MDEV-14441 InnoDB hangs when setting innodb_adaptive_hash_index=OFF during UPDATE

This race condition is a regression caused by MDEV-12121.

btr_cur_update_in_place(): Determine block->index!=NULL only once
in order to determine whether an adaptive hash index bucket needs
to be exclusively locked and unlocked.

If we evaluated block->index multiple times, and the adaptive hash
index was disabled before we locked the adaptive hash index, then
we would never release the adaptive hash index bucket latch, which
would eventually lead to InnoDB hanging.

storage/innobase/btr/btr0cur.cc

@@ -3,7 +3,7 @@
 Copyright (c) 1994, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2015, 2017, MariaDB Corporation.
+Copyright (c) 2015, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3692,34 +3692,38 @@ btr_cur_update_in_place(
 	      || row_get_rec_trx_id(rec, index, offsets));
 
 #ifdef BTR_CUR_HASH_ADAPT
-	if (block->index) {
-		/* TO DO: Can we skip this if none of the fields
-		index->search_info->curr_n_fields
-		are being updated? */
-
-		/* The function row_upd_changes_ord_field_binary works only
-		if the update vector was built for a clustered index, we must
-		NOT call it if index is secondary */
-
-		if (!dict_index_is_clust(index)
-		    || row_upd_changes_ord_field_binary(index, update, thr,
-							NULL, NULL)) {
-
-			/* Remove possible hash index pointer to this record */
-			btr_search_update_hash_on_delete(cursor);
+	{
+		rw_lock_t* ahi_latch = block->index
+			? btr_get_search_latch(block->index) : NULL;
+		if (ahi_latch) {
+			/* TO DO: Can we skip this if none of the fields
+			index->search_info->curr_n_fields
+			are being updated? */
+
+			/* The function row_upd_changes_ord_field_binary
+			does not work on a secondary index. */
+
+			if (!dict_index_is_clust(index)
+			    || row_upd_changes_ord_field_binary(
+				    index, update, thr, NULL, NULL)) {
+
+				/* Remove possible hash index pointer
+				to this record */
+				btr_search_update_hash_on_delete(cursor);
+			}
 		}
 
-		btr_search_x_lock(index);
-	}
+		rw_lock_x_lock(ahi_latch);
 
-	assert_block_ahi_valid(block);
+		assert_block_ahi_valid(block);
 #endif /* BTR_CUR_HASH_ADAPT */
 
-	row_upd_rec_in_place(rec, index, offsets, update, page_zip);
+		row_upd_rec_in_place(rec, index, offsets, update, page_zip);
 
 #ifdef BTR_CUR_HASH_ADAPT
-	if (block->index) {
-		btr_search_x_unlock(index);
+		if (ahi_latch) {
+			rw_lock_x_unlock(ahi_latch);
+		}
 	}
 #endif /* BTR_CUR_HASH_ADAPT */