Applying InnoDB snashot 5.0-ss4900 part 1, Fixes BUG#42279

1) BUG#42279 - Race condition in btr_search_drop_page_hash_when_freed()

Detailed revision comments:

r4031 | marko | 2009-01-23 15:33:46 +0200 (Fri, 23 Jan 2009) | 8 lines
branches/5.0: btr_search_drop_page_hash_when_freed(): Check if
buf_page_get_gen() returns NULL.  The page may have been evicted
from the buffer pool between buf_page_peek_if_search_hashed() and
buf_page_get_gen(), because the buffer pool mutex will be released
between these two calls. (Bug #42279)

rb://82 approved by Heikki Tuuri

innobase/btr/btr0sea.c

@@ -1101,12 +1101,20 @@ btr_search_drop_page_hash_when_freed(
 	page = buf_page_get_gen(space, page_no, RW_S_LATCH, NULL,
 				BUF_GET_IF_IN_POOL, __FILE__, __LINE__,
 				&mtr);
+	/* Because the buffer pool mutex was released by
+	buf_page_peek_if_search_hashed(), it is possible that the
+	block was removed from the buffer pool by another thread
+	before buf_page_get_gen() got a chance to acquire the buffer
+	pool mutex again.  Thus, we must check for a NULL return. */
+
+	if (UNIV_LIKELY(page != NULL)) {
 
 #ifdef UNIV_SYNC_DEBUG
-	buf_page_dbg_add_level(page, SYNC_TREE_NODE_FROM_HASH);
+		buf_page_dbg_add_level(page, SYNC_TREE_NODE_FROM_HASH);
 #endif /* UNIV_SYNC_DEBUG */
 
-	btr_search_drop_page_hash_index(page);
+		btr_search_drop_page_hash_index(page);
+	}
 
 	mtr_commit(&mtr);
 }