Subject: Re: [BUG] git-rebase: reword squashes commits in case of merge-conflicts

From: Elijah Newren <[email-masked]>
Date: Sat, 16 Jun 2018 09:08:04 -0700

On Mon, Jun 11, 2018 at 06:06:11PM +0200, ch wrote:

> During a recent rebase operation on one of my repositories a number of commits
> unexpectedly ended up getting squashed into other commits. After some
> experiments it turned out that the 'reword' instruction seems to squash the
> referenced commit into the preceding commit if there's a merge-conflict.
> 
> Here's a small reproduction recipe to illustrate the behavior:

Indeed I can reproduce.  It bisects back to when we switched to using the
rebase-helper builtin, so this started with git-2.13.0.  I can also slim down
your reproduction testcase a bit; you don't need any additional commits after
the reword, and the only reason to have commits before it is to induce a merge
conflict by dropping them.  The fact that there are lots of fixups floating
around is not necessary.

Anyway, patch below.

-- >8 --
Subject: [PATCH] sequencer: do not squash 'reword' commits when we hit
 conflicts

Ever since commit 18633e1a22 ("rebase -i: use the rebase--helper builtin",
2017-02-09), when a commit marked as 'reword' in an interactive rebase
has conflicts and fails to apply, when the rebase is resumed that commit
will be squashed into its parent with its commit message taken.

The issue can be understood better by looking at commit 56dc3ab04b
("sequencer (rebase -i): implement the 'edit' command", 2017-01-02), which
introduced error_with_patch() for the edit command.  For the edit command,
it needs to stop the rebase whether or not the patch applies cleanly.  If
the patch does apply cleanly, then when it resumes it knows it needs to
amend all changes into the previous commit.  If it does not apply cleanly,
then the changes should not be amended.  Thus, it passes !res (success of
applying the 'edit' commit) to error_with_patch() for the to_amend flag.

The problematic line of code actually came from commit 04efc8b57c
("sequencer (rebase -i): implement the 'reword' command", 2017-01-02).
Note that to get to this point in the code:
  * !!res (i.e. patch application failed)
  * item->command < TODO_SQUASH
  * item->command != TODO_EDIT
  * !is_fixup(item->command) [i.e. not squash or fixup]
So that means this can only be a failed patch application that is either a
pick, revert, or reword.  For any of those cases we want a new commit, so
we should not set the to_amend flag.

Reported-by: ch <[email-masked]>
Signed-off-by: Elijah Newren <[email-masked]>
---
 sequencer.c              |  2 +-
 t/t3423-rebase-reword.sh | 44 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100755 t/t3423-rebase-reword.sh

diff --git a/sequencer.c b/sequencer.c
index cca968043e..9e6d1ee368 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3217,7 +3217,7 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 			} else if (res && is_rebase_i(opts) && item->commit)
 				return res | error_with_patch(item->commit,
 					item->arg, item->arg_len, opts, res,
-					item->command == TODO_REWORD);
+					0);
 		} else if (item->command == TODO_EXEC) {
 			char *end_of_arg = (char *)(item->arg + item->arg_len);
 			int saved = *end_of_arg;
diff --git a/t/t3423-rebase-reword.sh b/t/t3423-rebase-reword.sh
new file mode 100755
index 0000000000..31c09efd22
--- /dev/null
+++ b/t/t3423-rebase-reword.sh
@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='git rebase interactive with rewording'
+
+. ./test-lib.sh
+
+. "$TEST_DIRECTORY"/lib-rebase.sh
+
+test_expect_success 'setup' '
+	test_commit master file-1 test &&
+
+	git checkout -b stuff &&
+
+	test_commit feature_a file-2 aaa &&
+	test_commit feature_b file-2 ddd
+'
+
+test_expect_success 'reword without issues functions as intended' '
+	git checkout stuff^0 &&
+
+	set_fake_editor &&
+	FAKE_LINES="pick 1 reword 2" FAKE_COMMIT_MESSAGE="feature_b_reworded" \
+		git rebase -i -v master &&
+
+	test "$(git log -1 --format=%B)" = "feature_b_reworded" &&
+	test $(git rev-list --count HEAD) = 3
+'
+
+test_expect_success 'reword comes after a conflict preserves commit' '
+	git checkout stuff^0 &&
+
+	set_fake_editor &&
+	test_must_fail env FAKE_LINES="reword 2" \
+		git rebase -i -v master &&
+
+	git checkout --theirs file-2 &&
+	git add file-2 &&
+	FAKE_COMMIT_MESSAGE="feature_b_reworded" git rebase --continue &&
+
+	test "$(git log -1 --format=%B)" = "feature_b_reworded" &&
+	test $(git rev-list --count HEAD) = 2
+'
+
+test_done
-- 
2.18.0.rc2.3.g7fc0d4cb57