Does git revert also use the 3-way-merge?

Question

When I run git revert, it can happen, that a conflict occurs. Does git rely on the 3-way-merge, as it is depicted in the question merge internals (cf. table below) also for revert?

What is the merge base for a revert? In What are the three files in a 3-way merge for interactive rebasing using git and meld? it is quite clear, but its hard to imagine this for a revert.

A - B - C - D - C^-1

(If I want to revert C at the end.)

Answer 1

Yes, there is a base. (Side note: this code has changed a lot since I looked at it years ago. I picked up some of this for my recent cherry-pick answer, which you have linked here.)

Both git cherry-pick and git revert are implemented by the same source files (builtin/revert.c and sequencer.c).

As you say, the tricky part is deciding what to fake up for the merge base. In your example, we're undoing the B-to-C diffs. Here's the actual source code (in sequencer.c), stripped down somewhat:

if (opts->action == REPLAY_REVERT) {
        base = commit;
        base_label = msg.label;
        next = parent;
        next_label = msg.parent_label;
        strbuf_addstr(&msgbuf, "Revert \"");
        strbuf_addstr(&msgbuf, msg.subject);
        strbuf_addstr(&msgbuf, "\"\n\nThis reverts commit ");
        strbuf_addstr(&msgbuf, oid_to_hex(&commit->object.oid));

        if (commit->parents && commit->parents->next) {
                strbuf_addstr(&msgbuf, ", reversing\nchanges made to ");
                strbuf_addstr(&msgbuf, oid_to_hex(&parent->object.oid));
        }
        strbuf_addstr(&msgbuf, ".\n");
} else {

[this is the cherry-pick case, included just for completeness]

        const char *p;

        base = parent;
        base_label = msg.parent_label;
        next = commit;
        next_label = msg.label;

When we enter here, commit points to data for C and parent points to data for B. The assignment to variable base is what sets the merge base, and next-vs-base is what to bring in. For cherry-pick, the commit's parent (possibly chosen via -m) is the merge base. For revert, the commit itself is the merge base and the parent (again possibly from -m) is what-to-bring-in.

The other way to get the same effect (which is how this was done many years ago, and until recently, I thought this was still being used) is to reverse-apply a commit as produced by git format-patch. In this case, the constructed base version is the second hash (the B part from the A..B part of a textual diff):

/*
 * This represents a "patch" to a file, both metainfo changes
 * such as creation/deletion, filemode and content changes represented
 * as a series of fragments.
 */
struct patch {
[snip]
    char old_sha1_prefix[41];
    char new_sha1_prefix[41];

static void reverse_patches(struct patch *p)
{
[snip]
            swap(p->old_sha1_prefix, p->new_sha1_prefix);

The reverse_patches function is called after extracting the text into a series of patches, i.e., after the code that extracts the hashes from the index lines, putting the A and B parts into the old and new prefix fields. Then (after reverse_patches), when actually applying each patch, git uses the saved old and new sha1 values to fake a 3-way merge (if git am is given --3way). So by reverse-applying a text patch, we would get the new file as the base and the original as the target, just as with the sequencer.c code.