[PATCH v3 2/2] blame: allow blame --reverse --first-parent when it makes sense

[Max Kirillov <max@max630.net>]

Do not die immediately when the two flags are specified. Instead
check that the specified range is along first-parent chain. Exploit
how prepare_revision_walk() handles first_parent_only flag: the commits
outside of first-parent chain are either unknown (and do not have any
children recorded) or appear as non-first parent of a commit along the
first-parent chain.

Since the check seems fragile, add test which verifies that blame dies
in both cases.

Signed-off-by: Max Kirillov <max@max630.net>
---
 builtin/blame.c          | 11 +++++++++--
 t/t8009-blame-reverse.sh |  7 ++++++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/builtin/blame.c b/builtin/blame.c
index 295ce92..eb348f0 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -2692,8 +2692,6 @@ parse_done:
 	}
 	else if (contents_from)
 		die("--contents and --children do not blend well.");
-	else if (revs.first_parent_only)
-		die("combining --first-parent and --reverse is not supported");
 	else {
 		final_commit_name = prepare_initial(&sb);
 		sb.commits.compare = compare_commits_by_reverse_commit_date;
@@ -2721,6 +2719,15 @@ parse_done:
 	if (prepare_revision_walk(&revs))
 		die(_("revision walk setup failed"));
 
+	if (reverse && revs.first_parent_only) {
+		struct commit_list *final_children = lookup_decoration(&revs.children,
+								       &sb.final->object);
+		if (!final_children ||
+		    hashcmp(final_children->item->parents->item->object.sha1,
+			    sb.final->object.sha1))
+			die("--reverse --first-parent together require range along first-parent chain");
+	}
+
 	if (is_null_sha1(sb.final->object.sha1)) {
 		o = sb.final->util;
 		sb.final_buf = xmemdupz(o->file.ptr, o->file.size);
diff --git a/t/t8009-blame-reverse.sh b/t/t8009-blame-reverse.sh
index 9f40613..042863b 100755
--- a/t/t8009-blame-reverse.sh
+++ b/t/t8009-blame-reverse.sh
@@ -24,11 +24,16 @@ test_expect_failure 'blame --reverse finds B1, not C1' '
 	test_cmp expect actual
 	'
 
-test_expect_failure 'blame --reverse --first-parent finds A1' '
+test_expect_success 'blame --reverse --first-parent finds A1' '
 	git blame --porcelain --reverse --first-parent A0..A3 -- file.t >actual_full &&
 	head -1 <actual_full | sed -e "sX .*XX" >actual &&
 	git rev-parse A1 >expect &&
 	test_cmp expect actual
 	'
 
+test_expect_success 'blame --reverse --first-parse dies if no first parent chain' '
+	test_must_fail git blame --porcelain --reverse --first-parent B1..A3 -- file.t &&
+	test_must_fail git blame --porcelain --reverse --first-parent B2..A3 -- file.t
+	'
+
 test_done
-- 
2.3.4.2801.g3d0809b