferGetBlockNumber(buf),
+	PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
-						  scan->xs_snapshot);
 
 	_bt_initialize_more_data(so, dir);