branches/zip: Non-functional change: Add some debug assertions and comments.

buf_page_t: Note that the LRU fields are protected by buf_pool_mutex
only, not block->mutex or buf_pool_zip_mutex.

buf_page_get_freed_page_clock(): Note that this is sometimes invoked
without mutex protection.

buf_pool_get_oldest_modification(): Note that the result may be out of
date.

buf_page_get_LRU_position(), buf_page_is_old(): Assert that the buffer
pool mutex is being held.

buf_page_release(): Assert that dirty blocks are in the flush list.

include/buf0buf.h

@@ -1053,7 +1053,8 @@ struct buf_page_struct{
 					not yet been flushed on disk; zero if
 					all modifications are on disk */
 
-	/* 3. LRU replacement algorithm fields; protected by buf_pool_mutex */
+	/* 3. LRU replacement algorithm fields; protected by
+	buf_pool_mutex only (not buf_pool_zip_mutex or block->mutex) */
 
 	UT_LIST_NODE_T(buf_page_t) LRU;
 					/* node of the LRU list */

include/buf0buf.ic

@@ -20,6 +20,7 @@ buf_page_get_freed_page_clock(
 					/* out: freed_page_clock */
 	const buf_page_t*	bpage)	/* in: block */
 {
+	/* This is sometimes read without holding buf_pool_mutex. */
 	return(bpage->freed_page_clock);
 }
 
@@ -89,6 +90,9 @@ buf_pool_get_oldest_modification(void)
 
 	buf_pool_mutex_exit();
 
+	/* The returned answer may be out of date: the flush_list can
+	change after the mutex has been released. */
+
 	return(lsn);
 }
 
@@ -261,6 +265,7 @@ buf_page_get_LRU_position(
 	const buf_page_t*	bpage)	/* in: control block */
 {
 	ut_ad(buf_page_in_file(bpage));
+	ut_ad(buf_pool_mutex_own());
 
 	return(bpage->LRU_position);
 }
@@ -429,6 +434,7 @@ buf_page_is_old(
 	const buf_page_t*	bpage)	/* in: control block */
 {
 	ut_ad(buf_page_in_file(bpage));
+	ut_ad(buf_pool_mutex_own());
 
 	return(bpage->old);
 }
@@ -1041,6 +1047,10 @@ buf_page_release(
 #endif
 	block->page.buf_fix_count--;
 
+	/* Dirty blocks should be in the flush list. */
+	ut_ad(!block->page.oldest_modification
+	      || block->page.in_flush_list);
+
 	mutex_exit(&block->mutex);
 
 	if (rw_latch == RW_S_LATCH) {