[PATCH 3/4] rebase: add --update-refs option

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

When working on a large feature, it can be helpful to break that feature
into multiple smaller parts that become reviewed in sequence. During
development or during review, a change to one part of the feature could
affect multiple of these parts. An interactive rebase can help adjust
the multi-part "story" of the branch.

However, if there are branches tracking the different parts of the
feature, then rebasing the entire list of commits can create commits not
reachable from those "sub branches". It can take a manual step to update
those branches.

Add a new --update-refs option to 'git rebase -i' that adds 'git
update-ref' exec steps to the todo file whenever a commit that is being
rebased is decorated with that <ref>. This allows the user to rebase a
long list of commits in a multi-part feature and keep all of their
pointers to those parts.

Use the new for_each_decoration() while iterating over the existing todo
list. Be sure to iterate after any squashing or fixups are placed.
Update the branch only after those squashes and fixups are complete.
This allows a --fixup commit at the tip of the feature to apply
correctly to the sub branch, even if it is fixing up the most-recent
commit in that part.

One potential problem here is that refs decorating commits that are
already marked as "fixup!" or "squash!" will not be included in this
list. Generally, the reordering of the "fixup!" and "squash!" is likely
to change the relative order of these refs, so it is not recommended.
The workflow here is intended to allow these kinds of commits at the tip
of the rebased branch while the other sub branches come along for the
ride without intervention.

Be careful to not attempt updating any branch that is checked out. The
most common example is the branch being rebased is checked out and
decorates the tip commit. If the user is rebasing commits reachable from
a different branch that is checked out in a different worktree, then
they may be surprised to not see that ref update. However, it's probably
best to not optimize for this scenario and do the safest thing that will
result in a successful rebase. A comment is left in the TODO list that
signals that these refs are currently checked out.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
 Documentation/git-rebase.txt  |  7 +++
 builtin/rebase.c              |  5 ++
 sequencer.c                   | 99 +++++++++++++++++++++++++++++++++++
 sequencer.h                   |  1 +
 t/t3404-rebase-interactive.sh | 29 ++++++++++
 5 files changed, 141 insertions(+)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 262fb01aec0..866554fc978 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -609,6 +609,13 @@ provided. Otherwise an explicit `--no-reschedule-failed-exec` at the
 start would be overridden by the presence of
 `rebase.rescheduleFailedExec=true` configuration.
 
+--update-refs::
+--no-update-refs::
+	Automatically force-update any branches that point to commits that
+	are being rebased. Any branches that are checked out in a worktree
+	or point to a `squash! ...` or `fixup! ...` commit are not updated
+	in this way.
+
 INCOMPATIBLE OPTIONS
 --------------------
 
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 7ab50cda2ad..56d82a52106 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -102,6 +102,7 @@ struct rebase_options {
 	int reschedule_failed_exec;
 	int reapply_cherry_picks;
 	int fork_point;
+	int update_refs;
 };
 
 #define REBASE_OPTIONS_INIT {			  	\
@@ -298,6 +299,7 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
 		ret = complete_action(the_repository, &replay, flags,
 			shortrevisions, opts->onto_name, opts->onto,
 			&opts->orig_head, &commands, opts->autosquash,
+			opts->update_refs,
 			&todo_list);
 	}
 
@@ -1124,6 +1126,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "autosquash", &options.autosquash,
 			 N_("move commits that begin with "
 			    "squash!/fixup! under -i")),
+		OPT_BOOL(0, "update-refs", &options.update_refs,
+			 N_("update local refs that point to commits "
+			    "that are being rebased")),
 		{ OPTION_STRING, 'S', "gpg-sign", &gpg_sign, N_("key-id"),
 			N_("GPG-sign commits"),
 			PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
diff --git a/sequencer.c b/sequencer.c
index 8c3ed3532ac..d6151af9849 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -35,6 +35,8 @@
 #include "commit-reach.h"
 #include "rebase-interactive.h"
 #include "reset.h"
+#include "branch.h"
+#include "log-tree.h"
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
@@ -5603,10 +5605,104 @@ static int skip_unnecessary_picks(struct repository *r,
 	return 0;
 }
 
