From: John Garry Date: Thu, 24 Mar 2022 12:59:45 GMT Subject: Re: [Question] .git folder file updates for changing head commit Message-ID: <5ceff005-bbe4-37f5-c453-17008c2167bc@huawei.com> In-Reply-To: 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 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