Re: git pull --rebase should use fast forward merge if possible

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

>> Is it possible to change the behavior of "git pull 
>> --rebase=true|preserve|interactive" to use a fast forward merge if the 
>> remote branch is ahead and the local branch contains no new commits? 
>
> Interesting.  I do not think of a reason why we shouldn't.
>
> If we were still working in scripted Porcelain, it would have been a
> five minute hack, perhaps like this.
>
>  contrib/examples/git-pull.sh | 8 ++++++++
>  1 file changed, 8 insertions(+)

... and if we have to work with built-ins, it becomes a lot larger
than a five-minute hack, unfortunately.

Something like this may have a chance of working ;-)

 builtin/pull.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/builtin/pull.c b/builtin/pull.c
index bf3fd3f..777ae56 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -878,10 +878,24 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		if (merge_heads.nr > 1)
 			die(_("Cannot merge multiple branches into empty head."));
 		return pull_into_void(*merge_heads.sha1, curr_head);
-	} else if (opt_rebase) {
-		if (merge_heads.nr > 1)
-			die(_("Cannot rebase onto multiple branches."));
+	}
+	if (opt_rebase && merge_heads.nr > 1)
+		die(_("Cannot rebase onto multiple branches."));
+
+	if (opt_rebase) {
+		struct commit_list *list = NULL;
+		struct commit *merge_head, *head;
+
+		head = lookup_commit_reference(orig_head);
+		commit_list_insert(head, &list);
+		merge_head = lookup_commit_reference(merge_heads.sha1[0]);
+		if (is_descendant_of(merge_head, list)) {
+			/* we can fast-forward this without invoking rebase */
+			opt_ff = "--ff-only";
+			return run_merge();
+		}
 		return run_rebase(curr_head, *merge_heads.sha1, rebase_fork_point);
-	} else
+	}
+	else
 		return run_merge();
 }