[PATCH 3/7] sequencer: handle single-commit pick as special case

[Ramkumar Ramachandra <artagnon@gmail.com>]

Prior to v1.7.2-rc1~4^2~7 (revert: allow cherry-picking more than one
commit, 2010-06-02), 'git cherry-pick' could only pick one commit at a
time, and it used '.git/CHERRY_PICK_HEAD' to pass on information to a
subsequent invocation in case of a conflict.  While
'.git/CHERRY_PICK_HEAD' can only keep information about one commit,
the sequencer uses '.git/sequencer' to persist information in the
general case.

A problem arises because a single-commit cherry-pick operation can be
completed successfully using 'git commit'.  This removes
'.git/CHERRY_PICK_HEAD' without informing the sequencer, leaving
behind a stale sequencer state as a result.  We have worked around
this problem already by prematurely removing the sequencer state in
d3f4628e (revert: Remove sequencer state when no commits are pending,
2011-06-06).  However, this gets in the way of our future plan to
eliminate a glaring workflow inconsistency:

  $ git cherry-pick foo
  ... .git/sequencer is created ....
  ... .git/CHERRY_PICK_HEAD is created ...
  ... conflict ...
  .... .git/sequencer is prematurely removed ...
  $ echo "resolved" >problematicfile
  $ git add problematicfile
  $ git commit
  ... .git/CHERRY_PICK_HEAD is removed ...
  $ git cherry-pick --continue
  error: No cherry-pick in progress

  $ git cherry-pick foo..bar
  ... .git/sequencer is created ....
  ... CHERRY_PICK_HEAD is created ...
  ... conflict in bar~1 ...
  $ echo "resolved" >problematicfile
  $ git add problematicfile
  $ git commit
  ... CHERRY_PICK_HEAD is removed ...
  $ git cherry-pick --continue # Success!

To eliminate this inconsistency, we have decided to make '--continue'
continue any general sequencer operation bypassing 'git commit'
completely (although preserving the existing workflow for backward
compatibility).  For '--continue' and '--reset' to work uniformly,
they must use the information in:

1. '.git/sequencer/head', '.git/sequencer/opts', '.git/sequencer/todo'
   in the general case.
2. '.git/sequencer/head', '.git/sequencer/opts', '.git/CHERRY_PICK_HEAD'
   in case of a single-commit cherry-pick.

As a start, handle cherry-picking a single commit as a special case by
not creating '.git/sequencer/todo' in the first place.  This will
eliminate the need for prematurely removing it in d3f4628e.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 sequencer.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 8b2518c..b35fcc7 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -741,6 +741,14 @@ static int pick_commits(struct replay_insn_list *todo_list,
 				opts->record_origin || opts->edit));
 	read_and_refresh_cache(opts);
 
+	/*
+	 * Backward compatibility hack: handle single-commit pick as a
+	 * special case.
+	 */
+	if (opts->subcommand == REPLAY_NONE &&
+		todo_list->next == NULL && todo_list->action == REPLAY_PICK)
+		return do_pick_commit(todo_list->operand, REPLAY_PICK, opts);
+
 	for (cur = todo_list; cur; cur = cur->next) {
 		save_todo(cur);
 		res = do_pick_commit(cur->operand, cur->action, opts);
-- 
1.7.6.351.gb35ac.dirty