Re: [PATCH RFC] rebase: respect --ff-only option

[Phillip Wood <phillip.wood123@gmail.com>]

On 05/07/2021 09:53, Phillip Wood wrote:
> Hi Alex
> 
> On 05/07/2021 05:45, Alex Henrie wrote:
>> The warning about pulling without specifying how to reconcile divergent
>> branches says that after setting pull.rebase to true, --ff-only can
>> still be passed on the command line to require a fast-forward. Make that
>> actually work.
> 
> As I understand it the motivation for this change is to have 'git -c 
> pull.rebase=true pull --ff-only' actually fast forward. Why cant we just 
> change pull not to rebase in that case? 

Looking at origin/seen:builtin/pull.c we already check if we can 
fast-forward and unconditionally merge in that case irrespective of any 
'--rebase' option or pull.rebase config. It should be simple for pull to 
error out if '--ff-only' is given and we cannot fast-forward.

Best Wishes

Phillip

> I've left some comments about 
> the rebase changes below
> 
>  > [...]
>> diff --git a/builtin/rebase.c b/builtin/rebase.c
>> index 12f093121d..b88f0cbcca 100644
>> --- a/builtin/rebase.c
>> +++ b/builtin/rebase.c
>> @@ -1345,12 +1349,14 @@ int cmd_rebase(int argc, const char **argv, 
>> const char *prefix)
>>                N_("ignore changes in whitespace")),
>>           OPT_PASSTHRU_ARGV(0, "whitespace", &options.git_am_opts,
>>                     N_("action"), N_("passed to 'git apply'"), 0),
>> -        OPT_BIT('f', "force-rebase", &options.flags,
>> -            N_("cherry-pick all commits, even if unchanged"),
>> -            REBASE_FORCE),
>> -        OPT_BIT(0, "no-ff", &options.flags,
>> -            N_("cherry-pick all commits, even if unchanged"),
>> -            REBASE_FORCE),
>> +        OPT_SET_INT_F('f', "force-rebase", &options.fast_forward,
>> +                  N_("cherry-pick all commits, even if unchanged"),
>> +                  FF_NO, PARSE_OPT_NONEG),
> 
> Why does this change rebase to start rejecting --no-force-rebase? This 
> is the sort of behavior change that deserves a mention in the commit 
> message.
> 
>> +        OPT_SET_INT(0, "ff", &options.fast_forward, N_("allow 
>> fast-forward"),
>> +                FF_ALLOW),
> 
> The useful option when rebasing is '--no-ff', now that will no longer 
> appear in the output of 'git rebase -h'
> 
>> +        OPT_SET_INT_F(0, "ff-only", &options.fast_forward,
>> +                  N_("abort if fast-forward is not possible"),
>> +                  FF_ONLY, PARSE_OPT_NONEG),
> 
> Is there a use for this outside of 'git pull --ff-only'? I'm far from 
> convinced we want this new option but if we do end up adding it I think 
> it should error out in combination with '-i' or '-x' as '-i' implies the 
> user wants to change the existing commits and '-x' can end up changing 
> them as well.
> 
> I think this patch addresses a valid problem but it seems to me that the 
> approach taken pushes complexity into rebase to handle a case when pull 
> does not need to invoke rebase in the first place.
> 
> Best Wishes
> 
> Phillip
> 
>>           OPT_CMDMODE(0, "continue", &action, N_("continue"),
>>                   ACTION_CONTINUE),
>>           OPT_CMDMODE(0, "skip", &action,
>> @@ -1635,7 +1641,7 @@ int cmd_rebase(int argc, const char **argv, 
>> const char *prefix)
>>           allow_preemptive_ff = 0;
>>       }
>>       if (options.committer_date_is_author_date || options.ignore_date)
>> -        options.flags |= REBASE_FORCE;
>> +        options.fast_forward = FF_NO;
>>       for (i = 0; i < options.git_am_opts.nr; i++) {
>>           const char *option = options.git_am_opts.v[i], *p;
>> @@ -1809,7 +1815,7 @@ int cmd_rebase(int argc, const char **argv, 
>> const char *prefix)
>>               die("cannot combine '--signoff' with "
>>                   "'--preserve-merges'");
>>           strvec_push(&options.git_am_opts, "--signoff");
>> -        options.flags |= REBASE_FORCE;
>> +        options.fast_forward = FF_NO;
>>       }
>>       if (options.type == REBASE_PRESERVE_MERGES) {
>> @@ -1952,6 +1958,24 @@ int cmd_rebase(int argc, const char **argv, 
>> const char *prefix)
>>       if (repo_read_index(the_repository) < 0)
>>           die(_("could not read index"));
>> +    /*
>> +     * 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 merge_base, so we 
>> have to
>> +     * call it before checking allow_preemptive_ff.
