Re: [PATCH v9 2/7] builtin/replay: move core logic into "libgit.a"

>
> Move the core logic used to replay commits into "libgit.a" so that it
> can be easily reused by other commands. It will be used in a subsequent
> commit where we're about to introduce a new git-history(1) command.
>
> Note that with this change we have no sign-comparison warnings anymore,
> and neither do we depend on `the_repository`.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  Makefile         |   1 +
>  builtin/replay.c | 368 +------------------------------------------------------
>  meson.build      |   1 +
>  replay.c         | 355 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  replay.h         |  64 ++++++++++
>  5 files changed, 422 insertions(+), 367 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index b7eba509c6..1c64a5d2ae 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1285,6 +1285,7 @@ LIB_OBJS += repack-geometry.o
>  LIB_OBJS += repack-midx.o
>  LIB_OBJS += repack-promisor.o
>  LIB_OBJS += replace-object.o
> +LIB_OBJS += replay.o
>  LIB_OBJS += repo-settings.o
>  LIB_OBJS += repository.o
>  LIB_OBJS += rerere.o
> diff --git a/builtin/replay.c b/builtin/replay.c
> index df3b32a52d..da8b7202f6 100644
> --- a/builtin/replay.c
> +++ b/builtin/replay.c
> @@ -2,257 +2,22 @@
>   * "git replay" builtin command
>   */
>
> -#define USE_THE_REPOSITORY_VARIABLE
> -#define DISABLE_SIGN_COMPARE_WARNINGS
> -
>  #include "git-compat-util.h"
>
>  #include "builtin.h"
>  #include "config.h"
> -#include "environment.h"
>  #include "hex.h"
> -#include "lockfile.h"
> -#include "merge-ort.h"
>  #include "object-name.h"
>  #include "parse-options.h"
>  #include "refs.h"
> +#include "replay.h"
>  #include "revision.h"
> -#include "strmap.h"
> -#include <oidset.h>
> -#include <tree.h>
>
>  enum ref_action_mode {
>         REF_ACTION_UPDATE,
>         REF_ACTION_PRINT,
>  };
>
> -static const char *short_commit_name(struct repository *repo,
> -                                    struct commit *commit)
> -{
> -       return repo_find_unique_abbrev(repo, &commit->object.oid,
> -                                      DEFAULT_ABBREV);
> -}
> -
> -static struct commit *peel_committish(struct repository *repo,
> -                                     const char *name,
> -                                     const char *mode)
> -{
> -       struct object *obj;
> -       struct object_id oid;
> -
> -       if (repo_get_oid(repo, name, &oid))
> -               die(_("'%s' is not a valid commit-ish for %s"), name, mode);
> -       obj = parse_object_or_die(repo, &oid, name);
> -       return (struct commit *)repo_peel_to_type(repo, name, 0, obj,
> -                                                 OBJ_COMMIT);
> -}
> -
> -static char *get_author(const char *message)
> -{
> -       size_t len;
> -       const char *a;
> -
> -       a = find_commit_header(message, "author", &len);
> -       if (a)
> -               return xmemdupz(a, len);
> -
> -       return NULL;
> -}
> -
> -static struct commit *create_commit(struct repository *repo,
> -                                   struct tree *tree,
> -                                   struct commit *based_on,
> -                                   struct commit *parent)
> -{
> -       struct object_id ret;
> -       struct object *obj = NULL;
> -       struct commit_list *parents = NULL;
> -       char *author;
> -       char *sign_commit = NULL; /* FIXME: cli users might want to sign again */
> -       struct commit_extra_header *extra = NULL;
> -       struct strbuf msg = STRBUF_INIT;
> -       const char *out_enc = get_commit_output_encoding();
> -       const char *message = repo_logmsg_reencode(repo, based_on,
> -                                                  NULL, out_enc);
> -       const char *orig_message = NULL;
> -       const char *exclude_gpgsig[] = { "gpgsig", "gpgsig-sha256", NULL };
> -
> -       commit_list_insert(parent, &parents);
> -       extra = read_commit_extra_headers(based_on, exclude_gpgsig);
> -       find_commit_subject(message, &orig_message);
> -       strbuf_addstr(&msg, orig_message);
> -       author = get_author(message);
> -       reset_ident_date();
> -       if (commit_tree_extended(msg.buf, msg.len, &tree->object.oid, parents,
> -                                &ret, author, NULL, sign_commit, extra)) {
