From: Junio C Hamano <gitster@pobox.com>
Date: Sun, 20 Jul 2008 18:31:38 GMT
Subject: Re: [PATCH] Enable git rev-list to parse --quiet
Message-ID: <7vwsjg76gl.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <20080720120437.GC15586@mail.local.tull.net>

Nick Andrew <nick@nick-andrew.net> writes:

> Exiting a process from within a callback function seems to me to violate
> the principle of least surprise.

Huh?  Who is surprised?

I do not know who taught you that "do not exit in a callback" dogma, but I
suspect it was misrepresented when it was taught to you.

A library that calls your function back could be structured this way:

	lib() {
        	perform some set-up that affects external world;
                call your callback function;
                clean-up the effect of previous set-up action;
	}

and exiting from your callback function is not a good idea as it prevents
the library from doing the necessary clean-up in such a case.

But that is true just in a(n extremely) general case.  Your generalization
is not particularly useful, methinks, and use of exit(0) in the patch is
very well justified (rather, I do not think they even need justifying).

 - The callback you are looking at is not a general purpose callback for
   other program's use, but written for a specific use of rev-list;

 - The purpose of that exit(0) is to signal "there is something" as
   quickly as possible, which was what you wanted out of rev-list;

 - Revision traversal is a read-only operation and we know that there is
   no externally visible set-up done in the function you are calling to
   get your callback called, that needs cleaning up later --- this is not
   expected to change, as there are longstanding existing callback
   functions supplied to traverse_commit_list() that die() upon seeing
   errors already.