If an empty .ibd file is not being copied into a backup, then the backup will have to be adjusted somehow. Maybe during mariabackup --prepare, we should actually create data files in recv_sys_t::recover_deferred() if that is not already the case.

Somewhat related to this, MDEV-25909 has been filed for avoiding unnecessary repeated reads of the first page of a data file. That could affect mariabackup --prepare as well.

I just analyzed an rr replay trace of a failed mariabackup --prepare . An assertion would fail in my development branch due to buf_flush_discard_page() being invoked while deferred_spaces still has not been converted to tablespaces.

I was unable to reproduce the race when restoring the same backup very many times. I think that a simple fix is as follows (possibly to be revised in MDEV-14481):

diff --git a/storage/innobase/buf/buf0flu.cc b/storage/innobase/buf/buf0flu.cc

index 1f720989f6f..30f46e6024f 100644

--- a/storage/innobase/buf/buf0flu.cc

+++ b/storage/innobase/buf/buf0flu.cc

@@ -1301,7 +1301,7 @@ static void buf_flush_LRU_list_batch(ulint max, flush_counters_t *n)

         space= nullptr;

       }

-      if (!space)

+      if (!space && !recv_recovery_is_on())

         buf_flush_discard_page(bpage);

       else if (neighbors && space->is_rotational())

       {

A cleaner fix could be to retain an additional X-latch on the page, instead of only retaining a buffer-fix. Then, no change to the page cleaner should be needed. But, this requires the page latch to always exist also for ROW_FORMAT=COMPRESSED blocks. (That is possible in my development branch.)

The link in the description is stale, and no summary of the failure is available. On a quick search of the cross-reference, I found some failures of the test mariabackup.xb_compressed_encrypted that fail like this:

mariabackup.xb_compressed_encrypted '16k,innodb' w3 [ fail ]

        Test ended at 2021-09-17 07:00:03

CURRENT_TEST: mariabackup.xb_compressed_encrypted

/mnt/buildbot/build/mariadb-10.6.5/extra/mariabackup/mariabackup based on MariaDB server 10.6.5-MariaDB Linux (x86_64)

…

[00] 2021-09-17 07:00:02 completed OK!

mysqltest: At line 29: query 'select sum(c1) from t1' failed: ER_GET_ERRNO (1030): Got error 194 "Tablespace is missing for a table" from storage engine InnoDB

In fact, mariabackup.xb_compressed_encrypted is the only mariabackup.* test that has with Tablespace is missing for table in the "Failure Output" section. There were a couple more tests failing for bb-10.6-MDEV-24626_1 like this.

In MDEV-27424, I posted a rough mtr test case that could explain this scenario, and is indeed caused by MDEV-24626.

This issue should be fixed by patch for MDEV-27424 (bb-10.6-thiru)

The fix looks OK to push. I would suggest to refer to this bug in the commit and let the later report MDEV-27424 remain open, because it is unclear which version that report was filed for.