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

[John Garry <john.garry@huawei.com>]

On 23/03/2022 20:50, Glen Choo wrote:
> Hi John, I'm not a big expert on Make (or even Git :p) but think I can
> answer some of your questions.
> 
> John Garry <john.garry@huawei.com> writes:
> 
>> Hi guys,
>>
>> I have a question about which files in the .git folder are updated as we
>> change the head commit. I could check the codebase myself but prob will
>> make a mistake and maybe some expert would be so kind as to just kindly
>> tell me.
>>
>> 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.
> 
> You're on the right track because if .git/HEAD doesn't change, the HEAD
> is still pointing to either the same commit (if in detached HEAD) or
> branch (if not in detached HEAD).
> 
> The problem is that when HEAD points to a branch, the branch's 'head
> commit' can change, implicitly changing the commit that .git/HEAD
> actually points to.
> 

ok

>> The problem is that actions like git cherry-pick and git reset --hard
>> HEAD^ may not update .git/HEAD (so don't trigger a rebuild).
> 
> That's what's happening here. If your HEAD is pointing to
> "refs/heads/main", "cherry-pick" and "reset --hard" will change where
> "refs/heads/main" points, but HEAD still points to "refs/heads/main".
> 
> Branches are often located in "loose form" under .git/refs/heads/*,
> *but* refs can also be stored in a "packed form" under .git/packed-refs
> (https://git-scm.com/docs/git-pack-refs). There is also a relatively
> new ref storage format called reftable that I'm not familiar with, but
> presumably stores its refs in other locations too.
> 
> All of this is to say that depending on specific files under .git/
> exposes you to a lot of Git internals that really aren't meant for
> external consumption. I suggest finding a different approach than
> watching Git-internal files for changes.

Thanks for the detailed explanation.

I agree with your suggestion to find a new method (which may be 
reverting to the old method to just run proper git commands).

> 
>> Is there some more suitable file(s) which we could use as a dependency?
>>   From my limited experimentation, .git/index seems to always update when
>> the changing head commit.
> 
> I think users typically expect the index (and therefore .git/index) to
> change when the head commit changes, but this isn't always true e.g.
> "reset --soft" will move the branch without changing the index.

ok, good to know. I was going to try that...

> 
> P.S. Even if you knew the value of .git/index or .git/HEAD, that
> wouldn't tell you whether or not the files in your working directory
> have changed, so I'm not sure if you want to use either as Makefile
> dependency.
> 

ok, thanks again for all this info,
John