branches/zip: Allow dirty compressed-only blocks to exist in the buffer pool

and to be flushed to disk.

buf_LRU_free_block(): Enable the freeing of uncompressed pages of
compressed tablespaces.

trx_doublewrite->buf_block_arr[]: Change the type from buf_block_t*
to buf_page_t*.

buf_flush_ready_for_flush(): Add debug assertion.

buf_flush_buffered_writes(), buf_flush_try_page(): Support blocks of type
BUF_BLOCK_ZIP_DIRTY.

buf_flush_post_to_doublewrite_buf(), buf_flush_write_block_low():
Change the type of the parameter from buf_block_t* to buf_page_t*.

buf_flush_init_for_writing(): Allow page to be NULL if page_zip_ is non-NULL.

buf/buf0lru.c

@@ -929,7 +929,7 @@ buf_LRU_free_block(
 			return(FALSE);
 		}
 
-		// b = buf_buddy_alloc(sizeof *b, FALSE); // TODO: enable this
+		b = buf_buddy_alloc(sizeof *b, FALSE);
 
 		if (!b) {
 			return(FALSE);

include/buf0flu.h

@@ -48,7 +48,8 @@ Initializes a page for writing to the tablespace. */
 void
 buf_flush_init_for_writing(
 /*=======================*/
-	byte*		page,		/* in/out: page */
+	byte*		page,		/* in/out: page, may be NULL
+					if page_zip_ is non-NULL */
 	void*		page_zip_,	/* in/out: compressed page, or NULL */
 	ib_uint64_t	newest_lsn);	/* in: newest modification lsn
 					to the page */

include/trx0sys.h

@@ -408,7 +408,7 @@ struct trx_doublewrite_struct{
 				address divisible by UNIV_PAGE_SIZE
 				(which is required by Windows aio) */
 	byte*	write_buf_unaligned; /* pointer to write_buf, but unaligned */
-	buf_block_t**
+	buf_page_t**
 		buf_block_arr;	/* array to store pointers to the buffer
 				blocks which have been cached to write_buf */
 };