> -               error(_("failed to write commit object"));
> -               goto out;
> -       }
> -
> -       obj = parse_object(repo, &ret);
> -
> -out:
> -       repo_unuse_commit_buffer(the_repository, based_on, message);
> -       free_commit_extra_headers(extra);
> -       free_commit_list(parents);
> -       strbuf_release(&msg);
> -       free(author);
> -       return (struct commit *)obj;
> -}
> -
> -struct ref_info {
> -       struct commit *onto;
> -       struct strset positive_refs;
> -       struct strset negative_refs;
> -       int positive_refexprs;
> -       int negative_refexprs;
> -};
> -
> -static void get_ref_information(struct repository *repo,
> -                               struct rev_cmdline_info *cmd_info,
> -                               struct ref_info *ref_info)
> -{
> -       int i;
> -
> -       ref_info->onto = NULL;
> -       strset_init(&ref_info->positive_refs);
> -       strset_init(&ref_info->negative_refs);
> -       ref_info->positive_refexprs = 0;
> -       ref_info->negative_refexprs = 0;
> -
> -       /*
> -        * When the user specifies e.g.
> -        *   git replay origin/main..mybranch
> -        *   git replay ^origin/next mybranch1 mybranch2
> -        * we want to be able to determine where to replay the commits.  In
> -        * these examples, the branches are probably based on an old version
> -        * of either origin/main or origin/next, so we want to replay on the
> -        * newest version of that branch.  In contrast we would want to error
> -        * out if they ran
> -        *   git replay ^origin/master ^origin/next mybranch
> -        *   git replay mybranch~2..mybranch
> -        * the first of those because there's no unique base to choose, and
> -        * the second because they'd likely just be replaying commits on top
> -        * of the same commit and not making any difference.
> -        */
> -       for (i = 0; i < cmd_info->nr; i++) {
> -               struct rev_cmdline_entry *e = cmd_info->rev + i;
> -               struct object_id oid;
> -               const char *refexpr = e->name;
> -               char *fullname = NULL;
> -               int can_uniquely_dwim = 1;
> -
> -               if (*refexpr == '^')
> -                       refexpr++;
> -               if (repo_dwim_ref(repo, refexpr, strlen(refexpr), &oid, &fullname, 0) != 1)
> -                       can_uniquely_dwim = 0;
> -
> -               if (e->flags & BOTTOM) {
> -                       if (can_uniquely_dwim)
> -                               strset_add(&ref_info->negative_refs, fullname);
> -                       if (!ref_info->negative_refexprs)
> -                               ref_info->onto = lookup_commit_reference_gently(repo,
> -                                                                               &e->item->oid, 1);
> -                       ref_info->negative_refexprs++;
> -               } else {
> -                       if (can_uniquely_dwim)
> -                               strset_add(&ref_info->positive_refs, fullname);
> -                       ref_info->positive_refexprs++;
> -               }
> -
> -               free(fullname);
> -       }
> -}
> -
> -static void set_up_replay_mode(struct repository *repo,
> -                              struct rev_cmdline_info *cmd_info,
> -                              const char *onto_name,
> -                              char **advance_name,
> -                              struct commit **onto,
> -                              struct strset **update_refs)
> -{
> -       struct ref_info rinfo;
> -
> -       get_ref_information(repo, cmd_info, &rinfo);
> -       if (!rinfo.positive_refexprs)
> -               die(_("need some commits to replay"));
> -
> -       die_for_incompatible_opt2(!!onto_name, "--onto",
> -                                 !!*advance_name, "--advance");
> -       if (onto_name) {
> -               *onto = peel_committish(repo, onto_name, "--onto");
> -               if (rinfo.positive_refexprs <
> -                   strset_get_size(&rinfo.positive_refs))
> -                       die(_("all positive revisions given must be references"));
> -               *update_refs = xcalloc(1, sizeof(**update_refs));
> -               **update_refs = rinfo.positive_refs;
