[PATCH 3/6] revert: don't let revert continue a cherry-pick

[Ramkumar Ramachandra <artagnon@gmail.com>]

When we allow mixing "revert" and "pick" instructions in the same
sheet in the next patch, the following workflow would be perfectly
valid:

  $ git cherry-pick base..latercommit
  [conflict occurs]
  $ edit problematicfile
  $ git add problematicfile
  $ git revert --continue
  [finishes successfully]

This is confusing to the operator, because the sequencer is an
implementation detail hidden behind the 'git cherry-pick' and 'git
revert' builtins.  So, disallow this workflow by keeping track of the
builtin command executed (either "revert" or "cherry-pick") in
'.git/sequencer/cmd'.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 builtin/revert.c                |   57 +++++++++++++++++++++++++++++++++++++++
 sequencer.h                     |    1 +
 t/t3510-cherry-pick-sequence.sh |   11 +++++++
 3 files changed, 69 insertions(+), 0 deletions(-)

diff --git a/builtin/revert.c b/builtin/revert.c
index 3ac6da0..52fa115 100644
--- a/builtin/revert.c
+++ b/builtin/revert.c
@@ -871,6 +871,50 @@ static int create_seq_dir(void)
 	return 0;
 }
 
+static enum replay_command read_cmd(void)
+{
+	const char *cmd_file = git_path(SEQ_CMD_FILE);
+	struct strbuf buf = STRBUF_INIT;
+	enum replay_command res;
+	int fd;
+
+	fd = open(cmd_file, O_RDONLY);
+	if (fd < 0)
+		die_errno(_("Could not open %s"), cmd_file);
+	if (strbuf_read(&buf, fd, 0) < 0) {
+		close(fd);
+		strbuf_release(&buf);
+		die(_("Could not read %s."), cmd_file);
+	}
+	close(fd);
+
+	if (!strcmp(buf.buf, "revert\n"))
+		res = REPLAY_CMD_REVERT;
+	else if (!strcmp(buf.buf, "cherry-pick\n"))
+		res = REPLAY_CMD_CHERRY_PICK;
+	else {
+		strbuf_release(&buf);
+		die(_("Malformed command file: %s"), cmd_file);
+	}
+	strbuf_release(&buf);
+	return res;
+}
+
+static void save_cmd(struct replay_opts *opts)
+{
+	const char *cmd_file = git_path(SEQ_CMD_FILE);
+	static struct lock_file cmd_lock;
+	struct strbuf buf = STRBUF_INIT;
+	int fd;
+
+	fd = hold_lock_file_for_update(&cmd_lock, cmd_file, LOCK_DIE_ON_ERROR);
+	strbuf_addf(&buf, "%s\n", command_name(opts));
+	if (write_in_full(fd, buf.buf, buf.len) < 0)
+		die_errno(_("Could not write to %s"), cmd_file);
+	if (commit_lock_file(&cmd_lock) < 0)
+		die(_("Error wrapping up %s."), cmd_file);
+}
+
 static void save_head(const char *head)
 {
 	const char *head_file = git_path(SEQ_HEAD_FILE);
@@ -1043,9 +1087,21 @@ static int continue_single_pick(void)
 static int sequencer_continue(struct replay_opts *opts)
 {
 	struct commit_list *todo_list = NULL;
+	enum replay_command cmd;
 
 	if (!file_exists(git_path(SEQ_TODO_FILE)))
 		return continue_single_pick();
+
+	/*
+	 * Disallow continuing a cherry-pick with 'git revert
+	 * --continue' and viceversa
+	 */
+	cmd = read_cmd();
+	if (cmd != opts->command)
+		return error(_("cannot %s: a %s is in progress."),
+			command_name(opts),
+			cmd == REPLAY_CMD_REVERT ? "revert" : "cherry-pick");
+
 	read_populate_opts(&opts);
 	read_populate_todo(&todo_list, opts);
 
@@ -1126,6 +1182,7 @@ static int pick_revisions(struct replay_opts *opts)
 			return error(_("Can't revert as initial commit"));
 		return error(_("Can't cherry-pick into empty head"));
 	}
+	save_cmd(opts);
 	save_head(sha1_to_hex(sha1));
 	save_opts(opts);
 	return pick_commits(todo_list, opts);
diff --git a/sequencer.h b/sequencer.h
index 07e0639..00ab685 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -2,6 +2,7 @@
 #define SEQUENCER_H
 
 #define SEQ_DIR		"sequencer"
+#define SEQ_CMD_FILE	"sequencer/cmd"
 #define SEQ_HEAD_FILE	"sequencer/head"
 #define SEQ_TODO_FILE	"sequencer/todo"
 #define SEQ_OPTS_FILE	"sequencer/opts"
diff --git a/t/t3510-cherry-pick-sequence.sh b/t/t3510-cherry-pick-sequence.sh
index 97f3710..73298cf 100755
--- a/t/t3510-cherry-pick-sequence.sh
+++ b/t/t3510-cherry-pick-sequence.sh
@@ -48,6 +48,7 @@ test_expect_success 'cherry-pick persists data on failure' '
 	pristine_detach initial &&
 	test_expect_code 1 git cherry-pick -s base..anotherpick &&
 	test_path_is_dir .git/sequencer &&
+	test_path_is_file .git/sequencer/cmd &&
 	test_path_is_file .git/sequencer/head &&
 	test_path_is_file .git/sequencer/todo &&
 	test_path_is_file .git/sequencer/opts
@@ -69,6 +70,7 @@ test_expect_success 'cherry-pick persists opts correctly' '
 	pristine_detach initial &&
 	test_expect_code 128 git cherry-pick -s -m 1 --strategy=recursive -X patience -X ours initial..anotherpick &&
 	test_path_is_dir .git/sequencer &&
+	test_path_is_file .git/sequencer/cmd &&
 	test_path_is_file .git/sequencer/head &&
 	test_path_is_file .git/sequencer/todo &&
 	test_path_is_file .git/sequencer/opts &&
@@ -517,4 +519,13 @@ test_expect_success 'commit descriptions in insn sheet are optional' '
 	test_line_count = 4 commits
 '
 
+test_expect_success 'revert --continue refuses to follow cherry-pick' '
+       pristine_detach initial &&
+       test_expect_code 1 git cherry-pick base..anotherpick &&
+       echo "c" >foo &&
+       git add foo &&
+       git commit &&
+       test_expect_code 128 git revert --continue
+'
+
 test_done
-- 
1.7.8.2