The mysql command-line tool inteprets Ctrl+C as a request to interrupt the running query. But the new wrapper also gets the SIGINT and exits back to the shell, leaving both mysql and the shell trying to use the terminal.
A lesser issue is that Ctrl+Z properly suspends both mysql and the wrapper, but executing fg does not seem to send SIGCONT through to mysql.
| Project | Branch | Lines +/- | Subject | |
|---|---|---|---|---|
| mediawiki/core | master | +6 -0 | In mysql.php ignore SIGINT | |
| operations/puppet | production | +7 -0 | Make "sql wikishared" work again |
Change 446528 had a related patch set uploaded (by Tim Starling; owner: Tim Starling):
[mediawiki/core@master] In mysql.php ignore SIGINT
A lesser issue is that Ctrl+Z properly suspends both mysql and the wrapper, but executing fg does not seem to send SIGCONT through to mysql.
Could not reproduce. After Ctrl+Z I see mysql stopped (T state), after fg it goes back to the S state.
On my local machine, with bash as the shell, before Ctrl+Z:
brad 10280 0.9 1.2 230104 45680 pts/7 S+ 10:12 0:00 php maintenance brad 10281 0.0 0.0 2368 752 pts/7 S+ 10:12 0:00 sh -c 'mysql' ' brad 10282 0.2 0.1 23288 7432 pts/7 S+ 10:12 0:00 mysql --default
After Ctrl+Z:
brad 10280 0.4 1.2 230104 45680 pts/7 T 10:12 0:00 php maintenance brad 10281 0.0 0.0 2368 752 pts/7 T 10:12 0:00 sh -c 'mysql' ' brad 10282 0.1 0.1 23288 7432 pts/7 T 10:12 0:00 mysql --default
After fg:
brad 10280 0.2 1.2 230104 45680 pts/7 S+ 10:12 0:00 php maintenance brad 10281 0.0 0.0 2368 752 pts/7 S+ 10:12 0:00 sh -c 'mysql' ' brad 10282 0.0 0.1 23288 7432 pts/7 T+ 10:12 0:00 mysql --default
On terbium mwmaint1001 the situation is even worse, mysql doesn't even get suspended. After Ctrl+Z:
anomie 7843 0.0 0.0 11192 3124 pts/0 T 14:10 0:00 /bin/bash /usr/local/bin/mwscript mysql.php --wiki=metawiki --wikidb=enwiki root 7847 0.0 0.0 49256 3668 pts/0 T 14:10 0:00 sudo -u www-data php /srv/mediawiki-staging/multiversion/MWScript.php mysql.php --wiki=metawiki --wikidb=enwiki www-data 7848 0.6 0.1 699072 110276 pts/0 T 14:10 0:00 php /srv/mediawiki-staging/multiversion/MWScript.php mysql.php --wiki=metawiki --wikidb=enwiki www-data 7849 0.0 0.0 362916 24464 ? Ss 14:10 0:00 php /srv/mediawiki-staging/multiversion/MWScript.php mysql.php --wiki=metawiki --wikidb=enwiki www-data 7850 0.0 0.0 362916 21088 ? Ss 14:10 0:00 php /srv/mediawiki-staging/multiversion/MWScript.php mysql.php --wiki=metawiki --wikidb=enwiki www-data 7855 0.0 0.0 4288 708 ? S 14:10 0:00 sh -c 'mysql' '--defaults-extra-file=/tmp/redacted.ini' '--user=wikiadmin' '--database=enwiki' '--host=10.64.32.115' www-data 7856 0.0 0.0 40468 7860 ? S 14:10 0:00 mysql --defaults-extra-file=/tmp/redacted.ini --user=wikiadmin --database=enwiki --host=10.64.32.115
Hmm. If I use PHP 7.0 on mwmaint1001, I get different behavior. Sometimes it works right, sometimes the fg unsuspends all the processes but doesn't correctly make them the foreground process, and sometimes it manages to not unsuspend the mysql process.
HHVM's behavior is apparently because it has the mysql process running in a different process group and must be manually forwarding signals or something.
In production, mwscript/sudo is causing its own set of problems with ctrl-Z even when running eval.php. To test it locally I skipped my own wrappers and ran php directly, since I figured the (evident) problems with the wrappers were out of scope.
Indeed HHVM on mwmaint1001 is creating a process group, which creates very special problems, like background mysql and the controlling shell simultaneously reading from a single terminal:
[0400][tstarling@mwmaint1001:~]$ ls /etc/hhvm -bash: l: command not found
With hhvm.server.light_process_count=0, it doesn't do that. But even with hhvm.server.light_process_count=0, I can still reproduce the problem of mysql remaining stopped after "fg". With strace, I was able to isolate the problem. mysql is installing SIGTSTP and SIGTTOU handlers. The intended function of these handlers is to do something, disable the handler, then resend the signal, stopping itself with kill(). But the actual sequence is as follows:
This is how it goes down with strace attached only to mysql. I'm not sure why mysql is receiving SIGTTOU. If I attach strace to hhvm or sh, the problem is not reproducible, there is no SIGTTOU signal.
SIGTTIN/SIGTTOU being blocked would explain how we can end up with background processes reading from the terminal, these signals are meant to stop that.
I think the workaround is to not press ctrl-Z in this script and leave it at that. This is a really stupid problem to spend half a day on.
Change 446530 had a related patch set uploaded (by Tim Starling; owner: Tim Starling):
[operations/puppet@production] Make "sql wikishared" work again
Change 446530 merged by Tim Starling:
[operations/puppet@production] Make "sql wikishared" work again
Change 446528 merged by jenkins-bot:
[mediawiki/core@master] In mysql.php ignore SIGINT