> -               memset(&rinfo.positive_refs, 0, sizeof(**update_refs));
> -       } else {
> -               struct object_id oid;
> -               char *fullname = NULL;
> -
> -               if (!*advance_name)
> -                       BUG("expected either onto_name or *advance_name in this function");
> -
> -               if (repo_dwim_ref(repo, *advance_name, strlen(*advance_name),
> -                            &oid, &fullname, 0) == 1) {
> -                       free(*advance_name);
> -                       *advance_name = fullname;
> -               } else {
> -                       die(_("argument to --advance must be a reference"));
> -               }
> -               *onto = peel_committish(repo, *advance_name, "--advance");
> -               if (rinfo.positive_refexprs > 1)
> -                       die(_("cannot advance target with multiple sources because ordering would be ill-defined"));
> -       }
> -       strset_clear(&rinfo.negative_refs);
> -       strset_clear(&rinfo.positive_refs);
> -}
> -
> -static struct commit *mapped_commit(kh_oid_map_t *replayed_commits,
> -                                   struct commit *commit,
> -                                   struct commit *fallback)
> -{
> -       khint_t pos = kh_get_oid_map(replayed_commits, commit->object.oid);
> -       if (pos == kh_end(replayed_commits))
> -               return fallback;
> -       return kh_value(replayed_commits, pos);
> -}
> -
> -static struct commit *pick_regular_commit(struct repository *repo,
> -                                         struct commit *pickme,
> -                                         kh_oid_map_t *replayed_commits,
> -                                         struct commit *onto,
> -                                         struct merge_options *merge_opt,
> -                                         struct merge_result *result)
> -{
> -       struct commit *base, *replayed_base;
> -       struct tree *pickme_tree, *base_tree;
> -
> -       base = pickme->parents->item;
> -       replayed_base = mapped_commit(replayed_commits, base, onto);
> -
> -       result->tree = repo_get_commit_tree(repo, replayed_base);
> -       pickme_tree = repo_get_commit_tree(repo, pickme);
> -       base_tree = repo_get_commit_tree(repo, base);
> -
> -       merge_opt->branch1 = short_commit_name(repo, replayed_base);
> -       merge_opt->branch2 = short_commit_name(repo, pickme);
> -       merge_opt->ancestor = xstrfmt("parent of %s", merge_opt->branch2);
> -
> -       merge_incore_nonrecursive(merge_opt,
> -                                 base_tree,
> -                                 result->tree,
> -                                 pickme_tree,
> -                                 result);
> -
> -       free((char*)merge_opt->ancestor);
> -       merge_opt->ancestor = NULL;
> -       if (!result->clean)
> -               return NULL;
> -       return create_commit(repo, result->tree, pickme, replayed_base);
> -}
> -
>  static enum ref_action_mode parse_ref_action_mode(const char *ref_action, const char *source)
>  {
>         if (!ref_action || !strcmp(ref_action, "update"))
> @@ -278,137 +43,6 @@ static enum ref_action_mode get_ref_action_mode(struct repository *repo, const c
>         return REF_ACTION_UPDATE;
>  }
>
> -struct replay_revisions_options {
> -       const char *advance;
> -       const char *onto;
> -       int contained;
> -};
> -
> -struct replay_result {
> -       struct replay_ref_update {
> -               char *refname;
> -               struct object_id old_oid;
> -               struct object_id new_oid;
> -       } *updates;
> -       size_t updates_nr, updates_alloc;
> -
> -       bool merge_conflict;
> -};
> -
> -static void replay_result_release(struct replay_result *result)
> -{
> -       for (size_t i = 0; i < result->updates_nr; i++)
> -               free(result->updates[i].refname);
> -       free(result->updates);
> -}
> -
> -static void replay_result_queue_update(struct replay_result *result,
> -                                      const char *refname,
> -                                      const struct object_id *old_oid,
> -                                      const struct object_id *new_oid)
> -{
> -       ALLOC_GROW(result->updates, result->updates_nr + 1, result->updates_alloc);