+struct todo_add_branch_context {
+	struct todo_list new_list;
+	struct strbuf *buf;
+	struct commit *commit;
+};
+
+static int add_branch_for_decoration(const struct name_decoration *d, void *data)
+{
+	struct todo_add_branch_context *ctx = data;
+	size_t base_offset = ctx->buf->len;
+	int i = ctx->new_list.nr;
+	struct todo_item *item;
+	char *path;
+
+	ALLOC_GROW(ctx->new_list.items,
+		   ctx->new_list.nr + 1,
+		   ctx->new_list.alloc);
+	item = &ctx->new_list.items[i];
+
+	/* If the branch is checked out, then leave a comment instead. */
+	if (branch_checked_out(d->name, &path)) {
+		item->command = TODO_COMMENT;
+		strbuf_addf(ctx->buf, "# Ref %s checked out at '%s'\n",
+			    d->name, path);
+		free(path);
+	} else {
+		item->command = TODO_EXEC;
+		strbuf_addf(ctx->buf, "git update-ref %s HEAD %s\n",
+			    d->name, oid_to_hex(&ctx->commit->object.oid));
+	}
+
+	item->commit = NULL;
+	item->offset_in_buf = base_offset;
+	item->arg_offset = base_offset;
+	item->arg_len = ctx->buf->len - base_offset;
+	ctx->new_list.nr++;
+
+	return 0;
+}
+
+/*
+ * For each 'pick' command, find out if the commit has a decoration in
+ * refs/heads/. If so, then add a 'git branch -f' exec command after
+ * that 'pick' (plus any following 'squash' or 'fixup' commands).
+ */
+static int todo_list_add_branch_commands(struct todo_list *todo_list)
+{
+	int i;
+	static struct string_list decorate_refs_exclude = STRING_LIST_INIT_NODUP;
+	static struct string_list decorate_refs_exclude_config = STRING_LIST_INIT_NODUP;
+	static struct string_list decorate_refs_include = STRING_LIST_INIT_NODUP;
+	struct decoration_filter decoration_filter = {
+		.include_ref_pattern = &decorate_refs_include,
+		.exclude_ref_pattern = &decorate_refs_exclude,
+		.exclude_ref_config_pattern = &decorate_refs_exclude_config
+	};
+	struct todo_add_branch_context ctx = {
+		.new_list = TODO_LIST_INIT,
+		.buf = &todo_list->buf,
+	};
+
+	string_list_append(&decorate_refs_include, "refs/heads/");
+	load_ref_decorations(&decoration_filter, 0);
+
+	for (i = 0; i < todo_list->nr; ) {
+		struct todo_item *item = &todo_list->items[i];
+
+		do {
+			/* insert ith item into new list */
+			ALLOC_GROW(ctx.new_list.items,
+				   ctx.new_list.nr + 1,
+				   ctx.new_list.alloc);
+
+			memcpy(&ctx.new_list.items[ctx.new_list.nr++],
+			       &todo_list->items[i],
+			       sizeof(struct todo_item));
+
+			i++;
+		} while (i < todo_list->nr &&
+			 todo_list->items[i].command != TODO_PICK);
+
+		ctx.commit = item->commit;
+		for_each_decoration(item->commit, add_branch_for_decoration, &ctx);
+	}
+
+	free(todo_list->items);
+	todo_list->items = ctx.new_list.items;
+	todo_list->nr = ctx.new_list.nr;
+	todo_list->alloc = ctx.new_list.alloc;
+
+	return 0;
+}
+
 int complete_action(struct repository *r, struct replay_opts *opts, unsigned flags,
 		    const char *shortrevisions, const char *onto_name,
 		    struct commit *onto, const struct object_id *orig_head,
 		    struct string_list *commands, unsigned autosquash,
+		    unsigned keep_decorations,
 		    struct todo_list *todo_list)
 {
 	char shortonto[GIT_MAX_HEXSZ + 1];
@@ -5628,6 +5724,9 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 	if (autosquash && todo_list_rearrange_squash(todo_list))
 		return -1;
 
+	if (keep_decorations && todo_list_add_branch_commands(todo_list))
+		return -1;
+
 	if (commands->nr)
 		todo_list_add_exec_commands(todo_list, commands);
 
diff --git a/sequencer.h b/sequencer.h
index da64473636b..1add3c7119f 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -166,6 +166,7 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 		    const char *shortrevisions, const char *onto_name,
 		    struct commit *onto, const struct object_id *orig_head,
 		    struct string_list *commands, unsigned autosquash,
+		    unsigned keep_decorations,
 		    struct todo_list *todo_list);
 int todo_list_rearrange_squash(struct todo_list *todo_list);
 
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index f31afd4a547..38c7ef95e0e 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1743,6 +1743,35 @@ test_expect_success 'ORIG_HEAD is updated correctly' '
 	test_cmp_rev ORIG_HEAD test-orig-head@{1}
 '
 
+test_expect_success '--update-refs adds git branch commands' '
+	git checkout -b update-refs no-conflict-branch &&
+	test_commit extra fileX &&
+	git commit --amend --fixup=L &&
+	(
+		set_cat_todo_editor &&
+		git branch -f base A &&
+		git branch -f first J &&
+		git branch -f second J &&
+		git branch -f third L &&
+
+		test_must_fail git rebase -i --autosquash --update-refs primary >todo &&
+
+		cat >expect <<-EOF &&
+		pick $(git log -1 --format=%h J) J
+		exec git update-ref refs/heads/second HEAD $(git rev-parse J)
+		exec git update-ref refs/heads/first HEAD $(git rev-parse  J)
+		pick $(git log -1 --format=%h K) K
+		pick $(git log -1 --format=%h L) L
+		fixup $(git log -1 --format=%h update-refs) fixup! L
+		exec git update-ref refs/heads/third HEAD $(git rev-parse L)
+		pick $(git log -1 --format=%h M) M
+		exec git update-ref refs/heads/no-conflict-branch HEAD $(git rev-parse M)
+		EOF
+
+		test_cmp expect todo
+	)
+'
+
 # This must be the last test in this file
 test_expect_success '$EDITOR and friends are unchanged' '
 	test_editor_unchanged
-- 
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