>> +     */
>> +    allow_preemptive_ff = can_fast_forward(options.onto, 
>> options.upstream,
>> +                           options.restrict_revision,
>> +                           &options.orig_head, &merge_base)
>> +                  && allow_preemptive_ff;
>> +
>> +    if (!allow_preemptive_ff && options.fast_forward == FF_ONLY) {
>> +        ret = error_ff_impossible();
>> +        goto cleanup;
>> +    }
>> +
>>       if (options.autostash) {
>>           create_autostash(the_repository, state_dir_path("autostash", 
>> &options),
>>                    DEFAULT_REFLOG_ACTION);
>> @@ -1968,20 +1992,10 @@ int cmd_rebase(int argc, const char **argv, 
>> const char *prefix)
>>        * everything leading up to orig_head) on top of onto.
>>        */
>> -    /*
>> -     * 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 merge_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, &merge_base) &&
>> -        allow_preemptive_ff) {
>> +    if (allow_preemptive_ff) {
>>           int flag;
>> -        if (!(options.flags & REBASE_FORCE)) {
>> +        if (options.fast_forward != FF_NO) {
>>               /* Lazily switch to the target branch if needed... */
>>               if (options.switch_to) {
>>                   strbuf_reset(&buf);
>> diff --git a/builtin/revert.c b/builtin/revert.c
>> index 237f2f18d4..f9b61478a2 100644
>> --- a/builtin/revert.c
>> +++ b/builtin/revert.c
>> @@ -123,7 +123,7 @@ static int run_sequencer(int argc, const char 
>> **argv, struct replay_opts *opts)
>>       if (opts->action == REPLAY_PICK) {
>>           struct option cp_extra[] = {
>>               OPT_BOOL('x', NULL, &opts->record_origin, N_("append 
>> commit name")),
>> -            OPT_BOOL(0, "ff", &opts->allow_ff, N_("allow 
>> fast-forward")),
>> +            OPT_SET_INT(0, "ff", &opts->fast_forward, N_("allow 
>> fast-forward"), FF_ALLOW),
>>               OPT_BOOL(0, "allow-empty", &opts->allow_empty, 
>> N_("preserve initially empty commits")),
>>               OPT_BOOL(0, "allow-empty-message", 
>> &opts->allow_empty_message, N_("allow commits with empty messages")),
>>               OPT_BOOL(0, "keep-redundant-commits", 
>> &opts->keep_redundant_commits, N_("keep redundant, empty commits")),
>> @@ -166,7 +166,7 @@ static int run_sequencer(int argc, const char 
>> **argv, struct replay_opts *opts)
>>                   "--strategy", opts->strategy ? 1 : 0,
>>                   "--strategy-option", opts->xopts ? 1 : 0,
>>                   "-x", opts->record_origin,
>> -                "--ff", opts->allow_ff,
>> +                "--ff", opts->fast_forward == FF_ALLOW,
>>                   "--rerere-autoupdate", opts->allow_rerere_auto == 
>> RERERE_AUTOUPDATE,
>>                   "--no-rerere-autoupdate", opts->allow_rerere_auto == 
>> RERERE_NOAUTOUPDATE,
>>                   NULL);
>> @@ -177,7 +177,7 @@ static int run_sequencer(int argc, const char 
>> **argv, struct replay_opts *opts)
>>           opts->default_strategy = NULL;
>>       }
>> -    if (opts->allow_ff)
>> +    if (opts->fast_forward == FF_ALLOW)
>>           verify_opt_compatible(me, "--ff",
>>                   "--signoff", opts->signoff,
>>                   "--no-commit", opts->no_commit,
>> diff --git a/sequencer.c b/sequencer.c
>> index 0bec01cf38..84447937d2 100644
>> --- a/sequencer.c
>> +++ b/sequencer.c
>> @@ -2120,18 +2120,23 @@ static int do_pick_commit(struct repository *r,
>>           return error(_("cannot get commit message for %s"),
>>               oid_to_hex(&commit->object.oid));
>> -    if (opts->allow_ff && !is_fixup(command) &&
>> -        ((parent && oideq(&parent->object.oid, &head)) ||
>> -         (!parent && unborn))) {
>> -        if (is_rebase_i(opts))
>> -            write_author_script(msg.message);
>> -        res = fast_forward_to(r, &commit->object.oid, &head, unborn,
>> -            opts);
>> -        if (res || command != TODO_REWORD)
