Re: [PATCH v2 02/10] sequencer: introduce new commands to reset the revision

> In the upcoming commits, we will teach the sequencer to recreate merges.
> This will be done in a very different way from the unfortunate design of
> `git rebase --preserve-merges` (which does not allow for reordering
> commits, or changing the branch topology).
>
> The main idea is to introduce new todo list commands, to support
> labeling the current revision with a given name, resetting the current
> revision to a previous state, merging labeled revisions.
>
> This idea was developed in Git for Windows' Git garden shears (that are
> used to maintain the "thicket of branches" on top of upstream Git), and
> this patch is part of the effort to make it available to a wider
> audience, as well as to make the entire process more robust (by
> implementing it in a safe and portable language rather than a Unix shell
> script).
>
> This commit implements the commands to label, and to reset to, given
> revisions. The syntax is:
>
>         label <name>
>         reset <name>
>
> Internally, the `label <name>` command creates the ref
> `refs/rewritten/<name>`. This makes it possible to work with the labeled
> revisions interactively, or in a scripted fashion (e.g. via the todo
> list command `exec`).
>
> Later in this patch series, we will mark the `refs/rewritten/` refs as
> worktree-local, to allow for interactive rebases to be run in parallel in
> worktrees linked to the same repository.
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  git-rebase--interactive.sh |   2 +
>  sequencer.c                | 180 ++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 179 insertions(+), 3 deletions(-)
>
> diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
> index fcedece1860..7e5281e74aa 100644
> --- a/git-rebase--interactive.sh
> +++ b/git-rebase--interactive.sh
> @@ -162,6 +162,8 @@ s, squash <commit> = use commit, but meld into previous commit
>  f, fixup <commit> = like \"squash\", but discard this commit's log message
>  x, exec <commit> = run command (the rest of the line) using shell
>  d, drop <commit> = remove commit
> +l, label <label> = label current HEAD with a name
> +t, reset <label> = reset HEAD to a label
>
>  These lines can be re-ordered; they are executed from top to bottom.
>  " | git stripspace --comment-lines >>"$todo"
> diff --git a/sequencer.c b/sequencer.c
> index 4d3f60594cb..92ca8d2adee 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -21,6 +21,8 @@
>  #include "log-tree.h"
>  #include "wt-status.h"
>  #include "hashmap.h"
> +#include "unpack-trees.h"
> +#include "worktree.h"
>
>  #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
>
> @@ -116,6 +118,13 @@ static GIT_PATH_FUNC(rebase_path_stopped_sha, "rebase-merge/stopped-sha")
>  static GIT_PATH_FUNC(rebase_path_rewritten_list, "rebase-merge/rewritten-list")
>  static GIT_PATH_FUNC(rebase_path_rewritten_pending,
>         "rebase-merge/rewritten-pending")
> +
> +/*
> + * The path of the file listing refs that need to be deleted after the rebase
> + * finishes. This is used by the `merge` command.
> + */

