Subject: Re: Feature request: Allow to update commit ID in messages when rebasing

From: Jakub Narebski <[email-masked]>
Date: Thu, 18 Apr 2019 19:32:26 +0200

Junio C Hamano <[email-masked]> writes:
> Ævar Arnfjörð Bjarmason <[email-masked]> writes:
>> On Wed, Apr 17 2019, Giuseppe Crinò wrote:
>>
>>> The feature I'm asking is to add an extra-step during rebasing,
>>> checking whether there's a reference to a commit that's not going to
>>> be included in history and asks the user whether the heuristics is
>>> correct and if she wants to update those references.
>>>
>>> Scenario: it can happen for a commit message to contain the ID of an
>>> ancestor commit. A typical example is a commit with the message
>>> "revert 01a9fe8". If 01a9fe8 and the commit that reverts it are
>>> involved in a rebase the message "revert 01a9fe8" is no longer valid
>>> -- the old 01a9fe8 has now a different hash. This will most likely be
>>> ignored by the person who's rebasing but will let the other people
>>> reading history confused.
>>
>> This would be useful. Done properly we'd need some machinery/command to
>> extract the commit id parts from the free-text of the commit
>> message. That would be useful for other parts of git, e.g. as discussed
>> here:
>> https://public-inbox.org/git/[email-masked]/
>
> That's a helpful input.
>
> But in general we do not have an infrastructure to systematically
> keep track of "this commit was rewritten to produce that other
> commit", so even if a mention of an old/superseded commit can be
> identified reliably, there is no reliable source to rewrite it to
> the name of the corresponding commit in the new world.
>
> For that mapping, we'd need something like the "git change/evolve"
> Stefan Xenos was working on, which hasn't materialized.

Well, what about limiting changes and rewriting only to the commits
being rewritten by [interactive] rebase?  I mean that we would rewrite
"revert 01a9fe8" only if:

a.) the commit with this message is undergoing rewrite
b.) the commit 01a9fe8 is undergoing rewrite in the same command

We could use the infrastructure from git-filter-branch for this.

It is serious limitation, but that might be good enough for Giuseppe
Crinò use case.  Though for example there is a question what to do if
referred-to commit (01a9fe8 in the example) is simply dropped, or is
gets split in two?  Ask user?

Another possibility would be to provide a command line option to rebase
which would automatically generate replacements (in git-replace meaning)
from old pre-rebase name to new post-rebase name (assuming no splitting,
no dropping commits).  This would make references just work... well, as
long as refs/replace/* are in place (they are not copied by default).

On the other hand some of our performance-improving features, like the
commit-graph, do not work if there are replacements.

Best,
--
Jakub Narębski