[PATCH v3 4/7] sequencer: treat error reading HEAD as unborn branch

[Brian Lyles <brianmlyles@gmail.com>]

When using git-cherry-pick(1) with `--allow-empty` while on an unborn
branch, an error is thrown. This is inconsistent with the same
cherry-pick when `--allow-empty` is not specified.

Treat a failure reading HEAD as an unborn branch in
`is_index_unchanged`. This is consistent with other sequencer logic such
as `do_pick_commit`. When on an unborn branch, use the `empty_tree` as
the tree to compare against.

Add a new test to cover this scenario. While modelled off of the
existing 'cherry-pick on unborn branch' test, some improvements can be
made:

- Use `git switch --orphan unborn` instead of `git checkout --orphan
  unborn` to avoid the need for a separate `rm -rf *` call
- Avoid using `--quiet` in the `git diff` call to make debugging easier
  in the event of a failure

Make these improvements to the existing test as well as the new test.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
---

Differences from v2:

- Minor code and test cleanup per [1] and the other replies in that
  thread

[1]: https://lore.kernel.org/git/62247a1c-0249-4ce1-8626-fe97b89c23dc@gmail.com/

 sequencer.c                   | 35 +++++++++++++++++++++--------------
 t/t3501-revert-cherry-pick.sh | 14 +++++++++++---
 2 files changed, 32 insertions(+), 17 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index f49a871ac0..a62ce244c1 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -770,29 +770,36 @@ static struct object_id *get_cache_tree_oid(struct index_state *istate)
 static int is_index_unchanged(struct repository *r)
 {
 	struct object_id head_oid, *cache_tree_oid;
+	const struct object_id *head_tree_oid;
 	struct commit *head_commit;
 	struct index_state *istate = r->index;
 
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL))
-		return error(_("could not resolve HEAD commit"));
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL)) {
+		/*
+		 * Treat an error reading HEAD as an unborn branch.
+		 */
+		head_tree_oid = the_hash_algo->empty_tree;
+	} else {
+		head_commit = lookup_commit(r, &head_oid);
 
-	head_commit = lookup_commit(r, &head_oid);
+		/*
+		 * If head_commit is NULL, check_commit, called from
+		 * lookup_commit, would have indicated that head_commit is not
+		 * a commit object already.  repo_parse_commit() will return failure
+		 * without further complaints in such a case.  Otherwise, if
+		 * the commit is invalid, repo_parse_commit() will complain.  So
+		 * there is nothing for us to say here.  Just return failure.
+		 */
+		if (repo_parse_commit(r, head_commit))
+			return -1;
 
-	/*
-	 * If head_commit is NULL, check_commit, called from
-	 * lookup_commit, would have indicated that head_commit is not
-	 * a commit object already.  repo_parse_commit() will return failure
-	 * without further complaints in such a case.  Otherwise, if
-	 * the commit is invalid, repo_parse_commit() will complain.  So
-	 * there is nothing for us to say here.  Just return failure.
-	 */
-	if (repo_parse_commit(r, head_commit))
-		return -1;
+		head_tree_oid = get_commit_tree_oid(head_commit);
+	}
 
 	if (!(cache_tree_oid = get_cache_tree_oid(istate)))
 		return -1;
 
-	return oideq(cache_tree_oid, get_commit_tree_oid(head_commit));
+	return oideq(cache_tree_oid, head_tree_oid);
 }
 
 static int write_author_script(const char *message)
diff --git a/t/t3501-revert-cherry-pick.sh b/t/t3501-revert-cherry-pick.sh
index aeab689a98..8a1d154ca6 100755
--- a/t/t3501-revert-cherry-pick.sh
+++ b/t/t3501-revert-cherry-pick.sh
@@ -104,11 +104,19 @@ test_expect_success 'revert forbidden on dirty working tree' '
 '
 
 test_expect_success 'cherry-pick on unborn branch' '
-	git checkout --orphan unborn &&
+	git switch --orphan unborn &&
 	git rm --cached -r . &&
-	rm -rf * &&
 	git cherry-pick initial &&
-	git diff --quiet initial &&
+	git diff initial &&
+	test_cmp_rev ! initial HEAD
+'
+
+test_expect_success 'cherry-pick on unborn branch with --allow-empty' '
+	git checkout --detach &&
+	git branch -D unborn &&
+	git switch --orphan unborn &&
+	git cherry-pick initial --allow-empty &&
+	git diff initial &&
 	test_cmp_rev ! initial HEAD
 '
 
-- 
2.43.0