>> +    if (opts->fast_forward != FF_NO) {
>> +        if (!is_fixup(command) &&
>> +            ((parent && oideq(&parent->object.oid, &head)) ||
>> +             (!parent && unborn))) {
>> +            if (is_rebase_i(opts))
>> +                write_author_script(msg.message);
>> +            res = fast_forward_to(r, &commit->object.oid, &head, unborn,
>> +                opts);
>> +            if (res || command != TODO_REWORD)
>> +                goto leave;
>> +            reword = 1;
>> +            msg_file = NULL;
>> +            goto fast_forward_edit;
>> +        } else if (opts->fast_forward == FF_ONLY) {
>> +            res = error_ff_impossible();
>>               goto leave;
>> -        reword = 1;
>> -        msg_file = NULL;
>> -        goto fast_forward_edit;
>> +        }
>>       }
>>       if (parent && parse_commit(parent) < 0)
>>           /* TRANSLATORS: The first %s will be a "todo" command like
>> @@ -2764,7 +2769,7 @@ static int populate_opts_cb(const char *key, 
>> const char *value, void *data)
>>       else if (!strcmp(key, "options.record-origin"))
>>           opts->record_origin = git_config_bool_or_int(key, value, 
>> &error_flag);
>>       else if (!strcmp(key, "options.allow-ff"))
>> -        opts->allow_ff = git_config_bool_or_int(key, value, 
>> &error_flag);
>> +        opts->fast_forward = git_config_bool_or_int(key, value, 
>> &error_flag) ? FF_ALLOW : FF_NO;
>>       else if (!strcmp(key, "options.mainline"))
>>           opts->mainline = git_config_int(key, value);
>>       else if (!strcmp(key, "options.strategy"))
>> @@ -2853,17 +2858,17 @@ static int read_populate_opts(struct 
>> replay_opts *opts)
>>               opts->quiet = 1;
>>           if (file_exists(rebase_path_signoff())) {
>> -            opts->allow_ff = 0;
>> +            opts->fast_forward = FF_NO;
>>               opts->signoff = 1;
>>           }
>>           if (file_exists(rebase_path_cdate_is_adate())) {
>> -            opts->allow_ff = 0;
>> +            opts->fast_forward = FF_NO;
>>               opts->committer_date_is_author_date = 1;
>>           }
>>           if (file_exists(rebase_path_ignore_date())) {
>> -            opts->allow_ff = 0;
>> +            opts->fast_forward = FF_NO;
>>               opts->ignore_date = 1;
>>           }
>> @@ -3320,7 +3325,7 @@ static int save_opts(struct replay_opts *opts)
>>       if (opts->record_origin)
>>           res |= git_config_set_in_file_gently(opts_file,
>>                       "options.record-origin", "true");
>> -    if (opts->allow_ff)
>> +    if (opts->fast_forward == FF_ALLOW)
>>           res |= git_config_set_in_file_gently(opts_file,
>>                       "options.allow-ff", "true");
>>       if (opts->mainline) {
>> @@ -3849,9 +3854,9 @@ 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);
>> +    can_fast_forward = opts->fast_forward != FF_NO && 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
>> @@ -3885,6 +3890,11 @@ static int do_merge(struct repository *r,
>>           goto leave_merge;
>>       }
>> +    if (opts->fast_forward == FF_ONLY && !can_fast_forward) {
>> +        ret = error_ff_impossible();
>> +        goto leave_merge;
>> +    }
>> +
>>       if (strategy || to_merge->next) {
>>           /* Octopus merge */
>>           struct child_process cmd = CHILD_PROCESS_INIT;
>> @@ -4276,7 +4286,7 @@ static int pick_commits(struct repository *r,
>>       /* Note that 0 for 3rd parameter of setenv means set only if not 
>> set */
>>       setenv(GIT_REFLOG_ACTION, action_name(opts), 0);
>>       prev_reflog_action = xstrdup(getenv(GIT_REFLOG_ACTION));
>> -    if (opts->allow_ff)
>> +    if (opts->fast_forward != FF_NO)
>>           assert(!(opts->signoff || opts->no_commit ||
>>                opts->record_origin || should_edit(opts) ||
>>                opts->committer_date_is_author_date ||
>> @@ -5646,7 +5656,8 @@ int complete_action(struct repository *r, struct 