> -       result->updates[result->updates_nr].refname = xstrdup(refname);
> -       result->updates[result->updates_nr].old_oid = *old_oid;
> -       result->updates[result->updates_nr].new_oid = *new_oid;
> -       result->updates_nr++;
> -}
> -
> -static int replay_revisions(struct repository *repo, struct rev_info *revs,
> -                           struct replay_revisions_options *opts,
> -                           struct replay_result *out)
> -{
> -       kh_oid_map_t *replayed_commits = NULL;
> -       struct strset *update_refs = NULL;
> -       struct commit *last_commit = NULL;
> -       struct commit *commit;
> -       struct commit *onto = NULL;
> -       struct merge_options merge_opt;
> -       struct merge_result result;
> -       char *advance;
> -       int ret;
> -
> -       advance = xstrdup_or_null(opts->advance);
> -       set_up_replay_mode(repo, &revs->cmdline, opts->onto, &advance,
> -                          &onto, &update_refs);
> -
> -       /* FIXME: Should allow replaying commits with the first as a root commit */
> -
> -       if (prepare_revision_walk(revs) < 0) {
> -               ret = error(_("error preparing revisions"));
> -               goto out;
> -       }
> -
> -       init_basic_merge_options(&merge_opt, repo);
> -       memset(&result, 0, sizeof(result));
> -       merge_opt.show_rename_progress = 0;
> -       last_commit = onto;
> -       replayed_commits = kh_init_oid_map();
> -       while ((commit = get_revision(revs))) {
> -               const struct name_decoration *decoration;
> -               khint_t pos;
> -               int hr;
> -
> -               if (!commit->parents)
> -                       die(_("replaying down from root commit is not supported yet!"));
> -               if (commit->parents->next)
> -                       die(_("replaying merge commits is not supported yet!"));
> -
> -               last_commit = pick_regular_commit(repo, commit, replayed_commits,
> -                                                 onto, &merge_opt, &result);
> -               if (!last_commit)
> -                       break;
> -
> -               /* Record commit -> last_commit mapping */
> -               pos = kh_put_oid_map(replayed_commits, commit->object.oid, &hr);
> -               if (hr == 0)
> -                       BUG("Duplicate rewritten commit: %s\n",
> -                           oid_to_hex(&commit->object.oid));
> -               kh_value(replayed_commits, pos) = last_commit;
> -
> -               /* Update any necessary branches */
> -               if (advance)
> -                       continue;
> -               decoration = get_name_decoration(&commit->object);
> -               if (!decoration)
> -                       continue;
> -               while (decoration) {
> -                       if (decoration->type == DECORATION_REF_LOCAL &&
> -                           (opts->contained || strset_contains(update_refs,
> -                                                               decoration->name))) {
> -                               replay_result_queue_update(out, decoration->name,
> -                                                          &commit->object.oid,
> -                                                          &last_commit->object.oid);
> -                       }
> -                       decoration = decoration->next;
> -               }
> -       }
> -
> -       if (!result.clean) {
> -               out->merge_conflict = true;
> -               ret = -1;
> -               goto out;
> -       }
> -
> -       /* In --advance mode, advance the target ref */
> -       if (advance)
> -               replay_result_queue_update(out, advance,
> -                                          &onto->object.oid,
> -                                          &last_commit->object.oid);
> -
> -       ret = 0;
> -
> -out:
> -       if (update_refs) {
> -               strset_clear(update_refs);
> -               free(update_refs);
> -       }
> -       kh_destroy_oid_map(replayed_commits);
> -       merge_finalize(&merge_opt, &result);
> -       free(advance);
> -       return ret;
> -}
> -
>  static int handle_ref_update(enum ref_action_mode mode,
>                              struct ref_transaction *transaction,
