Git pull without fetch

Dear maintainers,

I use the git-maintenance prefetch task with a certain repo to periodically fetch updates from the remote. I'm trying to figure out a way to have git "complete" a prefetch on command, via an option, alias, or other extension that mimics the behavior of git pull, but does not fetch from the remote. By comparison, `git pull` or `git fetch && git rebase` would fetch the remote and fast-forward my local copy to match, but I don't want to fetch the remote. Instead I want to fast-forward my current branch (and the remote tracking branch) to the prefetched tip as if git pull had been run at the time of the last prefetch.

For example, suppose my local copy has a branch master that tracks the remote branch master from origin. Suppose that the local copy (and the remote tracking branch) have commits A--B, the remote has commits A--B--C--D, and commit C has been prefetched and is pointed to by refs/prefetch/remotes/origin/master. The desired effect could be accomplished, I think, by:

git update-ref refs/remotes/origin/master refs/prefetch/remotes/origin/master refs/heads/master && git rebase origin/master master

My question is, how can I calculate the correct remote tracking branch to accomplish this operation in the general case, just as git pull without argument would have done? Or is there a simpler way to achieve my goal?

Cheers, Ronan

February 17, 2023 6:46 PM, "Chris Torek" <chris.torek@gmail.com> wrote:

No it isn't. Sorry, but I think you have misunderstood the problem. I'm well aware of the behavior of git pull.

In the scenario described, `git rebase` will always report "Already up to date" because the head of the current branch and the head of the remote tracking branch are the same. The prefetched reference lives under refs/prefetch/* and is unnoticed by git rebase. This is why I used `git update-ref ...` first: to update the remote tracking branch from the prefetched copy.

ronan@rjp.ie writes:

That is very much working as designed. Prefetching is merely a performance thing---until the user explicitly says "I want to update my view of the other side by fetching and updating the remote tracking branches" with "git fetch", it will not disrupt refs/remotes/origin/* refs, and because of prefetching, when the user finally gets around to say that, observation of the other side with a real fetch would go faster, requiring fewer objects to be transmit.

And if somebody wants to lie to the system and say "pretend we did not fetch anything", then yes, _knowing_ that implementation detail of refs/prefetch/* hierarchy and taking advantage of it by using "update-ref" would be a way to achieve that.

Yes, I realize. I'm not reporting a bug. I guess I just think it would be handy to more conveniently reach the prefetched updates when I'm offline, and I gather from conversation there is no method builtin to git.

For now I've settled on this to be "pretend fetch":

$ git for-each-ref refs/prefetch/ --format='update refs/%(refname:lstrip=2) (refname)' | git update-ref --stdin

which I can follow-up with a no-argument `git rebase`, similar to a `git pull` if the remote was available.

In my original question I had intended only to update the upstream of my current branch, but I've decided that's not necessary for me. However, I have found that it is possible to get the upstream reference for the current branch like so:

$ git rev-parse --symbolic-full-name '@{upstream}' refs/remotes/origin/master

using this it should be possible to find the corresponding prefetched reference in refs/prefetch if it exists and only update that reference without parsing "branch.<name>.remote", "remote.<name>.fetch", and "branch.<name>.merge" myself.