Saturday, June 15, 2013

Fun with Bugs #10 - recently reported bugs affecting MySQL 5.6.12

MySQL 5.6.12 is available to community for more than a week already, so people started to test and use it. And, no wonder, new bug reports started to appear. Let's concentrate on them in this issue.

I'd like to start with a funny one.  Bug #69413 had scared some of my Facebook readers to death, as we see kernel mutex mentioned clearly in the release notes for 5.6.12. What, kernel mutex comes back again? No, it's just a result of null merge and, probably, copy/paste from the release notes for 5.5.32.

It seems recent bug reports for 5.6.12 are mostly related to small details that may not be of any importance to a typical user. For example, Bug #69419 that was reported by my colleague almost immediately after release questions the way mtr is used in the release process. Change related to fix for other bug had broken few tests, but tests were neither updated nor temporary disabled it seems. This is strange, at best, and can mean many things (from simple mistake to "nobody cares", to switch to some other tools for internal regression testing).

"Nobody cares" does NOT apply though, as during this week Shane Bester had reported 2 public bugs related to potential performance improvements possible in 5.6.12. Check Bug #69420 and Bug #69422. Looks like he tries to find and eliminate reasons for even less than smallest slowdown in benchmarks.

He is not the only one. Check Bug #69451. Event the smallest chunk of redundant code can not hide these days from careful users...

One topic for bug reports is ages old: MySQL still do not use proper data type for integers in many parts of the code. Bug #69431 from Shane is one of recent examples. Bug #69469 (that is more or less a duplicate of Bug #69249 reported for 5.6.11 a month ago), is another one, but related to a new feature introduced in 5.6. It seems that topic is valid for a new code as much as for older one that Monty and Sinisa were reviewing a decade ago. Let's hope that for MySQL 5.7 GA the review of the entire code base is planned, with the aim to find and fix all problems of this kind (among others).

Unfortunately it's not only about minor and cosmetic things. If you use raw devices with InnoDB and plan to upgrade to 5.6, check Bug #69424. It's not yet verified, and previous bug of this kind, Bug #68860, was set to "Not a bug" in two days... But, well, how one should upgrade with existing raw decide containing data, when code of srv_file_check_mode() function clearly says:

/*********************************************************************//**
Check if a file can be opened in read-write mode.
@return true if it doesn't exist or can be opened in rw mode. */
static
bool
srv_file_check_mode(
/*================*/
        const char*     name)           /*!< in: filename to check */
{
        os_file_stat_t  stat;

        memset(&stat, 0x0, sizeof(stat));

        dberr_t         err = os_file_get_status(name, &stat, true);

        if (err == DB_FAIL) {

                ib_logf(IB_LOG_LEVEL_ERROR,
                        "os_file_get_status() failed on '%s'. Can't determine "
                        "file permissions", name);

                return(false);
 

        } else if (err == DB_SUCCESS) {

                /* Note: stat.rw_perm is only valid of files */

                if (stat.type == OS_FILE_TYPE_FILE) {
                        if (!stat.rw_perm) {

                                ib_logf(IB_LOG_LEVEL_ERROR,
                                        "%s can't be opened in %s mode",
                                        name,
                                        srv_read_only_mode
                                        ? "read" : "read-write");

                                return(false);
                        }
                } else {
                        /* Not a regular file, bail out. */

                        ib_logf(IB_LOG_LEVEL_ERROR,
                                "'%s' not a regular file.", name);

                        return(false);
                }
        } else {

                /* This is OK. If the file create fails on RO media, there
                is nothing we can do. */

                ut_a(err == DB_NOT_FOUND);
        }

        return(true);
}


That is, if file is not a regular file we unconditionally return false, and as soon as this function returns false in all places it is used we just assume error. (I have to check this myself eventually as I have no raw decide at hand for immediate test, but code like this does not present in MySQL 5.5, so it seems good old manual page just can not be used any more.)

It seems Oracle MySQL engineers should pay more attention to testing upgrade procedures (and reading community bug reports). Even if eventually this may not be the case, currently community QA efforts (and public bugs database) are still important and sometimes lead to findings that seem new and unexpected to Oracle MySQL engineers.



Another serious enough bug from recently reported and verified, Bug #69444, is related to replication. It seems to be not really crash safe when DDL statement is involved. Potentially when crash happens during "wrong" time, DDL is going to be executed again upon slave restart.

That's all for now. MySQL 5.6.12 is going to be the best release ever for 6+ more weeks it seems, so we all have plenty of time to check it and contribute to public bugs database...

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