branches/zip: buf_LRU_free_block(): When buf_page_is_old(b), we incremented

buf_pool->LRU_old_len.  However, we forgot to check if buf_pool->LRU_old
happens to point to b's successor in the LRU list.  If it does, we must
assign buf_pool->LRU_old = b.  The following invariants hold:

In the LRU list, the "old" flag should grow monotonically, i.e., it is 0
for the first few items and 1 from thereafter.

If buf_pool->LRU_old != NULL, it must point to the first item with old=1
in the LRU list, and there must be buf_pool->LRU_old_len old items in the list.

This should fix Mantis issue#50 and issue#68.

buf/buf0lru.c

@@ -1246,6 +1246,12 @@ buf_LRU_free_block(
 
 				if (buf_page_is_old(b)) {
 					buf_pool->LRU_old_len++;
+					if (UNIV_UNLIKELY
+					    (buf_pool->LRU_old
+					     == UT_LIST_GET_NEXT(LRU, b))) {
+
+						buf_pool->LRU_old = b;
+					}
 				}
 
 				lru_len = UT_LIST_GET_LEN(buf_pool->LRU);

include/buf0buf.h

@@ -1277,13 +1277,17 @@ struct buf_pool_struct{
 					/* base node of the LRU list */
 	buf_page_t*	LRU_old;	/* pointer to the about 3/8 oldest
 					blocks in the LRU list; NULL if LRU
-					length less than BUF_LRU_OLD_MIN_LEN */
+					length less than BUF_LRU_OLD_MIN_LEN;
+					NOTE: when LRU_old != NULL, its length
+					should always equal LRU_old_len */
 	ulint		LRU_old_len;	/* length of the LRU list from
 					the block to which LRU_old points
 					onward, including that block;
 					see buf0lru.c for the restrictions
 					on this value; not defined if
-					LRU_old == NULL */
+					LRU_old == NULL;
+					NOTE: LRU_old_len must be adjusted
+					whenever LRU_old shrinks or grows! */
 
 	UT_LIST_BASE_NODE_T(buf_block_t) unzip_LRU;
 					/* base node of the unzip_LRU list */