Bug#12595091 POSSIBLY INVALID ASSERTION IN BTR_CUR_PESSIMISTIC_UPDATE()

Facebook got a case where the page compresses really well so that
btr_cur_optimistic_update() returns DB_UNDERFLOW, but when a record
gets updated, the compression rate radically changes so that
btr_cur_insert_if_possible() can not insert in place despite
reorganizing/recompressing the page, leading to the assertion failing.

rb:1220 approved by Sunny Bains

storage/innodb_plugin/ChangeLog

+2012-08-16	The InnoDB Team
+
+	* btr/btr0cur.c:
+	Fix Bug#12595091 POSSIBLY INVALID ASSERTION IN
+	BTR_CUR_PESSIMISTIC_UPDATE()
+
 2012-08-16	The InnoDB Team
 
 	* btr/btr0btr.c, btr/btr0cur.c:

storage/innodb_plugin/btr/btr0cur.c

@@ -2326,7 +2326,12 @@ btr_cur_pessimistic_update(
 		err = DB_SUCCESS;
 		goto return_after_reservations;
 	} else {
-		ut_a(optim_err != DB_UNDERFLOW);
+		/* If the page is compressed and it initially
+		compresses very well, and there is a subsequent insert
+		of a badly-compressing record, it is possible for
+		btr_cur_optimistic_update() to return DB_UNDERFLOW and
+		btr_cur_insert_if_possible() to return FALSE. */
+		ut_a(page_zip || optim_err != DB_UNDERFLOW);
 
 		/* Out of space: reset the free bits. */
 		if (!dict_index_is_clust(index)