branches/innodb+: buf_flush_insert_sorted_into_flush_list(): Restore ut_ad()

assertions that were removed when introducing the flush list mutex,
and add comments that explain why these assertions are valid.

Discussed with Inaam Rana.

buf/buf0flu.c

@@ -280,6 +280,22 @@ buf_flush_insert_sorted_into_flush_list(
 
 	buf_flush_list_mutex_enter();
 
+	/* The field in_LRU_list is protected by buf_pool_mutex, which
+	we are not holding.  However, while a block is in the flush
+	list, it is dirty and cannot be discarded, not from the
+	page_hash or from the LRU list.  At most, the uncompressed
+	page frame of a compressed block may be discarded or created
+	(copying the block->page to or from a buf_page_t that is
+	dynamically allocated from buf_buddy_alloc()).  Because those
+	transitions hold block->mutex and the flush list mutex (via
+	buf_flush_relocate_on_flush_list()), there is no possibility
+	of a race condition in the assertions below. */
+	ut_ad(block->page.in_LRU_list);
+	ut_ad(block->page.in_page_hash);
+	/* buf_buddy_block_register() will take a block in the
+	BUF_BLOCK_MEMORY state, not a file page. */
+	ut_ad(!block->page.in_zip_hash);
+
 	ut_ad(!block->page.in_flush_list);
 	ut_d(block->page.in_flush_list = TRUE);
 	block->page.oldest_modification = lsn;