branches/zip: Remove a race condition.

buf_page_init_for_read(): Protect the buf_zip_decompress() call with
an X-lock on the block.

page_zip_validate(): Move the page_is_comp() assertion after the
page header comparison.

buf/buf0buf.c

@@ -2161,6 +2161,7 @@ buf_page_init_for_read(
 				}
 			}
 
+			rw_lock_x_lock(&block->lock);
 			mutex_exit(&block->mutex);
 			mutex_exit(&buf_pool->zip_mutex);
 
@@ -2174,6 +2175,7 @@ buf_page_init_for_read(
 			}
 
 			buf_zip_decompress(block, srv_use_checksums);
+			rw_lock_x_unlock(&block->lock);
 
 			return(NULL);
 		case BUF_BLOCK_FILE_PAGE:

page/page0zip.c

@@ -2401,8 +2401,6 @@ page_zip_validate(
 	page_t*		temp_page;
 	ibool		valid;
 
-	ut_a(page_is_comp(page));
-
 	if (memcmp(page_zip->data + FIL_PAGE_PREV, page + FIL_PAGE_PREV,
 		   FIL_PAGE_LSN - FIL_PAGE_PREV)
 	    || memcmp(page_zip->data + FIL_PAGE_TYPE, page + FIL_PAGE_TYPE, 2)
@@ -2412,6 +2410,8 @@ page_zip_validate(
 		return(FALSE);
 	}
 
+	ut_a(page_is_comp(page));
+
 	if (page_zip_validate_header_only) {
 		return(TRUE);
 	}