>                              const char *refname,
> diff --git a/meson.build b/meson.build
> index dd52efd1c8..a5a4e99b25 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -471,6 +471,7 @@ libgit_sources = [
>    'repack-midx.c',
>    'repack-promisor.c',
>    'replace-object.c',
> +  'replay.c',
>    'repo-settings.c',
>    'repository.c',
>    'rerere.c',
> diff --git a/replay.c b/replay.c
> new file mode 100644
> index 0000000000..fc7186ef09
> --- /dev/null
> +++ b/replay.c
> @@ -0,0 +1,355 @@
> +#define USE_THE_REPOSITORY_VARIABLE
> +#define DISABLE_SIGN_COMPARE_WARNINGS
> +
> +#include "git-compat-util.h"
> +#include "environment.h"
> +#include "hex.h"
> +#include "merge-ort.h"
> +#include "object-name.h"
> +#include "oidset.h"

> +#include "tree.h"
> +
> +static const char *short_commit_name(struct repository *repo,
> +                                    struct commit *commit)
> +{
> +       return repo_find_unique_abbrev(repo, &commit->object.oid,
> +                                      DEFAULT_ABBREV);
> +}
> +
> +static struct commit *peel_committish(struct repository *repo,
> +                                     const char *name,
> +                                     const char *mode)
> +{
> +       struct object *obj;
> +       struct object_id oid;
> +
> +       if (repo_get_oid(repo, name, &oid))
> +               die(_("'%s' is not a valid commit-ish for %s"), name, mode);
> +       obj = parse_object_or_die(repo, &oid, name);
> +       return (struct commit *)repo_peel_to_type(repo, name, 0, obj,
> +                                                 OBJ_COMMIT);
> +}
> +
> +static char *get_author(const char *message)
> +{
> +       size_t len;
> +       const char *a;
> +
> +       a = find_commit_header(message, "author", &len);
> +       if (a)
> +               return xmemdupz(a, len);
> +
> +       return NULL;
> +}
> +
> +static struct commit *create_commit(struct repository *repo,
> +                                   struct tree *tree,
> +                                   struct commit *based_on,
> +                                   struct commit *parent)
> +{
> +       struct object_id ret;
> +       struct object *obj = NULL;
> +       struct commit_list *parents = NULL;
> +       char *author;
> +       char *sign_commit = NULL; /* FIXME: cli users might want to sign again */
> +       struct commit_extra_header *extra = NULL;
> +       struct strbuf msg = STRBUF_INIT;
> +       const char *out_enc = get_commit_output_encoding();
> +       const char *message = repo_logmsg_reencode(repo, based_on,
> +                                                  NULL, out_enc);
> +       const char *orig_message = NULL;
> +       const char *exclude_gpgsig[] = { "gpgsig", "gpgsig-sha256", NULL };
> +
> +       commit_list_insert(parent, &parents);
> +       extra = read_commit_extra_headers(based_on, exclude_gpgsig);
> +       find_commit_subject(message, &orig_message);
> +       strbuf_addstr(&msg, orig_message);
> +       author = get_author(message);
> +       reset_ident_date();
> +       if (commit_tree_extended(msg.buf, msg.len, &tree->object.oid, parents,
> +                                &ret, author, NULL, sign_commit, extra)) {
> +               error(_("failed to write commit object"));
> +               goto out;
> +       }
> +
> +       obj = parse_object(repo, &ret);
> +
> +out:
> +       repo_unuse_commit_buffer(the_repository, based_on, message);
> +       free_commit_extra_headers(extra);
> +       free_commit_list(parents);
> +       strbuf_release(&msg);
> +       free(author);
> +       return (struct commit *)obj;
> +}
> +
> +struct ref_info {
> +       struct commit *onto;
> +       struct strset positive_refs;
> +       struct strset negative_refs;
> +       int positive_refexprs;
> +       int negative_refexprs;
> +};
> +
> +static void get_ref_information(struct repository *repo,
> +                               struct rev_cmdline_info *cmd_info,
> +                               struct ref_info *ref_info)
> +{
> +       int i;
> +
> +       ref_info->onto = NULL;
> +       strset_init(&ref_info->positive_refs);
> +       strset_init(&ref_info->negative_refs);
> +       ref_info->positive_refexprs = 0;
> +       ref_info->negative_refexprs = 0;
> +
> +       /*
> +        * When the user specifies e.g.
