Re: [PATCH 3/3] trace: add GIT_TRACE_STDIN

> On Wed, Jun 17, 2015 at 05:04:04PM +0700, Duy Nguyen wrote:
>
>> I wonder if we could do it a bit differently. Instead of
>> GIT_TRACE_STDIN, I would add GIT_TRACE_HOOK that points to a script.
>> Whenever a command is run via run-command interface, the actual
>> command line to be executed would be "<hook script> <command>
>> <arguments>".
>
> Hmm, yeah, I like that. It's even more flexible, and it is much more
> obvious why it works only for run-command. If we feed the resulting
> "hooked" command to the shell, I think you could do:
>
>   GIT_TRACE_HOOK='
>     f() {
>       case "$1 $2" in
>       git pack-objects)
>         tee /tmp/foo.out | "$@"
>         ;;
>       esac
>     }; f
>   '
>
> That is not 100% correct (you would miss "git --some-arg pack-objects"),

> but it is probably fine in practice for debugging sessions. It is a bit
> more complicated to use, but I really like the flexibility (I can
> imagine that "GIT_TRACE_HOOK=gdbserver localhost:1234" would come in
> handy).
>
>> Because this script is given full command line, it can decide to trace
>> something if the command name is matched (or arguments are matched) or
>> just execute the original command. It's more flexible that trace.*
>> config keys. We also have an opportunity to replace builtin commands,
>> like pack-objects, in command pipeline in fetch or push with something
>> else, to inject errors or whatever. It can be done manually, but it's
>> not easy or convenient.
>
> My other motive for trace.* was that we could have something like
> "trace.prune", and have git-prune provide verbose debugging information.
> We have custom patches like that on GitHub servers, which we've used to
> debug occasional weirdness (e.g., you find that an object is missing
> from a repo, but you have no clue why it went away; was it never there,
> did somebody prune it, did it get dropped from a pack?).
>
> I can send those upstream, but it would be nice not to introduce a
> totally separate tracing facility when trace_* is so close. But it
> needs:
>
>   1. To be enabled by config, not environment.
>
>   2. To support some basic output filename flexibility so the output can
>      be organized (we write the equivalent of GIT_TRACE_FOO to
>      $GIT_DIR/ghlog_foo/YYYY-MM-DD/YYYY-MM-DDTHH:MM:SS.PID).
>
> For (1), we could just load trace.* in git_default_config; you couldn't
> use it with any "early" tracing that happens before then, but I think in
> practice it would be fine for most traces.
>
> For (2), I think we could accomplish that with %-placeholders (like my
> earlier patch), and the ability to write relative paths into $GIT_DIR
> (again, you couldn't do this for "early" traces, but you could for other
> stuff).
>
> Or we could just do nothing. I'm not sure if anybody else is actually
> interested in verbose-logging patches like these.