> +static GIT_PATH_FUNC(rebase_path_refs_to_delete, "rebase-merge/refs-to-delete")
> +
>  /*
>   * The following files are written by git-rebase just after parsing the
>   * command-line (and are only consumed, not modified, by the sequencer).
> @@ -767,6 +776,8 @@ enum todo_command {
>         TODO_SQUASH,
>         /* commands that do something else than handling a single commit */
>         TODO_EXEC,
> +       TODO_LABEL,
> +       TODO_RESET,
>         /* commands that do nothing but are counted for reporting progress */
>         TODO_NOOP,
>         TODO_DROP,
> @@ -785,6 +796,8 @@ static struct {
>         { 'f', "fixup" },
>         { 's', "squash" },
>         { 'x', "exec" },
> +       { 'l', "label" },
> +       { 't', "reset" },
>         { 0,   "noop" },
>         { 'd', "drop" },
>         { 0,   NULL }
> @@ -1253,7 +1266,8 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
>                 if (skip_prefix(bol, todo_command_info[i].str, &bol)) {
>                         item->command = i;
>                         break;
> -               } else if (bol[1] == ' ' && *bol == todo_command_info[i].c) {
> +               } else if ((bol + 1 == eol || bol[1] == ' ') &&
> +                          *bol == todo_command_info[i].c) {
>                         bol++;
>                         item->command = i;
>                         break;
> @@ -1279,7 +1293,8 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
>                 return error(_("missing arguments for %s"),
>                              command_to_string(item->command));
>
> -       if (item->command == TODO_EXEC) {
> +       if (item->command == TODO_EXEC || item->command == TODO_LABEL ||
> +           item->command == TODO_RESET) {
>                 item->commit = NULL;
>                 item->arg = bol;
>                 item->arg_len = (int)(eol - bol);
> @@ -1919,6 +1934,144 @@ static int do_exec(const char *command_line)
>         return status;
>  }
>
> +static int safe_append(const char *filename, const char *fmt, ...)
> +{
> +       va_list ap;
> +       struct lock_file lock = LOCK_INIT;
> +       int fd = hold_lock_file_for_update(&lock, filename, 0);
> +       struct strbuf buf = STRBUF_INIT;
> +
> +       if (fd < 0)
> +               return error_errno(_("could not lock '%s'"), filename);
> +
> +       if (strbuf_read_file(&buf, filename, 0) < 0 && errno != ENOENT)
> +               return error_errno(_("could not read '%s'"), filename);
> +       strbuf_complete(&buf, '\n');
> +       va_start(ap, fmt);
> +       strbuf_vaddf(&buf, fmt, ap);
> +       va_end(ap);
> +
> +       if (write_in_full(fd, buf.buf, buf.len) < 0) {
> +               rollback_lock_file(&lock);
> +               return error_errno(_("could not write to '%s'"), filename);
> +       }
> +       if (commit_lock_file(&lock) < 0) {
> +               rollback_lock_file(&lock);
> +               return error(_("failed to finalize '%s'"), filename);
> +       }
> +
> +       return 0;
> +}
> +
> +static int do_label(const char *name, int len)
> +{
> +       struct ref_store *refs = get_main_ref_store();
> +       struct ref_transaction *transaction;
> +       struct strbuf ref_name = STRBUF_INIT, err = STRBUF_INIT;
> +       struct strbuf msg = STRBUF_INIT;
> +       int ret = 0;
> +       struct object_id head_oid;
> +
> +       strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
> +       strbuf_addf(&msg, "rebase -i (label) '%.*s'", len, name);
> +
> +       transaction = ref_store_transaction_begin(refs, &err);
> +       if (!transaction) {
> +               error("%s", err.buf);
> +               ret = -1;
> +       } else if (get_oid("HEAD", &head_oid)) {
> +               error(_("could not read HEAD"));
> +               ret = -1;
> +       } else if (ref_transaction_update(transaction, ref_name.buf, &head_oid,
> +                                         NULL, 0, msg.buf, &err) < 0 ||
> +                  ref_transaction_commit(transaction, &err)) {
> +               error("%s", err.buf);
> +               ret = -1;
> +       }
> +       ref_transaction_free(transaction);
> +       strbuf_release(&err);
> +       strbuf_release(&msg);
> +
> +       if (!ret)
> +               ret = safe_append(rebase_path_refs_to_delete(),
> +                                 "%s\n", ref_name.buf);
> +       strbuf_release(&ref_name);
> +
> +       return ret;
> +}
> +
> +static int do_reset(const char *name, int len)
> +{
> +       struct strbuf ref_name = STRBUF_INIT;
> +       struct object_id oid;
> +       struct lock_file lock = LOCK_INIT;
> +       struct tree_desc desc;
> +       struct tree *tree;
> +       struct unpack_trees_options opts;
> +       int ret = 0, i;
> +
> +       if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
> +               return -1;
> +
> +       /* Determine the length of the label */
> +       for (i = 0; i < len; i++)
> +               if (isspace(name[i]))
> +                       len = i;
> +
> +       strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
> +       if (get_oid(ref_name.buf, &oid) &&
> +           get_oid(ref_name.buf + strlen("refs/rewritten/"), &oid)) {
> +               error(_("could not read '%s'"), ref_name.buf);
> +               rollback_lock_file(&lock);
> +               strbuf_release(&ref_name);
> +               return -1;
> +       }
> +
> +       memset(&opts, 0, sizeof(opts));
> +       opts.head_idx = 1;
> +       opts.src_index = &the_index;
> +       opts.dst_index = &the_index;
> +       opts.fn = oneway_merge;
> +       opts.merge = 1;
> +       opts.update = 1;
> +       opts.reset = 1;
> +
> +       read_cache_unmerged();

> +       if (!fill_tree_descriptor(&desc, &oid)) {
> +               error(_("failed to find tree of %s"), oid_to_hex(&oid));
> +               rollback_lock_file(&lock);
> +               free((void *)desc.buffer);
> +               strbuf_release(&ref_name);
> +               return -1;
> +       }
> +
> +       if (unpack_trees(1, &desc, &opts)) {
> +               rollback_lock_file(&lock);
> +               free((void *)desc.buffer);
> +               strbuf_release(&ref_name);
> +               return -1;
> +       }
> +
> +       tree = parse_tree_indirect(&oid);
> +       prime_cache_tree(&the_index, tree);
> +
> +       if (write_locked_index(&the_index, &lock, COMMIT_LOCK) < 0)
> +               ret = error(_("could not write index"));
> +       free((void *)desc.buffer);