Re: [Question] .git folder file updates for changing head commit

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Mar 27, 2022, 15:48 UTC
Message-ID: <220327.86tubje6bn.gmgdl@evledraar.gmail.com>
In-Reply-To: <YjuCSpAHD6xHkb5G@nand.local>

On Wed, Mar 23 2022, Taylor Blau wrote:

Show 18 quoted lines

> On Wed, Mar 23, 2022 at 03:19:06PM +0000, John Garry wrote:
>> For building the linux perf tool we use the git head commit id as part of
>> the tool version sting. To save time in re-building, the Makefile rule has a
>> dependency on .git/HEAD for rebuilding. An alternative approach would be to
>> compare git log output to check current versus previous build head commit,
>> but that is seen as inefficient time-wise.
>
> Having a Makefile recipe that depends on $GIT_DIR/HEAD seems strange to
> me.
>
> Presumably your Makefile rules would map out which parts of your program
> depend on each other, and would get invalidated when the source itself
> changes, no?
>
> Perhaps you also care about the commit you're building from in order to
> embed something into your program. But it seems like you could inject
> the output of "git rev-parse HEAD" when you construct the version
> identifier whenever you do need to rebuild.

Our very own build process for git.git relies on this, see how version.o needs a GIT-VERSION-FILE, which we generate by shelling out and including.

It is unfortunate that we don't have an advertised way to do this, with the ref backend I think trickery using $(file) to read the HEAD will work to do it in pure-make, i.e. you'd need to parse it to see if it's a symref, then depend on the target it points to.

Or you could recursively depend on a glob of the whole refspace for generating the version file ...

It would be nice if we had a way to guarantee that we'd write some file on HEAD updates, AFAIK not even the new reference-transaction hook will do that (due to "git reset --hard" and friends).

And yes, this does actually matter. There's a huge performance difference between a Makefile that needs to shell out for every little thing.

I've been optimizing our own Makefile incrementally from running in ~500ms down to 100-200ms for noop runs over over the last few months. It's possible to get it down to 10-20ms at least by getting rid of the remaining shell-outs.

That makes a big difference when e.g. using "make" with "git rebase -i -x".