[#MDEV-27924] page_zip_copy_recs() corrupts ROW_FORMAT=COMPRESSED block descriptor

A number of innodb_zip tests crash if the preprocessor symbol UNIV_ZIP_COPY is defined. (There currently is no cmake option for enabling that.)
Here is a simple example:

10.6 92f79a22e63a3fef71106c64dfd8559ee9bdda4a
innodb_zip.wl6344_compress_level '16k,innodb' [ pass ] 573
***Warnings generated in error logs during shutdown after running tests: innodb_zip.wl6344_compress_level

mariadbd: /mariadb/10.6/storage/innobase/buf/buf0flu.cc:1212: void buf_flush_discard_page(buf_page_t*): Assertion `state == buf_page_t::FREED \|\| state == buf_page_t::UNFIXED \|\| state == buf_page_t::IBUF_EXIST \|\| state == buf_page_t::REINIT' failed.

The cause is that page_zip_copy_recs() is invoking the default copy constructor of page_zip_des_t, which will also overwrite the fix data member, which we only stored in the page_zip_des_t object to avoid alignment loss.

The following fixes this block descriptor corruption that affects ROW_FORMAT=COMPRESSED pages in the event of a rather rare compression overflow:

diff --git a/storage/innobase/include/page0types.h b/storage/innobase/include/page0types.h

index 885d2290f7c..192e572543d 100644

--- a/storage/innobase/include/page0types.h

+++ b/storage/innobase/include/page0types.h

@@ -119,6 +119,16 @@ struct page_zip_des_t

 		       - reinterpret_cast<char*>(this));

+	page_zip_des_t() = default;

+	page_zip_des_t(const page_zip_des_t&) = default;

+	/* Initialize everything except the member "fix". */

+	page_zip_des_t(const page_zip_des_t& old, bool) {

+		memcpy((void*) this, (void*) &old,

+		       reinterpret_cast<char*>(&fix)

+		       - reinterpret_cast<char*>(this));

+	}

 private:

 	friend buf_pool_t;

 	friend buf_page_t;

diff --git a/storage/innobase/page/page0zip.cc b/storage/innobase/page/page0zip.cc

index ec90d73e765..73124b902f2 100644

--- a/storage/innobase/page/page0zip.cc

+++ b/storage/innobase/page/page0zip.cc

@@ -4562,7 +4562,7 @@ page_zip_copy_recs(

 	to the compressed data page. */

 		page_zip_t*	data = page_zip->data;

-		new (page_zip) page_zip_des_t(*src_zip);

+		new (page_zip) page_zip_des_t(*src_zip, false);

 		page_zip->data = data;

 	ut_ad(page_zip_get_trailer_len(page_zip, dict_index_is_clust(index))

With this patch (and UNIV_ZIP_COPY), only 2 tests fail with an expected result difference (~~MDEV-20752~~):

[MDEV-27924] page_zip_copy_recs() corrupts ROW_FORMAT=COMPRESSED block descriptor Created: 2022-02-23 Updated: 2022-02-23 Resolved: 2022-02-23
Status:	Closed
Project:	MariaDB Server
Component/s:	Storage Engine - InnoDB
Affects Version/s:	10.6.6, 10.7.2, 10.8.1, 10.6.7, 10.7.3, 10.8.2
Fix Version/s:	10.6.8, 10.7.4, 10.8.3

[MDEV-27924] page_zip_copy_recs() corrupts ROW_FORMAT=COMPRESSED block descriptor Created: 2022-02-23 Updated: 2022-02-23 Resolved: 2022-02-23