Check for GiST index tuples that don't fit on a page.

The page splitting code would go into infinite recursion if you try to
insert an index tuple that doesn't fit even on an empty page.

Per analysis and suggested fix by Andrew Gierth. Fixes bug #11555, reported
by Bryan Seitz (analysis happened over IRC). Backpatch to all supported
versions.

src/backend/access/gist/gist.c

@@ -1395,6 +1395,23 @@ gistSplit(Relation r,
 	int			i;
 	SplitedPageLayout *res = NULL;
 
+	/* this should never recurse very deeply, but better safe than sorry */
+	check_stack_depth();
+
+	/* there's no point in splitting an empty page */
+	Assert(len > 0);
+
+	/*
+	 * If a single tuple doesn't fit on a page, no amount of splitting will
+	 * help.
+	 */
+	if (len == 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+			errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
+				   IndexTupleSize(itup[0]), GiSTPageSize,
+				   RelationGetRelationName(r))));
+
 	memset(v.spl_lisnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
 	memset(v.spl_risnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
 	gistSplitByKey(r, page, itup, len, giststate, &v, 0);