[PATCH 4/5] Make sequencer abort safer

[Stephan Beyer <s-beyer@xxxxxxx>]

In contrast to "git am --abort", a sequencer abort did not check
whether the current HEAD is the one that is expected. This can
lead to loss of work (when not spotted and resolved using reflog
before the garbage collector chimes in).

This behavior is now changed by mimicking "git am --abort":
the abortion is done but HEAD is not changed when the current HEAD
is not the expected HEAD.

A new file "sequencer/current" is added to save the expected HEAD.

The new behavior is only active when --abort is invoked on multiple
picks. The problem does not occur for the single-pick case because
it is handled differently.

Signed-off-by: Stephan Beyer <s-beyer@xxxxxxx>
---
 sequencer.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/sequencer.c b/sequencer.c
index 30b10ba14..c9b560ac1 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -27,6 +27,7 @@ GIT_PATH_FUNC(git_path_seq_dir, "sequencer")
 static GIT_PATH_FUNC(git_path_todo_file, "sequencer/todo")
 static GIT_PATH_FUNC(git_path_opts_file, "sequencer/opts")
 static GIT_PATH_FUNC(git_path_head_file, "sequencer/head")
+static GIT_PATH_FUNC(git_path_curr_file, "sequencer/current")
 
 /*
  * A script to set the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
@@ -310,6 +311,20 @@ static int error_dirty_index(struct replay_opts *opts)
 	return -1;
 }
 
+static void update_curr_file()
+{
+	struct object_id head;
+
+	/* Do nothing on a single-pick */
+	if (!file_exists(git_path_seq_dir()))
+		return;
+
+	if (!get_oid("HEAD", &head))
+		write_file(git_path_curr_file(), "%s", oid_to_hex(&head));
+	else
+		write_file(git_path_curr_file(), "%s", "");
+}
+
 static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 			int unborn, struct replay_opts *opts)
 {
@@ -339,6 +354,7 @@ static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 	strbuf_release(&sb);
 	strbuf_release(&err);
 	ref_transaction_free(transaction);
+	update_curr_file();
 	return 0;
 }
 
@@ -813,6 +829,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 
 leave:
 	free_message(commit, &msg);
+	update_curr_file();
 
 	return res;
 }
@@ -1132,9 +1149,34 @@ static int save_head(const char *head)
 	return 0;
 }
 
+static int rollback_is_safe()
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct object_id expected_head, actual_head;
+
+	if (strbuf_read_file(&sb, git_path_curr_file(), 0) >= 0) {
+		strbuf_trim(&sb);
+		if (get_oid_hex(sb.buf, &expected_head)) {
+			strbuf_release(&sb);
+			die(_("could not parse %s"), git_path_curr_file());
+		}
+		strbuf_release(&sb);
+	}
+	else if (errno == ENOENT)
+		oidclr(&expected_head);
+	else
+		die_errno(_("could not read '%s'"), git_path_curr_file());
+
+	if (get_oid("HEAD", &actual_head))
+		oidclr(&actual_head);
+
+	return !oidcmp(&actual_head, &expected_head);
+}
+
 static int reset_for_rollback(const unsigned char *sha1)
 {
 	const char *argv[4];	/* reset --merge <arg> + NULL */
+
 	argv[0] = "reset";
 	argv[1] = "--merge";
 	argv[2] = sha1_to_hex(sha1);
@@ -1189,6 +1231,12 @@ int sequencer_rollback(struct replay_opts *opts)
 		error(_("cannot abort from a branch yet to be born"));
 		goto fail;
 	}
+
+	if (!rollback_is_safe()) {
+		/* Do not error, just do not rollback */
+		warning(_("You seem to have moved HEAD. "
+			  "Not rewinding, check your HEAD!"));
+	} else
 	if (reset_for_rollback(sha1))
 		goto fail;
 	strbuf_release(&buf);
@@ -1393,6 +1441,7 @@ int sequencer_pick_revisions(struct replay_opts *opts)
 		return -1;
 	if (save_opts(opts))
 		return -1;
+	update_curr_file();
 	res = pick_commits(&todo_list, opts);
 	todo_list_release(&todo_list);
 	return res;
-- 
2.11.0.27.g4eed97c