Eric Sunshine pointed out that I had such a commit message in
https://public-inbox.org/git/CAPig+cRrS0_nYJJY=o6cbov630snqhpv5qgrqdd8mw-syzn...@mail.gmail.com/
and I went on a hunt to figure out how the heck this happened.

Turns out that if there is a fixup/squash chain where the *last* command
fails with merge conflicts, and we either --skip ahead or resolve the
conflict to a clean tree and then --continue, our code does not do a
final cleanup.

Contrary to my initial gut feeling, this bug was not introduced by my
rewrite in C of the core parts of rebase -i, but it looks to me as if
that bug was with us for a very long time (at least the --skip part).

The developer (read: user of rebase -i) in me says that we would want to
fast-track this, but the author of rebase -i in me says that we should
be cautious and cook this in `next` for a while.

Fixes since v3 (thanks, Phillip, for the really fruitful discussion!):

- We now avoid using the commit message prepared for the skipped
  fixup/squash.

- Replaced the "rebase -i: Handle "combination of <n> commits" with
  GETTEXT_POISON" patch by a *real* fix instead of a work-around: Instead
  of parsing the first line of the commit message and punting when it is
  missing an ASCII-encoded number, we determine <n> separately
  (independent from any localized text).

- Fixed quite a couple more corner cases, using the `current-fixups`
  file introduced for the GETTEXT_POISON fix:

  * we only need to re-commit if this was the final fixup/squash in the
    fixup/squash chain,

  * we only need to commit interactively if there was *any* non-skipped
    squash,

  * if the fixup/squash chain continues, the <N> was incorrect in the
    "This is a combination of <N> commits" comment in the intermediate
    commit message (it included the now-skipped commits), and

  * even if a filed fixup/squash in the middle of a fixup/squash chain
    failed, and its merge conflicts were resolved and committed, the
    "This is a combination of <N> commits" comment was incorrect: we
    had already deleted message-fixup and message-squash, so the next
    update_squash_message() would mistakenly assume that we were
    starting afresh. Worse: if only fixup commands were remaining, but
    there had been a squash command, we would retain the "squash!" line
    in the commit message and not give the user a chance to clean things
    up in the final fixup!


Johannes Schindelin (4):
  rebase -i: demonstrate bugs with fixup!/squash! commit messages
  rebase -i: Handle "combination of <n> commits" with GETTEXT_POISON
  sequencer: always commit without editing when asked for
  rebase --skip: clean up commit message after a failed fixup/squash

 sequencer.c                | 193 ++++++++++++++++++++++++++++---------
 sequencer.h                |   6 +-
 t/t3418-rebase-continue.sh |  49 ++++++++++
 3 files changed, 200 insertions(+), 48 deletions(-)


base-commit: 1f1cddd558b54bb0ce19c8ace353fd07b758510d
Published-As: 
https://github.com/dscho/git/releases/tag/clean-msg-after-fixup-continue-v4
Fetch-It-Via: git fetch https://github.com/dscho/git 
clean-msg-after-fixup-continue-v4