> +        *   git replay origin/main..mybranch
> +        *   git replay ^origin/next mybranch1 mybranch2
> +        * we want to be able to determine where to replay the commits.  In
> +        * these examples, the branches are probably based on an old version
> +        * of either origin/main or origin/next, so we want to replay on the
> +        * newest version of that branch.  In contrast we would want to error
> +        * out if they ran
> +        *   git replay ^origin/master ^origin/next mybranch
> +        *   git replay mybranch~2..mybranch
> +        * the first of those because there's no unique base to choose, and
> +        * the second because they'd likely just be replaying commits on top
> +        * of the same commit and not making any difference.
> +        */
> +       for (i = 0; i < cmd_info->nr; i++) {
> +               struct rev_cmdline_entry *e = cmd_info->rev + i;
> +               struct object_id oid;
> +               const char *refexpr = e->name;
> +               char *fullname = NULL;
> +               int can_uniquely_dwim = 1;
> +
> +               if (*refexpr == '^')
> +                       refexpr++;
> +               if (repo_dwim_ref(repo, refexpr, strlen(refexpr), &oid, &fullname, 0) != 1)
> +                       can_uniquely_dwim = 0;
> +
> +               if (e->flags & BOTTOM) {
> +                       if (can_uniquely_dwim)
> +                               strset_add(&ref_info->negative_refs, fullname);
> +                       if (!ref_info->negative_refexprs)
> +                               ref_info->onto = lookup_commit_reference_gently(repo,
> +                                                                               &e->item->oid, 1);
> +                       ref_info->negative_refexprs++;
> +               } else {
> +                       if (can_uniquely_dwim)
> +                               strset_add(&ref_info->positive_refs, fullname);
> +                       ref_info->positive_refexprs++;
> +               }
> +
> +               free(fullname);
> +       }
> +}
> +
> +static void set_up_replay_mode(struct repository *repo,
> +                              struct rev_cmdline_info *cmd_info,
> +                              const char *onto_name,
> +                              char **advance_name,
> +                              struct commit **onto,
> +                              struct strset **update_refs)
> +{
> +       struct ref_info rinfo;
> +
> +       get_ref_information(repo, cmd_info, &rinfo);
> +       if (!rinfo.positive_refexprs)
> +               die(_("need some commits to replay"));
> +
> +       die_for_incompatible_opt2(!!onto_name, "--onto",
> +                                 !!*advance_name, "--advance");
> +       if (onto_name) {
> +               *onto = peel_committish(repo, onto_name, "--onto");
> +               if (rinfo.positive_refexprs <
> +                   strset_get_size(&rinfo.positive_refs))
> +                       die(_("all positive revisions given must be references"));
> +               *update_refs = xcalloc(1, sizeof(**update_refs));
> +               **update_refs = rinfo.positive_refs;
> +               memset(&rinfo.positive_refs, 0, sizeof(**update_refs));
> +       } else {
> +               struct object_id oid;
> +               char *fullname = NULL;
> +
> +               if (!*advance_name)
> +                       BUG("expected either onto_name or *advance_name in this function");
> +
> +               if (repo_dwim_ref(repo, *advance_name, strlen(*advance_name),
> +                            &oid, &fullname, 0) == 1) {
> +                       free(*advance_name);
> +                       *advance_name = fullname;
> +               } else {
> +                       die(_("argument to --advance must be a reference"));
> +               }
> +               *onto = peel_committish(repo, *advance_name, "--advance");
> +               if (rinfo.positive_refexprs > 1)
> +                       die(_("cannot advance target with multiple sources because ordering would be ill-defined"));
> +       }
> +       strset_clear(&rinfo.negative_refs);
> +       strset_clear(&rinfo.positive_refs);
> +}
> +
> +static struct commit *mapped_commit(kh_oid_map_t *replayed_commits,
> +                                   struct commit *commit,
