Re: Plugin mechanism(s) for Git?

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Jul 15, 2016, 12:46 UTC
Message-ID: <CACBZZX4TA0Ljf2aNZtZ5Sj8fFj-vhoLKq5RP5YbKSS9o9Ljg+A@mail.gmail.com>
In-Reply-To: <20160715121838.GA21968@sigill.intra.peff.net>

On Fri, Jul 15, 2016 at 2:18 PM, Jeff King <peff@peff.net> wrote:

Show 7 quoted lines

> Some features, like the index-helper, aren't quite so easy. One reason
> is that its data needs to persist as a cache between multiple git
> invocations. In general, I think it would be nice to solve that by
> communicating via on-disk files, rather than a running daemon (just
> because it has fewer moving parts). But that's only half of it for
> index-helper. It needs to monitor inotify while git isn't running at
> all; so it really _does_ need some kind of long-running daemon.

This *may* have changed in the recent versions of the series, but I'm fairly sure and for what it's worth to this discussion, that's not what the index-helper does. It's there to keep the index file in memory instead of reading it from disk.

It can *also* if you "git update-index --watchman" spawn a watchman daemon in the background, which is the thing that'll be doing the inotify calls and needs to stay persistent, the index-helper then communicates with the watchman daemon "what changed since X?" to compute a new index when requested.

I.e. if you:

    $ git config indexhelper.autorun true
    $ git update-index --watchman

You'll end up with both:

    $ ps ax|grep -e index-helper -e watchman|grep -v grep
    61958 ?        Ss     0:00 git-index-helper --detach --autorun
    62813 ?        Sl     0:04 watchman get-sockname

So for the purposes of your example the index-helper is one of those things that doesn't "need" to persist as a demon, only the third party watchman daemon does, but the index-helper speeds up reading the index since it keeps it alive in memory.

All or some of the above may be wrong, Nguyen/David, any comments?