Saturday, February 8, 2020

Fun with Bugs #93 - On MySQL Bug Reports I am Subscribed to, Part XXVII

No matter what I write and present about dynamic tracing, blog posts about MySQL bugs are more popular based on statistics. So, to make more readers happy, I'd like to continue my review of interesting bugs reported in November with this post on bugs reported during December, 2019.

As usual, I'll try to concentrate on bug reports related to InnoDB, replication and optimizer, but some other categories also got my attention:
  • Bug #97911 - "[FATAL] Semaphore wait has lasted > 600 seconds. We intentionally crash the serv...". This bug got marked as a duplicate of other, older long semaphore wait bug (in "Can't repeat" status!) without much analysis. I think all Oracle engineers who added comments to that bug missed one interesting point:
    ... [ERROR] [MY-012872] [InnoDB] Semaphore wait has lasted > 39052544 seconds.
    even though bug reporter highlighted it in a comment. Reported wait time is a problem and is surely a bug, no matter how to reproduce the long wait itself and what is its root cause!
  • Bug #97913 - "Undo logs growing during partitioning ALTER queries". This bug (affecting only MySQL 5.7.x) was reported by Przemyslaw Malkowski from Percona, who also presented useful examples of monitoring queries to the information_schema.innodb_metrics and performance_schema. Check also comments that may explain why 8.0 is not affected in a similar way.
  • Bug #97935 - "Memory leak in client connection using information_schema". It took some efforts (starting from but not limited to Valgrind Massif profiling of heap memory usage) and time for Daniel Nichter to prove the point and get this bug "Verified". It is also not clear if MySQL 8 is also affected.
  • Bug #97950 - "buf_read_page_handle_error can trigger assert failure". Bug reporter, Shu Lin, tried his best to make the point. It's clear enough how to repeat this, and one could use one of documented test synchonisation methods if gdb is too much for bug verification. I do not think this bug was handled properly or got the level of attention it truly deserved.
  • Bug #97966 - "XA COMMIT in another session will not write binlog event". This bug was reported by Lujun Wang and immediately verified, but again with no documented check if MySQL 8 is affected. This happens too often, unfortunately.
  • Bug #97971 - "Roles not handling column level privileges correctly; Can SELECT, but not UPDATE". Clear and simple bug report with a workaround from Travis Bement. It was immediately verified.
  • Bug #98014 - "Lossy implicit conversion in conditional breaks ONLY_FULL_GROUP_BY". Yet another case of (IMHO) improper bug processing. The argument presented (from the manual):
    "MySQL 5.7.5 and later also permits a nonaggregate column not named in a GROUP BY clause when ONLY_FULL_GROUP_BY SQL mode is enabled, provided that this column is limited to a single value"
    does not apply, as "single value" for = 0 is NOT selected, we have multiple Host values matching it due to conversion. This is how proper version (guess what it is) works:
    mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;
    ERROR 1055 (42000): 'mysql.user.Host' isn't in GROUP BY
    mysql> select @@sql_mode;
    | @@sql_mode         |
    1 row in set (0.001 sec)

    mysql> set session sql_mode='';
    Query OK, 0 rows affected (0.029 sec)

    mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;+---------------+-----------+----------+
    | User          | Host      | COUNT(*) |
    | data_engineer |           |        1 |
    | en            | localhost |        1 |
    | ro1           |           |        1 |
    | ro2           |           |        1 |
    | role-1        |           |        1 |
    | root          | ::1       |        3 |
    | user1         | %         |        1 |
    13 rows in set, 17 warnings (0.003 sec)
    I think this bug reported by Joshua Varner must be verified.
  • Bug #98046 - "Inconsistent behavior while logging a killed query in the slow query log". Bug reporter, Pranay Motupalli, provided a clear test case and a detailed analysis, including the gdb debugging session that proves the point. Nice bug report.
  • Bug #98055 - "MySQL Optimizer Bug not picking right index". Both the bug reporter (Sudheer Gadipathi) and engineer who verified the bug stated that MySQL 8.0.x is similarly affected (UNIQUE key is preferred for the partitioned table, even though there is a better non-unique index). But 8.0.x is NOT listed in the "Version:" filed. Weird.
  • Bug #98068 - "SELECT FOR UPDATE not-exist table in PERFORMANCE SCHEMA reports confusing error". This is a funny (but still a regression) bug report by William ZHANG. Proper versions work like this:
    mysql> select database();+--------------------+
    | database()         |
    | performance_schema |
    1 row in set (0.001 sec)

    mysql> select * from not_exist_table;
    ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist
    mysql> select * from not_exist_table for update;
    ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist
  • Bug #98072 - "innochecksum summary shows blob pages as other type of page for 8.0 tables". The bug was reported by SERGEY KUZMICHEV. This time "regression" tag is missing, even though it's clearly stated that MySQL 5.7 worked differently. This is from the proper version:
    ================PAGE TYPE SUMMARY==============
           1        Index page
           0        Undo log page
           1        Inode page
           0        Insert buffer free list page
         508        Freshly allocated page
           1        Insert buffer bitmap
           0        System page
           0        Transaction system page
           1        File Space Header
           0        Extent descriptor page
          64        BLOB page
           0        Compressed BLOB page
           0        Page compressed page
           0        Page compressed encrypted page
           0        Other type of page

  • Bug #98083 - "Restarting the computer when deleting the database will cause directory residues". One would expect that MySQL 8 with a data dictionary should have some means to figure out the remaining database directory for a dropped database upon startup (as it stores information about databases elsewhere) and do proper cleanup. I think this bug reported by Jinming Liao must be verified and fixed. There is no "... manual creation or deletion of tables or databases..." involved in this case.
  • Bug #98091 - "InnoDB does not initialize raw disk partitions". As simple as that and both 5.7.29 and 8.0.219 are surely affected. It was not always the case, I've used raw devices myself with older MySQL versions, so this bug reported by Saverio M is a regression. Still, "regression" tag is missing.
That's all bugs reported in December, 2019 that I cared to subscribe to  and mention here. Next time I'll check bugs reported in January, 2020. There are at least 16 in my list already, so stay tuned.

Follow the links in this post to get more details about profiling and creating off-CPU FlameGraphs for MySQL. This post is devoted to bugs, though :)

To summarize:
  1. I am happy to see bug reports from people whom I never noticed before. MySQL Community is alive.
  2. Some flexibility in following common sense based bugs verification procedures is still visible. Bugs reported for 5.7 are not checked on 8.0 (or the results of this check are not documented in public), nobody cares to read what bug reporter says carefully or go extra mile, "regression" tag not added, and so on. 
  3. Probably at this stage my writings are mostly ignored by Oracle's decision makers. But I keep watching them all anyway.

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