A Change to Commit IDs Too Ridiculous to Consider?

Those of us who write instructional material about Git all face the same problem. This is that we can't write step by step instructions that show the results of making a commit because users will always see different commit IDs. This is fundamental to the design of Git.

Even if the instructional material tells users to use standard author and committer information, (e.g. [email protected]) and shows the text of the file being committed and the commit message to add, the resulting commit ID will differ from reader to reader since the commit will presumably take place at different times.

What if it were possible, for instructional purposes only, to somehow tell Git to relax this requirement. By this I mean, the commit date would *not* be included when constructing the commit ID. This would allow tutorials to show exactly what to expect to see when running commands.

I realize that questions would remain such as how to turn on this behavior (e.g. command line flags, environment variables) and whether 'git log' (and maybe other commands) should somehow distinguish these mutant commits. There would probably be other issues to consider.

Again, this is for instructional purposes only, and only when the committer explicitly chooses to use this option. I'm *not* proposing a general change to Git's behavior.

Is such a thing to ridiculous to even consider? Is there a better way to achieve the same result?

Jon Forrest

Please try to keep to the 80-character lines.

W dniu 2016-07-24 o 20:12, Jon Forrest pisze:

There are two options: first, to tell the reader upfront that objects id would be different / would change. This has the advantage that you do not need to update those objects when you change instructions in the middle. Note that commit objects are not the only things that change; for example the result of ls -l would also be slightly different.

Another possibility is to set authordate and committerdate to some specified time by the way of appropriate environment variables.

What I think you don't realize is that "commit" objects are not treated in any way special. Object identifiers of all objects are SHA-1 hash of uncompressed loose representation of said object (type + length + contents).

Well, you could not record dates in commit object, but I think Git considers such objects broken.

IMVHO it would require heavy surgery of Git for little benefit (see the beginning of reply for alternate solutions).

Jakub Narębski

And what is the problem with that, if you are doing it with instructional purposes? Let's assume that this helps and not confuses later when the commits *do* change. What is the problem you face?

I mean, for some examples you can use HEAD, HEAD^, HEAD~4, etc. and that always works, no matter the commit id. In which cases do you want/need the commit ids to be equal? Can you be more specific?

Thanks a lot,
Rodrigo

Sorry.

That sounds like a great idea. Assuming it
works the way I envision, this wouldn't require
any changes to the source code.

I know this, but I thought that commit object IDs were the only ones that included a date in what gets run through the SHA-1 hash function. If there are others, then you're right - they'd need to be included in this proposal.

You mean that Git could, after the fact, detect commit IDs that didn't include a date? If this is true, then your idea of using fixed dates from environment variables would be the only way to do this.

Even using your environment variable solution that wouldn't require any code changes?

Jon

A lot of instructional material contains stuff like "Do [xxx] and you'll see [zzz]. If you don't then something went wrong so try to figure out what happened and do it again."

Git, as it stands, for good reason doesn't allow this approach.

I don't think a Git beginner, when using a version of Git that somehow works the way I proposed, will be confused. The fact that performing the same steps results in the same commit IDs won't be something that they'll care about or even notice. The material can include a callout mentioning the difference between "real" Git and "learners" Git.

This will work in some cases, but should come later in a Git book. But, in many cases using relative commit IDs, rather than absolute, will be less clear (I believe).

Sure. Take a look at the 2nd or 3rd chapter of Pro Git Reedited, 2nd Edition (or just Pro Git 2nd Edition - it doesn't matter). You see lots of output showing 'git commit' commands and the commit IDs that result. I suspect you'd see the same in almost any book about Git.

Jon

W dniu 2016-07-24 o 21:20, Jon Forrest pisze:

This would however require for user to write more.
The environment variables are GIT_AUTHOR_DATE and GIT_COMMITTER_DATE; their format is described in the "DATE FORMATS" section in the git-commit(1) manpage.

And it is not something that the user would do when working with Git themselves, for their own project.

The problem is that many function in Git are object-type agnostic. Changing how 'commit' objects are treated would require heavy code surgery... unless done as filter (see below), but even then extra code would be needed, for small benefit and large maintenance burden.

Now that I think about this, it could be done when displaying object names (in git log and git show), replacing true commit object with SHA-1 of those objects with dates stripped. Still needs work.

I think^H^H git fsck can check that objects are well formed, and warn if they are not.

$ git fsck
error in commit 6deb0829fecdf1feab0cc7c66061a92a93cb19e7: 
  missingSpaceBeforeDate: invalid author/committer line - missing space before date
error in commit 762f28c2567c07d378d485c3e2a498947d49f406: 
  badDate: invalid author/committer line - bad date

No, this do not need no changes to git code, of course.

Jakub Narębski

To follow up, Jakub's approach works great without requiring any changes to Git.

For example, the following test script always
produces the same commit ID:

----
export GIT_AUTHOR_DATE=2005-04-07T22:13:13
export GIT_COMMITTER_DATE=2005-04-07T22:13:13
mkdir -p /tmp/test
cd /tmp/test
rm -rf .git
git init
echo "Test" > README
git add README
git commit -m "test"
git log
----

As expected, commenting out the 2 export lines results in different commit IDs each time.

Case closed.

Thanks, Jakub!

Jon Forrest

I would think that the early-stage learners are better served that it is the norm, not anything strange, that the commit object name would be different when you do two identical sequence from scratch. Forcing them to know GIT_*_DATE variables, just to give them an impression as if setting of them is part of any normal workflow (or more importantly, stable commit IDs made by different people at different times is something expected), is doing them double disservice, IMHO.

Ehh, sent without proofreading. The above should say "... are better served if the book taught that it is the norm, ...".

From: "Jon Forrest" <[email protected]>

You may want to look at how the test suite handles the need for well defined commit sequences.

It's not something I've really studied, but I am aware of the test_tick to increment the time and similar helpers.

There is a big learning step that needs to be got over by many beginners who have no concept of a DVCS, nor of multiple master copies (which to most is an oxymoron!), nor why the sha is a good solution and serial numbers are a bad solution!.

Being able to do a few "Hello World" commits starting at unix t=0, and then progressing on to see how they differ when it's unix=now time, or they use their own user IDs could be a useful step for those that need it.

Philip