From: Jeff King <peff@peff.net>
Date: Tue, 01 Sep 2015 22:22:30 GMT
Subject: Re: [PATCH 2/2] pkt-line: show packets in async processes as "sideband"
Message-ID: <20150901222230.GB7862@sigill.intra.peff.net>
In-Reply-To: <xmqq1tehrdoa.fsf@gitster.mtv.corp.google.com>

On Tue, Sep 01, 2015 at 03:13:25PM -0700, Junio C Hamano wrote:

> > So instead, let us just assume that any async process is
> > handling sideband data. That's always true now, and is
> > likely to remain so in the future.
> 
> Hmm, does Stefan's thread-pool thing interact with this decision in
> any way?

I don't think so. It is true that:

  start_async(...);
  finish_async(...);
  pthread_create(foo);

would incorrectly report in_async() inside the foo function (because we
know that async has been kicked off, and we know that we are in a thread
that is not the main one).

But I don't think it matters in the current code base, because we tend
to use async for I/O tasks like muxing/demuxing, and real threads in
CPU-heavy tasks like pack-objects.

Still, I admit it is a little gross, and may be a problem in the future.

An alternate approach would be to have the async system provide a
thread-local storage abstraction. With pthreads, we'd build on
pthread_setspecific(), and without, it becomes a simple global variable.

That's more work, but a lot less error-prone, and it may come in handy
in the future.

-Peff