Right now MOZ_LOG's default output does not include process identifiers,
so messages from different processes look misleadingly similar:
[Main Thread]: D/nsBlah Message Message
[Main Thread]: D/nsBlah Message Message
They will soon instead look like:
[4372:Main Thread]: D/nsBlah Message Message
[4452:Main Thread]: D/nsBlah Message Message
If you mainly interact with things through mach, you may be thinking,
"Hey! What about those PIDs things get tagged with?":
GECKO(3972) | [Main Thread]: D/nsBlah Message Message
GECKO(3972) | [Main Thread]: D/nsBlah Message Message
AFAICT, mach and/or its helpers are doing a best-effort tagging of the
stdout/stderr of the spawned parent process. Since the content
processes inherit the descriptors of the parent, mach/friends are unable
to magically figure out the true writer without additional machinery to
give each content process its own descriptors that are pushed out to
mach via sendmsg or something else fancy. So everything gets labeled
with the parent PID. It would be cool to have a mechanism so that the
structured logging output could be properly labeled, but that arguably
is a different bug.
The change to add the PID is tracked on
https://bugzilla.mozilla.org/show_bug.cgi?id=1428979 so if you have
thoughts/proposals/patches, please chime in there. This change will
likely land sometime late today or tomorrow otherwise. This may break
log parsers that use very specific regexps, although the location was
chosen to leverage the enclosing "[]" and the one I know about is fine[1].
If you would like to learn more about Gecko logging in general, please
see
https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Gecko_Logging
If you're wondering why this didn't seem to be a problem for Necko's
cool about:networking logging as documented at
https://developer.mozilla.org/en-US/docs/Mozilla/Debugging/HTTP_logging
it's because MOZ_LOG did already have a mechanism for partitioning log
files by labeled process, see
https://searchfox.org/mozilla-central/rev/03877052c151a8f062eea177f684a2743cd7b1d5/xpcom/base/Logging.cpp#136
Andrew
1: https://github.com/mayhemer/logan/blob/master/logan-rules.js#L5
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