Re: multiple working directories for long-running builds (was: "git merge" merges too much!)

From: Dmitry Potapov <dpotapov@gmail.com>
Date: Dec 1, 2009, 18:51 UTC
Message-ID: <20091201185114.GC11235@dpotapov.dyndns.org>
In-Reply-To: <m1NFX19-000kn4C@most.weird.com>

On Tue, Dec 01, 2009 at 12:59:58PM -0500, Greg A. Woods wrote:

Show 19 quoted lines

> At Tue, 1 Dec 2009 08:47:34 +0300, Dmitry Potapov <dpotapov@gmail.com> wrote:
> Subject: Re: "git merge" merges too much!
> > 
> > On Mon, Nov 30, 2009 at 07:24:14PM -0500, Greg A. Woods wrote:
> > > 
> > > Things get even weirder if you happen to be playing with older branches
> > > too -- most build tools don't have ability to follow files that go back
> > > in time as they assume any product files newer than the sources are
> > > already up-to-date, no matter how much older the sources might become on
> > > a second build.
> > 
> > No, files do not go back in time when you switch between branches. The
> > timestamp on files is the time when they are written to your working
> > tree
> 
> Hmmm, I didn't really say anything in particular about file timestamps
> -- I meant the file content may go back in time.  More correctly I
> should have said that the file content may become inconsistent with the
> state of other files that have just been compiled.

There is no difference of content going back in time or forth. If a file is changed, any decent build system should recompile the corresponding files. If the build does not handle dependencies properly, you can end up with inconsistent state just by editing some files.

> If the timestamps do not get set back to commit time, but rather are
> simply updated to move the last modify time to the time each change is
> made to a working file (which is as you said, to be expected),

More precisely, Git does not anything about modification time during checkout. The system automatically updates the modification time when a file is written, and Git does not mess with it.

> regardless of whether its content goes back in time or not, then this
> may or may not help a currently running build to figure out what really
> needs to be re-compiled.

Obviously, switching branches while running build may produce very confusing results, but it is not any different than editing files by hands during built -- any concurrent modification may confuse the build system.

> I just disagreed that "git archive" was a reasonable alternative to
> leaving the working directory alone during the entire time of the build.

Using "git archive" allows you avoid running long time procedure such as full clean build and testing in the working tree. Also, it is guaranteed that you test exactly what you put in Git and some other garbage in your working tree does not affect the result. But my point was that switching between branches and recompile a few changed files may be faster than going to another working tree.

Dmitry