> +                                   struct commit *fallback)
> +{
> +       khint_t pos = kh_get_oid_map(replayed_commits, commit->object.oid);
> +       if (pos == kh_end(replayed_commits))
> +               return fallback;
> +       return kh_value(replayed_commits, pos);
> +}
> +
> +static struct commit *pick_regular_commit(struct repository *repo,
> +                                         struct commit *pickme,
> +                                         kh_oid_map_t *replayed_commits,
> +                                         struct commit *onto,
> +                                         struct merge_options *merge_opt,
> +                                         struct merge_result *result)
> +{
> +       struct commit *base, *replayed_base;
> +       struct tree *pickme_tree, *base_tree;
> +
> +       base = pickme->parents->item;
> +       replayed_base = mapped_commit(replayed_commits, base, onto);
> +
> +       result->tree = repo_get_commit_tree(repo, replayed_base);
> +       pickme_tree = repo_get_commit_tree(repo, pickme);
> +       base_tree = repo_get_commit_tree(repo, base);
> +
> +       merge_opt->branch1 = short_commit_name(repo, replayed_base);
> +       merge_opt->branch2 = short_commit_name(repo, pickme);
> +       merge_opt->ancestor = xstrfmt("parent of %s", merge_opt->branch2);
> +
> +       merge_incore_nonrecursive(merge_opt,
> +                                 base_tree,
> +                                 result->tree,
> +                                 pickme_tree,
> +                                 result);
> +
> +       free((char*)merge_opt->ancestor);
> +       merge_opt->ancestor = NULL;
> +       if (!result->clean)
> +               return NULL;
> +       return create_commit(repo, result->tree, pickme, replayed_base);
> +}
> +
> +void replay_result_release(struct replay_result *result)
> +{
> +       for (size_t i = 0; i < result->updates_nr; i++)
> +               free(result->updates[i].refname);
> +       free(result->updates);
> +}
> +
> +static void replay_result_queue_update(struct replay_result *result,
> +                                      const char *refname,
> +                                      const struct object_id *old_oid,
> +                                      const struct object_id *new_oid)
> +{
> +       ALLOC_GROW(result->updates, result->updates_nr + 1, result->updates_alloc);
> +       result->updates[result->updates_nr].refname = xstrdup(refname);
> +       result->updates[result->updates_nr].old_oid = *old_oid;
> +       result->updates[result->updates_nr].new_oid = *new_oid;
> +       result->updates_nr++;
> +}
> +
> +int replay_revisions(struct repository *repo, struct rev_info *revs,
> +                    struct replay_revisions_options *opts,
> +                    struct replay_result *out)
> +{
> +       kh_oid_map_t *replayed_commits = NULL;
> +       struct strset *update_refs = NULL;
> +       struct commit *last_commit = NULL;
> +       struct commit *commit;
> +       struct commit *onto = NULL;
> +       struct merge_options merge_opt;
> +       struct merge_result result;
> +       char *advance;
> +       int ret;
> +
> +       advance = xstrdup_or_null(opts->advance);
> +       set_up_replay_mode(repo, &revs->cmdline, opts->onto, &advance,
> +                          &onto, &update_refs);
> +
> +       /* FIXME: Should allow replaying commits with the first as a root commit */
> +
> +       if (prepare_revision_walk(revs) < 0) {
> +               ret = error(_("error preparing revisions"));
> +               goto out;
> +       }
> +
> +       init_basic_merge_options(&merge_opt, repo);
> +       memset(&result, 0, sizeof(result));
> +       merge_opt.show_rename_progress = 0;
> +       last_commit = onto;
> +       replayed_commits = kh_init_oid_map();
> +       while ((commit = get_revision(revs))) {
> +               const struct name_decoration *decoration;
> +               khint_t pos;
> +               int hr;
> +
> +               if (!commit->parents)
> +                       die(_("replaying down from root commit is not supported yet!"));
> +               if (commit->parents->next)
> +                       die(_("replaying merge commits is not supported yet!"));
> +
> +               last_commit = pick_regular_commit(repo, commit, replayed_commits,