Interdiff vs v3:
 diff --git a/sequencer.c b/sequencer.c
 index e1efb0ebf31..cec180714ef 100644
 --- a/sequencer.c
 +++ b/sequencer.c
 @@ -74,13 +74,6 @@ static GIT_PATH_FUNC(rebase_path_message, 
"rebase-merge/message")
   * previous commit and from the first squash/fixup commit are written
   * to it. The commit message for each subsequent squash/fixup commit
   * is appended to the file as it is processed.
 - *
 - * The first line of the file is of the form
 - *     # This is a combination of $count commits.
 - * where $count is the number of commits whose messages have been
 - * written to the file so far (including the initial "pick" commit).
 - * Each time that a commit message is processed, this line is read and
 - * updated. It is deleted just before the combined commit is made.
   */
  static GIT_PATH_FUNC(rebase_path_squash_msg, "rebase-merge/message-squash")
  /*
 @@ -91,6 +84,11 @@ static GIT_PATH_FUNC(rebase_path_squash_msg, 
"rebase-merge/message-squash")
   * commit without opening the editor.)
   */
  static GIT_PATH_FUNC(rebase_path_fixup_msg, "rebase-merge/message-fixup")
 +/*
 + * This file contains the list fixup/squash commands that have been
 + * accumulated into message-fixup or message-squash so far.
 + */
 +static GIT_PATH_FUNC(rebase_path_current_fixups, 
"rebase-merge/current-fixups")
  /*
   * A script to set the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
   * GIT_AUTHOR_DATE that will be used for the commit that is currently
 @@ -106,13 +104,6 @@ static GIT_PATH_FUNC(rebase_path_author_script, 
"rebase-merge/author-script")
   * command is processed, this file is deleted.
   */
  static GIT_PATH_FUNC(rebase_path_amend, "rebase-merge/amend")
 -/*
 - * If there was a merge conflict in a fixup/squash series, we need to
 - * record the type so that a `git rebase --skip` can clean up the commit
 - * message as appropriate. This file will contain that type (`fixup` or
 - * `squash`), and not exist otherwise.
 - */
 -static GIT_PATH_FUNC(rebase_path_amend_type, "rebase-merge/amend-type")
  /*
   * When we stop at a given patch via the "edit" command, this file contains
   * the abbreviated commit name of the corresponding patch.
 @@ -260,6 +251,7 @@ int sequencer_remove_state(struct replay_opts *opts)
        for (i = 0; i < opts->xopts_nr; i++)
                free(opts->xopts[i]);
        free(opts->xopts);
 +      strbuf_release(&opts->current_fixups);
  
        strbuf_addstr(&dir, get_dir(opts));
        remove_dir_recursively(&dir, 0);
 @@ -725,6 +717,8 @@ static int run_git_commit(const char *defmsg, struct 
replay_opts *opts,
                argv_array_pushf(&cmd.args, "-S%s", opts->gpg_sign);
        if (defmsg)
                argv_array_pushl(&cmd.args, "-F", defmsg, NULL);
 +      else if (!(flags & EDIT_MSG))
 +              argv_array_pushl(&cmd.args, "-C", "HEAD", NULL);
        if ((flags & CLEANUP_MSG))
                argv_array_push(&cmd.args, "--cleanup=strip");
        if ((flags & EDIT_MSG))
 @@ -1336,33 +1330,23 @@ static int update_squash_messages(enum todo_command 
command,
                struct commit *commit, struct replay_opts *opts)
  {
        struct strbuf buf = STRBUF_INIT;
 -      int count, res;
 +      int res;
        const char *message, *body;
  
 -      if (file_exists(rebase_path_squash_msg())) {
 +      if (opts->current_fixup_count > 0) {
                struct strbuf header = STRBUF_INIT;
 -              char *eol, *p;
 +              char *eol;
  
 -              if (strbuf_read_file(&buf, rebase_path_squash_msg(), 2048) <= 0)
 +              if (strbuf_read_file(&buf, rebase_path_squash_msg(), 9) <= 0)
                        return error(_("could not read '%s'"),
                                rebase_path_squash_msg());
  
 -              p = buf.buf + 1;
 -              eol = strchrnul(buf.buf, '\n');
 -              if (buf.buf[0] != comment_line_char ||
 -                  (p += strcspn(p, "0123456789\n")) == eol)
 -                      count = -1;
 -              else
 -                      count = strtol(p, NULL, 10);
 +              eol = buf.buf[0] != comment_line_char ?
 +                      buf.buf : strchrnul(buf.buf, '\n');
  
                strbuf_addf(&header, "%c ", comment_line_char);
 -              if (count < 1)
 -                      strbuf_addf(&header, _("This is a combination of "
 -                                             "several commits."));
 -              else
 -                      strbuf_addf(&header,
 -                                  _("This is a combination of %d commits."),
 -                                  ++count);
 +              strbuf_addf(&header, _("This is a combination of %d commits."),
 +                          opts->current_fixup_count + 2);
                strbuf_splice(&buf, 0, eol - buf.buf, header.buf, header.len);
                strbuf_release(&header);
        } else {
 @@ -1385,10 +1369,8 @@ static int update_squash_messages(enum todo_command 
command,
                                     rebase_path_fixup_msg());
                }
  
