EventLog does not play well with forkProcess

[adept]

I am trying to see an eventlog of process that daemonizes itself via forkProcess.

Unfortunately, as parent process dies, he writes out his eventlog (or at least a header part, I haven't checked exactly), and eventlog of the child process is then appended to the same file.

As a result, neither ghc-events nor threadscope is able to read the eventlog file.

I've modified the program so that it would not fork, and generated "good" eventlog. I'm attaching it and "bad" eventlog as well so you can see the difference by yourself.

Perhaps "+RTS -ls" could write output to filename that includes the PID of the process, like "program.$PID.eventlog"?

[adept]

Good eventlog from modified version of the program that does not fork

[adept]

Bad eventlog from the version that uses forkProcess

[adept]

Well, here is a plan: when forked child process does initEventLogging, it should detect that prog_name.evenlog is already opened and being written to by another process.

It could be done with: 1)OS locks 2)file-based locking (*.pid files or something similar) 3)using "static int alreadyWriting", which would be populated by parent process.

I'm inclined to go with (3). Forked child will check to see whether parent process is alreadyWriting and will use different filename if necessary.

Comments?

[adept]

Attached patch fixes the problem. Please review.

[adept]

Reopening eventlog after fork()

[simonmar]

Replying to adept:

Attached patch fixes the problem. Please review.

Looks fine. I'll validate and push.

[simonmar]

Thanks, pushed:

Fri Dec  3 13:39:50 GMT 2010  Dmitry Astapov <dastapov@gmail.com>
  * Fixes for #4512: EventLog.c - provides ability to terminate event logging, Schedule.c - uses them in forkProcess.

[simonmar]

A couple more fixes were required to this patch:

Fri Dec 10 09:39:06 GMT 2010  Simon Marlow <marlowsd@gmail.com>
  * allocate enough room for the longer filename (addendum to #4512)

Fri Dec 10 09:39:51 GMT 2010  Simon Marlow <marlowsd@gmail.com>
  * Only reset the event log if logging is turned on (addendum to #4512)

[igloo]

First merged.

[igloo]

Last 2 also merged.