> +                                                 onto, &merge_opt, &result);
> +               if (!last_commit)
> +                       break;
> +
> +               /* Record commit -> last_commit mapping */
> +               pos = kh_put_oid_map(replayed_commits, commit->object.oid, &hr);
> +               if (hr == 0)
> +                       BUG("Duplicate rewritten commit: %s\n",
> +                           oid_to_hex(&commit->object.oid));
> +               kh_value(replayed_commits, pos) = last_commit;
> +
> +               /* Update any necessary branches */
> +               if (advance)
> +                       continue;
> +               decoration = get_name_decoration(&commit->object);
> +               if (!decoration)
> +                       continue;
> +               while (decoration) {
> +                       if (decoration->type == DECORATION_REF_LOCAL &&
> +                           (opts->contained || strset_contains(update_refs,
> +                                                               decoration->name))) {
> +                               replay_result_queue_update(out, decoration->name,
> +                                                          &commit->object.oid,
> +                                                          &last_commit->object.oid);
> +                       }
> +                       decoration = decoration->next;
> +               }
> +       }
> +
> +       if (!result.clean) {
> +               out->merge_conflict = true;
> +               ret = -1;
> +               goto out;
> +       }
> +
> +       /* In --advance mode, advance the target ref */
> +       if (advance)
> +               replay_result_queue_update(out, advance,
> +                                          &onto->object.oid,
> +                                          &last_commit->object.oid);
> +
> +       ret = 0;
> +
> +out:
> +       if (update_refs) {
> +               strset_clear(update_refs);
> +               free(update_refs);
> +       }
> +       kh_destroy_oid_map(replayed_commits);
> +       merge_finalize(&merge_opt, &result);
> +       free(advance);
> +       return ret;
> +}
> diff --git a/replay.h b/replay.h
> new file mode 100644
> index 0000000000..84bc8a7a5b
> --- /dev/null
> +++ b/replay.h
> @@ -0,0 +1,64 @@
> +#ifndef REPLAY_H
> +#define REPLAY_H
> +
> +#include "hash.h"
> +
> +struct repository;
> +struct rev_info;
> +
> +/*
> + * A set of options that can be passed to `replay_revisions()`.
> + */
> +struct replay_revisions_options {
> +       /*
> +        * Starting point at which to create the new commits; must be a branch
> +        * name. The branch will be updated to point to the rewritten commits.
> +        * This option is mutually exclusive with `onto`.
> +        */
> +       const char *advance;
> +
> +       /*
> +        * Starting point at which to create the new commits; must be a
> +        * committish. References pointing at decendants of `onto` will be
> +        * updated to point to the new commits.
> +        */
> +        const char *onto;
> +
> +       /*
> +        * Update branches that point at commits in the given revision range.
> +        * Requires `onto` to be set.
> +        */
> +       int contained;
> +};
> +
> +/* This struct is used as an out-parameter by `replay_revisions()`. */
> +struct replay_result {
> +       /*
> +        * The set of reference updates that are caused by replaying the
> +        * commits.
> +        */
> +       struct replay_ref_update {
> +               char *refname;
> +               struct object_id old_oid;
> +               struct object_id new_oid;
> +       } *updates;
> +       size_t updates_nr, updates_alloc;
> +
> +       /* Set to true in case the replay failed with a merge conflict. */
> +       bool merge_conflict;
> +};
> +
> +void replay_result_release(struct replay_result *result);
> +
> +/*
> + * Replay a set of commits onto a new location. Leaves both the working tree,
> + * index and references untouched. Reference updates caused by the replay will
> + * be recorded in the `updates` out pointer.
> + *
> + * Returns 0 on success, a negative error code otherwise.
> + */
> +int replay_revisions(struct repository *repo, struct rev_info *revs,
> +                    struct replay_revisions_options *opts,
> +                    struct replay_result *out);
> +