 -              count = 2;
                strbuf_addf(&buf, "%c ", comment_line_char);
 -              strbuf_addf(&buf, _("This is a combination of %d commits."),
 -                          count);
 +              strbuf_addf(&buf, _("This is a combination of %d commits."), 2);
                strbuf_addf(&buf, "\n%c ", comment_line_char);
                strbuf_addstr(&buf, _("This is the 1st commit message:"));
                strbuf_addstr(&buf, "\n\n");
 @@ -1405,22 +1387,14 @@ static int update_squash_messages(enum todo_command 
command,
        if (command == TODO_SQUASH) {
                unlink(rebase_path_fixup_msg());
                strbuf_addf(&buf, "\n%c ", comment_line_char);
 -              if (count < 2)
 -                      strbuf_addf(&buf, _("This is the next commit "
 -                                          "message:"));
 -              else
 -                      strbuf_addf(&buf, _("This is the commit message #%d:"),
 -                                  count);
 +              strbuf_addf(&buf, _("This is the commit message #%d:"),
 +                          ++opts->current_fixup_count);
                strbuf_addstr(&buf, "\n\n");
                strbuf_addstr(&buf, body);
        } else if (command == TODO_FIXUP) {
                strbuf_addf(&buf, "\n%c ", comment_line_char);
 -              if (count < 2)
 -                      strbuf_addf(&buf, _("The next commit message will be "
 -                                          "skipped:"));
 -              else
 -                      strbuf_addf(&buf, _("The commit message #%d will be "
 -                                          "skipped:"), count);
 +              strbuf_addf(&buf, _("The commit message #%d will be skipped:"),
 +                          ++opts->current_fixup_count);
                strbuf_addstr(&buf, "\n\n");
                strbuf_add_commented_lines(&buf, body, strlen(body));
        } else
 @@ -1429,6 +1403,17 @@ static int update_squash_messages(enum todo_command 
command,
  
        res = write_message(buf.buf, buf.len, rebase_path_squash_msg(), 0);
        strbuf_release(&buf);
 +
 +      if (!res) {
 +              strbuf_addf(&opts->current_fixups, "%s%s %s",
 +                          opts->current_fixups.len ? "\n" : "",
 +                          command_to_string(command),
 +                          oid_to_hex(&commit->object.oid));
 +              res = write_message(opts->current_fixups.buf,
 +                                  opts->current_fixups.len,
 +                                  rebase_path_current_fixups(), 0);
 +      }
 +
        return res;
  }
  
 @@ -1691,6 +1676,9 @@ static int do_pick_commit(enum todo_command command, 
struct commit *commit,
        if (!res && final_fixup) {
                unlink(rebase_path_fixup_msg());
                unlink(rebase_path_squash_msg());
 +              unlink(rebase_path_current_fixups());
 +              strbuf_reset(&opts->current_fixups);
 +              opts->current_fixup_count = 0;
        }
  
  leave:
 @@ -2067,6 +2055,16 @@ static int read_populate_opts(struct replay_opts *opts)
                read_strategy_opts(opts, &buf);
                strbuf_release(&buf);
  
 +              if (read_oneliner(&opts->current_fixups,
 +                                rebase_path_current_fixups(), 1)) {
 +                      const char *p = opts->current_fixups.buf;
 +                      opts->current_fixup_count = 1;
 +                      while ((p = strchr(p, '\n'))) {
 +                              opts->current_fixup_count++;
 +                              p++;
 +                      }
 +              }
 +
                return 0;
        }
  
 @@ -2413,21 +2411,9 @@ static int error_with_patch(struct commit *commit,
  static int error_failed_squash(struct commit *commit,
        struct replay_opts *opts, int subject_len, const char *subject)
  {
 -      const char *amend_type = "squash";
 -
 -      if (rename(rebase_path_squash_msg(), rebase_path_message()))
 -              return error(_("could not rename '%s' to '%s'"),
 +      if (copy_file(rebase_path_message(), rebase_path_squash_msg(), 0666))
 +              return error(_("could not copy '%s' to '%s'"),
                        rebase_path_squash_msg(), rebase_path_message());
 -
 -      if (file_exists(rebase_path_fixup_msg())) {
 -              unlink(rebase_path_fixup_msg());
 -              amend_type = "fixup";
 -      }
 -      if (write_message(amend_type, strlen(amend_type),
 -                     rebase_path_amend_type(), 0))
 -              return error(_("could not write '%s'"),
 -                           rebase_path_amend_type());
 -
        unlink(git_path_merge_msg());
        if (copy_file(git_path_merge_msg(), rebase_path_message(), 0666))
                return error(_("could not copy '%s' to '%s'"),
 @@ -2604,7 +2590,6 @@ static int pick_commits(struct todo_list *todo_list, 
struct replay_opts *opts)
                        unlink(rebase_path_author_script());
                        unlink(rebase_path_stopped_sha());
                        unlink(rebase_path_amend());
 -                      unlink(rebase_path_amend_type());
                        delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
                }
                if (item->command <= TODO_SQUASH) {
 @@ -2794,9 +2779,11 @@ static int continue_single_pick(void)
        return run_command_v_opt(argv, RUN_GIT_CMD);
  }
  
 -static int commit_staged_changes(struct replay_opts *opts)
 +static int commit_staged_changes(struct replay_opts *opts,
 +                               struct todo_list *todo_list)
  {
 -      unsigned int flags = ALLOW_EMPTY | EDIT_MSG, is_fixup = 0, is_clean;
 +      unsigned int flags = ALLOW_EMPTY | EDIT_MSG;
 +      unsigned int final_fixup = 0, is_clean;
  
        if (has_unstaged_changes(1))
                return error(_("cannot rebase: You have unstaged changes."));
 @@ -2819,21 +2806,69 @@ static int commit_staged_changes(struct replay_opts 
*opts)
                                       "working tree. Please, commit them\n"
                                       "first and then run 'git rebase "
                                       "--continue' again."));
 -              if (is_clean && !oidcmp(&head, &to_amend)) {
 -                      strbuf_reset(&rev);
 +              /*
 +               * When skipping a failed fixup/squash, we need to edit the
 +               * commit message, the current fixup list and count, and if it
 +               * was the last fixup/squash in the chain, we need to clean up
 +               * the commit message and if there was a squash, let the user
 +               * edit it.
 +               */
 +              if (is_clean && !oidcmp(&head, &to_amend) &&
 +                  opts->current_fixup_count > 0 &&
 +                  file_exists(rebase_path_stopped_sha())) {
 +                      const char *p = opts->current_fixups.buf;
 +                      int len = opts->current_fixups.len;
 +
 +                      opts->current_fixup_count--;
 +                      if (!len)
 +                              BUG("Incorrect current_fixups:\n%s", p);
 +                      while (len && p[len - 1] != '\n')
 +                              len--;
 +                      strbuf_setlen(&opts->current_fixups, len);
 +                      if (write_message(p, len, rebase_path_current_fixups(),
 +                                        0) < 0)
 +                              return error(_("could not write file: '%s'"),
 +                                           rebase_path_current_fixups());
 +
                        /*
 -                       * Clean tree, but we may need to finalize a
 -                       * fixup/squash chain. A failed fixup/squash leaves the
 -                       * file amend-type in rebase-merge/; It is okay if that
 -                       * file is missing, in which case there is no such
 -                       * chain to finalize.
 +                       * If a fixup/squash in a fixup/squash chain failed, the
 +                       * commit message is already correct, no need to commit
 +                       * it again.
 +                       *
 +                       * Only if it is the final command in the fixup/squash
 +                       * chain, and only if the chain is longer than a single
 +                       * fixup/squash command (which was just skipped), do we
 +                       * actually need to re-commit with a cleaned up commit
 +                       * message.
                         */
 -                      read_oneliner(&rev, rebase_path_amend_type(), 0);
 -                      if (!strcmp("squash", rev.buf))
 -                              is_fixup = TODO_SQUASH;
 -                      else if (!strcmp("fixup", rev.buf)) {
 -                              is_fixup = TODO_FIXUP;
 -                              flags = (flags & ~EDIT_MSG) | CLEANUP_MSG;
 +                      if (opts->current_fixup_count > 0 &&
 +                          !is_fixup(peek_command(todo_list, 0))) {
 +                              final_fixup = 1;
 +                              /*
 +                               * If there was not a single "squash" in the
 +                               * chain, we only need to clean up the commit
 +                               * message, no need to bother the user with
 +                               * opening the commit message in the editor.
 +                               */
 +                              if (!starts_with(p, "squash ") &&
 +                                  !strstr(p, "\nsquash "))
 +                                      flags = (flags & ~EDIT_MSG) | 
CLEANUP_MSG;
 +                      } else if (is_fixup(peek_command(todo_list, 0))) {
 +                              /*
 +                               * We need to update the squash message to skip
 +                               * the latest commit message.
 +                               */
 +                              struct commit *commit;
 +                              const char *path = rebase_path_squash_msg();
 +
 +                              if (parse_head(&commit) ||
 +                                  !(p = get_commit_buffer(commit, NULL)) ||
 +                                  write_message(p, strlen(p), path, 0)) {
 +                                      unuse_commit_buffer(commit, p);
 +                                      return error(_("could not write file: "
 +                                                     "'%s'"), path);
 +                              }
 +                              unuse_commit_buffer(commit, p);
                        }
                }
  
 @@ -2841,18 +2876,32 @@ static int commit_staged_changes(struct replay_opts 
*opts)
                flags |= AMEND_MSG;
        }
  
 -      if (is_clean && !is_fixup) {
 +      if (is_clean) {
                const char *cherry_pick_head = git_path_cherry_pick_head();
  
                if (file_exists(cherry_pick_head) && unlink(cherry_pick_head))
                        return error(_("could not remove CHERRY_PICK_HEAD"));
 -              return 0;
 +              if (!final_fixup)
 +                      return 0;
        }
  
 -      if (run_git_commit(rebase_path_message(), opts, flags))
 +      if (run_git_commit(final_fixup ? NULL : rebase_path_message(),
 +                         opts, flags))
                return error(_("could not commit staged changes."));
        unlink(rebase_path_amend());
 -      unlink(rebase_path_amend_type());
 +      if (final_fixup) {
 +              unlink(rebase_path_fixup_msg());
 +              unlink(rebase_path_squash_msg());
 +      }
 +      if (opts->current_fixup_count > 0) {
 +              /*
 +               * Whether final fixup or not, we just cleaned up the commit
 +               * message...
 +               */
 +              unlink(rebase_path_current_fixups());
 +              strbuf_reset(&opts->current_fixups);
 +              opts->current_fixup_count = 0;
 +      }
        return 0;
  }
  
 @@ -2864,14 +2913,16 @@ int sequencer_continue(struct replay_opts *opts)
        if (read_and_refresh_cache(opts))
                return -1;
  
