Subject: Re: [PATCH v2 04/13] fast-rebase: write conflict state to working tree, index, and HEAD

From: Derrick Stolee <[email-masked]>
Date: Mon, 17 May 2021 09:32:14 -0400

On 5/3/21 10:12 PM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <[email-masked]>
> 
> Previously, when fast-rebase hit a conflict, it simply aborted and left
> HEAD, the index, and the working tree where they were before the
> operation started.  While fast-rebase does not support restarting from a
> conflicted state, write the conflicted state out anyway as it gives us a
> way to see what the conflicts are and write tests that check for them.
> 
> This will be important in the upcoming commits, because sequencer.c is
> only superficially integrated with merge-ort.c; in particular, it calls
> merge_switch_to_result() after EACH merge instead of only calling it at
> the end of all the sequence of merges (or when a conflict is hit).  This
> not only causes needless updates to the working copy and index, but also
> causes all intermediate data to be freed and tossed, preventing caching
> information from one merge to the next.  However, integrating
> sequencer.c more deeply with merge-ort.c is a big task, and making this
> small extension to fast-rebase.c provides us with a simple way to test
> the edge and corner cases that we want to make sure continue working.

This seems a noble goal. I'm worried about the ramifications of such
a change, specifically about the state an automated process would be in
after hitting a conflict.

If I understand correctly, then the old code would abort the rebase and
go back to the previous HEAD location. The new code will pause the
rebase at the previous commit and leave the conflict markers in the
working directory.

The critical change is here:

> -	strbuf_addf(&reflog_msg, "finish rebase %s onto %s",
> -		    oid_to_hex(&last_picked_commit->object.oid),
> -		    oid_to_hex(&last_commit->object.oid));
> -	if (update_ref(reflog_msg.buf, branch_name.buf,
> -		       &last_commit->object.oid,
> -		       &last_picked_commit->object.oid,
> -		       REF_NO_DEREF, UPDATE_REFS_MSG_ON_ERR)) {
> -		error(_("could not update %s"), argv[4]);
> -		die("Failed to update %s", argv[4]);
> +	if (result.clean) {
> +		fprintf(stderr, "\nDone.\n");
> +		strbuf_addf(&reflog_msg, "finish rebase %s onto %s",
> +			    oid_to_hex(&last_picked_commit->object.oid),
> +			    oid_to_hex(&last_commit->object.oid));
> +		if (update_ref(reflog_msg.buf, branch_name.buf,
> +			       &last_commit->object.oid,
> +			       &last_picked_commit->object.oid,
> +			       REF_NO_DEREF, UPDATE_REFS_MSG_ON_ERR)) {
> +			error(_("could not update %s"), argv[4]);
> +			die("Failed to update %s", argv[4]);
> +		}
> +		if (create_symref("HEAD", branch_name.buf, reflog_msg.buf) < 0)
> +			die(_("unable to update HEAD"));
> +		strbuf_release(&reflog_msg);
> +		strbuf_release(&branch_name);
> +
> +		prime_cache_tree(the_repository, the_repository->index,
> +				 result.tree);
> +	} else {
> +		fprintf(stderr, "\nAborting: Hit a conflict.\n");
> +		strbuf_addf(&reflog_msg, "rebase progress up to %s",
> +			    oid_to_hex(&last_picked_commit->object.oid));
> +		if (update_ref(reflog_msg.buf, "HEAD",
> +			       &last_commit->object.oid,
> +			       &head,
> +			       REF_NO_DEREF, UPDATE_REFS_MSG_ON_ERR)) {
> +			error(_("could not update %s"), argv[4]);
> +			die("Failed to update %s", argv[4]);
> +		}
>  	}
> -	if (create_symref("HEAD", branch_name.buf, reflog_msg.buf) < 0)
> -		die(_("unable to update HEAD"));
> -	strbuf_release(&reflog_msg);
> -	strbuf_release(&branch_name);
> -
> -	prime_cache_tree(the_repository, the_repository->index, result.tree);

So perhaps this could use a test case to demonstrate the change in
behavior? Such a test would want to be created before this commit, then
the behavior change is provided as an edit to the test in this commit.

But maybe I'm misunderstanding the change here and such a test is
inappropriate.

Thanks,
-Stolee