>> replay_opts *opts, unsigned fla
>>           BUG("invalid todo list after expanding IDs:\n%s",
>>               new_todo.buf.buf);
>> -    if (opts->allow_ff && skip_unnecessary_picks(r, &new_todo, &oid)) {
>> +    if (opts->fast_forward != FF_NO
>> +        && skip_unnecessary_picks(r, &new_todo, &oid)) {
>>           todo_list_release(&new_todo);
>>           return error(_("could not skip unnecessary pick commands"));
>>       }
>> diff --git a/sequencer.h b/sequencer.h
>> index d57d8ea23d..e188dec312 100644
>> --- a/sequencer.h
>> +++ b/sequencer.h
>> @@ -28,6 +28,12 @@ enum commit_msg_cleanup_mode {
>>       COMMIT_MSG_CLEANUP_ALL
>>   };
>> +enum ff_type {
>> +    FF_NO,
>> +    FF_ALLOW,
>> +    FF_ONLY
>> +};
>> +
>>   struct replay_opts {
>>       enum replay_action action;
>> @@ -38,7 +44,6 @@ struct replay_opts {
>>       int record_origin;
>>       int no_commit;
>>       int signoff;
>> -    int allow_ff;
>>       int allow_rerere_auto;
>>       int allow_empty;
>>       int allow_empty_message;
>> @@ -50,6 +55,8 @@ struct replay_opts {
>>       int committer_date_is_author_date;
>>       int ignore_date;
>> +    enum ff_type fast_forward;
>> +
>>       int mainline;
>>       char *gpg_sign;
>> diff --git a/t/t3404-rebase-interactive.sh 
>> b/t/t3404-rebase-interactive.sh
>> index 66bcbbf952..858e406725 100755
>> --- a/t/t3404-rebase-interactive.sh
>> +++ b/t/t3404-rebase-interactive.sh
>> @@ -895,6 +895,11 @@ test_expect_success 'always cherry-pick with 
>> --no-ff' '
>>       test_must_be_empty out
>>   '
>> +test_expect_success 'interactive rebase prevents non-fast-forward 
>> with --ff-only' '
>> +    git checkout primary &&
>> +    test_must_fail git rebase -i --ff-only branch1
>> +'
>> +
>>   test_expect_success 'set up commits with funny messages' '
>>       git checkout -b funny A &&
>>       echo >>file1 &&
>> diff --git a/t/t3409-rebase-preserve-merges.sh 
>> b/t/t3409-rebase-preserve-merges.sh
>> index ec8062a66a..e656b5bd28 100755
>> --- a/t/t3409-rebase-preserve-merges.sh
>> +++ b/t/t3409-rebase-preserve-merges.sh
>> @@ -127,4 +127,8 @@ test_expect_success 'rebase -p ignores merge.log 
>> config' '
>>       )
>>   '
>> +test_expect_success 'rebase -p prevents non-fast-forward with 
>> --ff-only' '
>> +    test_must_fail git rebase -p --ff-only main
>> +'
>> +
>>   test_done
>> diff --git a/t/t3420-rebase-autostash.sh b/t/t3420-rebase-autostash.sh
>> index 43fcb68f27..69a85cb645 100755
>> --- a/t/t3420-rebase-autostash.sh
>> +++ b/t/t3420-rebase-autostash.sh
>> @@ -246,6 +246,15 @@ test_expect_success "rebase: fast-forward rebase" '
>>       git checkout feature-branch
>>   '
>> +test_expect_success "rebase: impossible fast-forward rebase" '
>> +    test_config rebase.autostash true &&
>> +    git reset --hard &&
>> +    echo dirty >>file1 &&
>> +    (git rebase --ff-only unrelated-onto-branch || true) &&
>> +    grep dirty file1 &&
>> +    git checkout feature-branch
>> +'
>> +
>>   test_expect_success "rebase: noop rebase" '
>>       test_config rebase.autostash true &&
>>       git reset --hard &&
>> diff --git a/t/t7601-merge-pull-config.sh b/t/t7601-merge-pull-config.sh
>> index 52e8ccc933..af260b9faa 100755
>> --- a/t/t7601-merge-pull-config.sh
>> +++ b/t/t7601-merge-pull-config.sh
>> @@ -183,6 +183,16 @@ test_expect_success 'pull prevents 
>> non-fast-forward with "only" in pull.ff' '
>>       test_must_fail git pull . c3
>>   '
>> +test_expect_success 'pull prevents non-fast-forward with --rebase 
>> --ff-only' '
>> +    git reset --hard c1 &&
>> +    test_must_fail git pull --rebase --ff-only . c3
>> +'
>> +
>> +test_expect_success 'pull prevents non-fast-forward with --no-rebase 
>> --ff-only' '
>> +    git reset --hard c1 &&
>> +    test_must_fail git pull --no-rebase --ff-only . c3
>> +'
>> +
>>   test_expect_success 'merge c1 with c2 (ours in pull.twohead)' '
>>       git reset --hard c1 &&
>>       git config pull.twohead ours &&
>>