 +      if (read_populate_opts(opts))
 +              return -1;
        if (is_rebase_i(opts)) {
 -              if (commit_staged_changes(opts))
 +              if ((res = read_populate_todo(&todo_list, opts)))
 +                      goto release_todo_list;
 +              if (commit_staged_changes(opts, &todo_list))
                        return -1;
        } else if (!file_exists(get_todo_path(opts)))
                return continue_single_pick();
 -      if (read_populate_opts(opts))
 -              return -1;
 -      if ((res = read_populate_todo(&todo_list, opts)))
 +      else if ((res = read_populate_todo(&todo_list, opts)))
                goto release_todo_list;
  
        if (!is_rebase_i(opts)) {
 diff --git a/sequencer.h b/sequencer.h
 index e45b178dfc4..1898158c52d 100644
 --- a/sequencer.h
 +++ b/sequencer.h
 @@ -44,10 +44,14 @@ struct replay_opts {
        char **xopts;
        size_t xopts_nr, xopts_alloc;
  
 +      /* Used by fixup/squash */
 +      struct strbuf current_fixups;
 +      int current_fixup_count;
 +
        /* Only used by REPLAY_NONE */
        struct rev_info *revs;
  };
 -#define REPLAY_OPTS_INIT { -1 }
 +#define REPLAY_OPTS_INIT { .action = -1, .current_fixups = STRBUF_INIT }
  
  /* Call this to setup defaults before parsing command line options */
  void sequencer_init_config(struct replay_opts *opts);
 diff --git a/t/t3418-rebase-continue.sh b/t/t3418-rebase-continue.sh
 index 693f92409ec..03bf1b8a3b3 100755
 --- a/t/t3418-rebase-continue.sh
 +++ b/t/t3418-rebase-continue.sh
 @@ -95,19 +95,46 @@ test_expect_success '--skip after failed fixup cleans 
commit message' '
        test_commit "fixup! wants-fixup" wants-fixup.t 1 wants-fixup-1 &&
        test_commit "fixup! wants-fixup" wants-fixup.t 2 wants-fixup-2 &&
        test_commit "fixup! wants-fixup" wants-fixup.t 3 wants-fixup-3 &&
 -      test_must_fail env FAKE_LINES="1 fixup 2 fixup 4" \
 +      test_must_fail env FAKE_LINES="1 fixup 2 squash 4" \
                git rebase -i HEAD~4 &&
  
        : now there is a conflict, and comments in the commit message &&
        git show HEAD >out &&
 -      test_i18ngrep "This is a combination of" out &&
 +      grep "fixup! wants-fixup" out &&
  
        : skip and continue &&
 -      git rebase --skip &&
 +      echo "cp \"\$1\" .git/copy.txt" | write_script copy-editor.sh &&
 +      (test_set_editor "$PWD/copy-editor.sh" && git rebase --skip) &&
 +
 +      : the user should not have had to edit the commit message &&
 +      test_path_is_missing .git/copy.txt &&
  
        : now the comments in the commit message should have been cleaned up &&
        git show HEAD >out &&
 -      test_i18ngrep ! "This is a combination of" out
 +      ! grep "fixup! wants-fixup" out &&
 +
 +      : now, let us ensure that "squash" is handled correctly &&
 +      git reset --hard wants-fixup-3 &&
 +      test_must_fail env FAKE_LINES="1 squash 4 squash 2 squash 4" \
 +              git rebase -i HEAD~4 &&
 +
 +      : the first squash failed, but there are two more in the chain &&
 +      (test_set_editor "$PWD/copy-editor.sh" &&
 +       test_must_fail git rebase --skip) &&
 +
 +      : not the final squash, no need to edit the commit message &&
 +      test_path_is_missing .git/copy.txt &&
 +
 +      : The first squash was skipped, therefore: &&
 +      git show HEAD >out &&
 +      test_i18ngrep "# This is a combination of 2 commits" out &&
 +
 +      (test_set_editor "$PWD/copy-editor.sh" && git rebase --skip) &&
 +      git show HEAD >out &&
 +      test_i18ngrep ! "# This is a combination" out &&
 +
 +      : Final squash failed, but there was still a squash &&
 +      test_i18ngrep "# This is a combination of 2 commits" .git/copy.txt
  '
  
  test_expect_success 'setup rerere database' '
-- 
2.17.0.windows.1.33.gfcbb1fa0445

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