[v2,5/7] rebase: factor out branch_base calculation

Commit Message

Phillip Wood Sept. 7, 2022, 2:37 p.m. UTC

From: Phillip Wood <[e-mail]>

Separate out calculating the merge base between 'onto' and 'HEAD' from
the check for whether we can fast-forward or not. This means we can skip
the fast-forward checks when the rebase is forced and avoid calculating
the merge-base between 'HEAD' and 'onto' when --keep-base is given.

Signed-off-by: Phillip Wood <[e-mail]>
---
 builtin/rebase.c | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

Comments

Junio C Hamano Sept. 7, 2022, 6:12 p.m. UTC | #1

"Phillip Wood via GitGitGadget" <[e-mail]> writes:

> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index dd5e0e1feb6..b5c78ce1fb0 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -871,13 +871,9 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
>  	struct commit_list *merge_bases = NULL;
>  	int res = 0;
>  
> -	merge_bases = get_merge_bases(onto, head);
> -	if (!merge_bases || merge_bases->next) {
> -		oidcpy(branch_base, null_oid());
> +	if (is_null_oid(branch_base))
>  		goto done;

Naïvely, I would have expected the condition of branch_base being
null to mean "the caller doesn't have branch base specified (like
parsing --onto), so we would need to compute one" and this
"branch_base is null, so leave without doing anything" looked
confusing at first.

After computing options.onto_name (given either with --onto or
falling back to .upstream_name), we make sure branch_base is
correctly filled, either from an explicit A...B notation or a single
committish object name given and converted with a call to
fill_branch_base().  And fill_branch_base() uses null object name as
a signal that it punted due to multiple merge bases.

> +static void fill_branch_base(struct rebase_options *options,
> +			    struct object_id *branch_base)
> +{
> +	struct commit_list *merge_bases = NULL;
> +
> +	merge_bases = get_merge_bases(options->onto, options->orig_head);
> +	if (!merge_bases || merge_bases->next)
> +		oidcpy(branch_base, null_oid());
> +	else
> +		oidcpy(branch_base, &merge_bases->item->object.oid);
> +
> +	free_commit_list(merge_bases);
> +}

So it is doing the same thing as the original, but unlike the
original, it is not immediately obvious why "goto done" is the right
thing to do here.  Perhaps it deserves a comment, e.g.

	if (is_null_oid(branch_base))
		goto done; /* fill_branch_base() already checked and punted */

or something.

Thanks.

Patch

diff --git a/builtin/rebase.c b/builtin/rebase.c
index dd5e0e1feb6..b5c78ce1fb0 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -871,13 +871,9 @@  static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	struct commit_list *merge_bases = NULL;
 	int res = 0;
 
-	merge_bases = get_merge_bases(onto, head);
-	if (!merge_bases || merge_bases->next) {
-		oidcpy(branch_base, null_oid());
+	if (is_null_oid(branch_base))
 		goto done;
-	}
 
-	oidcpy(branch_base, &merge_bases->item->object.oid);
 	if (!oideq(branch_base, &onto->object.oid))
 		goto done;
 
@@ -887,7 +883,6 @@  static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	if (!upstream)
 		goto done;
 
-	free_commit_list(merge_bases);
 	merge_bases = get_merge_bases(upstream, head);
 	if (!merge_bases || merge_bases->next)
 		goto done;
@@ -902,6 +897,20 @@  done:
 	return res && is_linear_history(onto, head);
 }
 
+static void fill_branch_base(struct rebase_options *options,
+			    struct object_id *branch_base)
+{
+	struct commit_list *merge_bases = NULL;
+
+	merge_bases = get_merge_bases(options->onto, options->orig_head);
+	if (!merge_bases || merge_bases->next)
+		oidcpy(branch_base, null_oid());
+	else
+		oidcpy(branch_base, &merge_bases->item->object.oid);
+
+	free_commit_list(merge_bases);
+}
+
 static int parse_opt_am(const struct option *opt, const char *arg, int unset)
 {
 	struct rebase_options *opts = opt->value;
@@ -1667,8 +1676,8 @@  int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (!options.onto)
 			die(_("Does not point to a valid commit '%s'"),
 				options.onto_name);
+		fill_branch_base(&options, &branch_base);
 	}
-
 	if (options.fork_point > 0)
 		options.restrict_revision =
 			get_fork_point(options.upstream_name, options.orig_head);
@@ -1696,13 +1705,10 @@  int cmd_rebase(int argc, const char **argv, const char *prefix)
 	 * Check if we are already based on onto with linear history,
 	 * in which case we could fast-forward without replacing the commits
 	 * with new commits recreated by replaying their changes.
-	 *
-	 * Note that can_fast_forward() initializes branch_base, so we have to
-	 * call it before checking allow_preemptive_ff.
 	 */
-	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
-		    options.orig_head, &branch_base) &&
-	    allow_preemptive_ff) {
+	if (allow_preemptive_ff &&
+	    can_fast_forward(options.onto, options.upstream, options.restrict_revision,
+			     options.orig_head, &branch_base)) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {