rebase -r: don't write .git/MERGE_MSG when fast-forwarding

When fast-forwarding we do not create a new commit so .git/MERGE_MSG
is not removed and can end up seeding the message of a commit made
after the rebase has finished. Avoid writing .git/MERGE_MSG when we
are fast-forwarding by writing the file after the fast-forward
checks. Note that there are no changes to the fast-forward code, it is
simply moved.

Note that the way this change is implemented means we no longer write
the author script when fast-forwarding either. I believe this is safe
for the reasons below but it is a departure from what we do when
fast-forwarding a non-merge commit. If we reword the merge then 'git
commit --amend' will keep the authorship of the commit we're rewording
as it ignores GIT_AUTHOR_* unless --reset-author is passed. It will
also export the correct GIT_AUTHOR_* variables to any hooks and we
already test the authorship of the reworded commit. If we are not
rewording then we no longer call spilt_ident() which means we are no
longer checking the commit author header looks sane. However this is
what we already do when fast-forwarding non-merge commits in
skip_unnecessary_picks() so I don't think we're breaking any promises
by not checking the author here.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/sequencer.c b/sequencer.c
index cc8a361cceb9387b3f889e7db4f597231a60aab5..c2cba5ed4b1771cddb94b998094bef1eaca8c408 100644 (file)
--- a/sequencer.c
+++ b/sequencer.c
@@ -983,7 +983,8 @@ static int run_git_commit(const char *defmsg,
 
        cmd.git_cmd = 1;
 
-       if (is_rebase_i(opts) && read_env_script(&cmd.env_array)) {
+       if (is_rebase_i(opts) && !(!defmsg && (flags & AMEND_MSG)) &&
+           read_env_script(&cmd.env_array)) {
                const char *gpg_opt = gpg_sign_opt_quoted(opts);
 
                return error(_(staged_changes_advice),
@@ -3815,6 +3816,45 @@ static int do_merge(struct repository *r,
                goto leave_merge;
        }
 
+       /*
+        * If HEAD is not identical to the first parent of the original merge
+        * commit, we cannot fast-forward.
+        */
+       can_fast_forward = opts->allow_ff && commit && commit->parents &&
+               oideq(&commit->parents->item->object.oid,
+                     &head_commit->object.oid);
+
+       /*
+        * If any merge head is different from the original one, we cannot
+        * fast-forward.
+        */
+       if (can_fast_forward) {
+               struct commit_list *p = commit->parents->next;
+
+               for (j = to_merge; j && p; j = j->next, p = p->next)
+                       if (!oideq(&j->item->object.oid,
+                                  &p->item->object.oid)) {
+                               can_fast_forward = 0;
+                               break;
+                       }
+               /*
+                * If the number of merge heads differs from the original merge
+                * commit, we cannot fast-forward.
+                */
+               if (j || p)
+                       can_fast_forward = 0;
+       }
+
+       if (can_fast_forward) {
+               rollback_lock_file(&lock);
+               ret = fast_forward_to(r, &commit->object.oid,
+                                     &head_commit->object.oid, 0, opts);
+               if (flags & TODO_EDIT_MERGE_MSG)
+                       goto fast_forward_edit;
+
+               goto leave_merge;
+       }
+
        if (commit) {
                const char *encoding = get_commit_output_encoding();
                const char *message = logmsg_reencode(commit, NULL, encoding);
@@ -3864,45 +3904,6 @@ static int do_merge(struct repository *r,
                }
        }
 
-       /*
-        * If HEAD is not identical to the first parent of the original merge
-        * commit, we cannot fast-forward.
-        */
-       can_fast_forward = opts->allow_ff && commit && commit->parents &&
-               oideq(&commit->parents->item->object.oid,
-                     &head_commit->object.oid);
-
-       /*
-        * If any merge head is different from the original one, we cannot
-        * fast-forward.
-        */
-       if (can_fast_forward) {
-               struct commit_list *p = commit->parents->next;
-
-               for (j = to_merge; j && p; j = j->next, p = p->next)
-                       if (!oideq(&j->item->object.oid,
-                                  &p->item->object.oid)) {
-                               can_fast_forward = 0;
-                               break;
-                       }
-               /*
-                * If the number of merge heads differs from the original merge
-                * commit, we cannot fast-forward.
-                */
-               if (j || p)
-                       can_fast_forward = 0;
-       }
-
-       if (can_fast_forward) {
-               rollback_lock_file(&lock);
-               ret = fast_forward_to(r, &commit->object.oid,
-                                     &head_commit->object.oid, 0, opts);
-               if (flags & TODO_EDIT_MERGE_MSG)
-                       goto fast_forward_edit;
-
-               goto leave_merge;
-       }
-
        if (strategy || to_merge->next) {
                /* Octopus merge */
                struct child_process cmd = CHILD_PROCESS_INIT;

diff --git a/t/lib-rebase.sh b/t/lib-rebase.sh
index 99d9e7efd2d4d45a9ca4e65a1ef92ed43ed20e1a..ec6b9b107da4eaab70e221b07c25c9f949f09b14 100644 (file)
--- a/t/lib-rebase.sh
+++ b/t/lib-rebase.sh
@@ -173,10 +173,16 @@ set_reword_editor () {
 
        write_script reword-editor.sh <<-EOF &&
        # Save the oid of the first reworded commit so we can check rebase
-       # fast-forwards to it
+       # fast-forwards to it. Also check that we do not write .git/MERGE_MSG
+       # when fast-forwarding
        if ! test -s reword-oid
        then
-               git rev-parse HEAD >reword-oid
+               git rev-parse HEAD >reword-oid &&
+               if test -f .git/MERGE_MSG
+               then
+                       echo 1>&2 "error: .git/MERGE_MSG exists"
+                       exit 1
+               fi
        fi &&
        # There should be no uncommited changes
        git diff --exit-code HEAD &&