Port r53 from branches/5.0:

Make comments about buf_block_t::index more accurate.

btr/btr0sea.c

@@ -889,7 +889,8 @@ Drops a page hash index. */
 void
 btr_search_drop_page_hash_index(
 /*============================*/
-	page_t*	page)	/* in: index page, s- or x-latched */
+	page_t*	page)	/* in: index page, s- or x-latched, or an index page
+			for which we know that block->buf_fix_count == 0 */
 {
 	hash_table_t*	table;
 	buf_block_t*	block;

include/buf0buf.h

@@ -831,7 +831,13 @@ struct buf_block_struct{
 					records with the same prefix should be
 					indexed in the hash index */
 					
-	/* The following 6 fields are protected by btr_search_latch: */
+	/* These 6 fields may only be modified when we have
+	an x-latch on btr_search_latch AND
+	a) we are holding an s-latch or x-latch on block->lock or
+	b) we know that block->buf_fix_count == 0.
+
+	An exception to this is when we init or create a page
+	in the buffer pool in buf0buf.c. */
 
 	ibool		is_hashed;	/* TRUE if hash index has already been
 					built on this page; note that it does
@@ -849,11 +855,7 @@ struct buf_block_struct{
 					BTR_SEARCH_RIGHT_SIDE in hash
 					indexing */
 	dict_index_t*	index;		/* Index for which the adaptive
-					hash index has been created.
-					This field may only be modified
-					while holding an s-latch or x-latch
-					on block->lock and an x-latch on
-					btr_search_latch. */
+					hash index has been created. */
 	/* 6. Debug fields */
 #ifdef UNIV_SYNC_DEBUG
 	rw_lock_t	debug_latch;	/* in the debug version, each thread