Method buildMergeObject

db/branch.go:294–338

(ctx context.Context, parent *branches.Config, retries int, author, message string)

}

func (b *Branch) buildMergeObject(ctx context.Context, parent *branches.Config, retries int, author, message string) (*commits.Object, error) {
	childPath, err := b.pool.commits.Path(ctx, b.Commit)
	if err != nil {
		return nil, err
	}
	parentPath, err := b.pool.commits.Path(ctx, parent.Commit)
	if err != nil {
		return nil, err
	}
	baseID := commonAncestor(parentPath, childPath)
	if baseID == ksuid.Nil {
		//XXX this shouldn't happen because because all of the branches
		// should live in a single tree.
		//XXX hmm, except if you branch main when it is empty...?
		// we shoudl detect this and not allow it...?
		return nil, errors.New("system error: cannot locate common ancestor for branch merge")
	}
	// Compute the snapshot of the common ancestor then compute patches
	// along each branch and make sure the two patches do not have a
	// delete conflict.  For now, this is the only kind of merge update
	// conflict we detect.
	base, err := b.pool.commits.Snapshot(ctx, baseID)
	if err != nil {
		return nil, err
	}
	childPatch, err := b.pool.commits.PatchOfPath(ctx, base, baseID, b.Commit)
	if err != nil {
		return nil, err
	}
	parentPatch, err := b.pool.commits.PatchOfPath(ctx, base, baseID, parent.Commit)
	if err != nil {
		return nil, err
	}
	if message == "" {
		message = fmt.Sprintf("merged %q into %q", b.Name, parent.Name)
	}
	// Now compute the diff between the parent patch and the child patch so that
	// the diff patch will reflect the changes from the child into the parent.
	// Diff() will also check for delete conflicts.
	diff, err := commits.Diff(parentPatch, childPatch)
	if err != nil {
		return nil, fmt.Errorf("error merging %q into %q: %w", b.Name, parent.Name, err)
	}
	return diff.NewCommitObject(parent.Commit, retries, author, message, super.Null), nil
}

func commonAncestor(a, b []ksuid.KSUID) ksuid.KSUID {
	m := make(map[ksuid.KSUID]struct{})