Fix race in SSI interaction with gin fast path.

The ginfast.c code previously checked for conflicts in before locking
the relevant buffer, leaving a window where a RW conflict could be
missed.  Re-order.

There was also a place where buffer ID and block number were confused
while trying to predicate-lock a page, noted by visual inspection.

Back-patch to all supported releases.  Fixes one more problem discovered
with the reproducer from bug #17949, in this case when Dmitry tried
other index types.

Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>
Reported-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/gin/ginfast.c

@@ -245,9 +245,10 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 	/*
 	 * An insertion to the pending list could logically belong anywhere in the
 	 * tree, so it conflicts with all serializable scans.  All scans acquire a
-	 * predicate lock on the metabuffer to represent that.
+	 * predicate lock on the metabuffer to represent that.  Therefore we'll
+	 * check for conflicts in, but not until we have the page locked and are
+	 * ready to modify the page.
 	 */
-	CheckForSerializableConflictIn(index, NULL, GIN_METAPAGE_BLKNO);
 
 	if (collector->sumsize + collector->ntuples * sizeof(ItemIdData) > GinListPageSize)
 	{
@@ -291,6 +292,8 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 		LockBuffer(metabuffer, GIN_EXCLUSIVE);
 		metadata = GinPageGetMeta(metapage);
 
+		CheckForSerializableConflictIn(index, NULL, GIN_METAPAGE_BLKNO);
+
 		if (metadata->head == InvalidBlockNumber)
 		{
 			/*
@@ -353,6 +356,8 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 		char	   *ptr;
 		char	   *collectordata;
 
+		CheckForSerializableConflictIn(index, NULL, GIN_METAPAGE_BLKNO);
+
 		buffer = ReadBuffer(index, metadata->tail);
 		LockBuffer(buffer, GIN_EXCLUSIVE);
 		page = BufferGetPage(buffer);

src/backend/access/gin/ginget.c

@@ -140,7 +140,9 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
 	 * Predicate lock entry leaf page, following pages will be locked by
 	 * moveRightIfItNeeded()
 	 */
-	PredicateLockPage(btree->index, stack->buffer, snapshot);
+	PredicateLockPage(btree->index,
+					  BufferGetBlockNumber(stack->buffer),
+					  snapshot);
 
 	for (;;)
 	{