v5, 3 partsblame: extract find_single_final

diff --git a/t/t8009-blame-vs-topicbranches.sh b/t/t8009-blame-vs-topicbranches.sh
new file mode 100755
index 0000000..175ad37
--- /dev/null
+++ b/t/t8009-blame-vs-topicbranches.sh
@@ -0,0 +1,36 @@
+#!/bin/sh
+
+test_description='blaming trough history with topic branches'
+. ./test-lib.sh
+
+# Creates the history shown below. '*'s mark the first parent in the merges.
+# The only line of file.t is changed in commit B2
+#
+#        +---C1
+#       /      \
+# A0--A1--*A2--*A3
+#   \     /
+#    B1-B2
+#
+test_expect_success setup '
+	test_commit A0 file.t line0 &&
+	test_commit A1 &&
+	git reset --hard A0 &&
+	test_commit B1 &&
+	test_commit B2 file.t line0changed &&
+	git reset --hard A1 &&
+	test_merge A2 B2 &&
+	git reset --hard A1 &&
+	test_commit C1 &&
+	git reset --hard A2 &&
+	test_merge A3 C1
+	'
+
+test_expect_failure 'blame --reverse --first-parent finds A1' '
+	git blame --porcelain --reverse --first-parent A0..A3 -- file.t >actual_full &&
+	head -n 1 <actual_full | sed -e "s/ .*//" >actual &&
+	git rev-parse A1 >expect &&
+	test_cmp expect actual
+	'
+
+test_done

diff --git a/builtin/blame.c b/builtin/blame.c
index 295ce92..38f6267 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -2401,16 +2401,11 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	return commit;
 }
 
-static char *prepare_final(struct scoreboard *sb)
+static struct object_array_entry *find_single_final(struct rev_info *revs)
 {
 	int i;
-	const char *final_commit_name = NULL;
-	struct rev_info *revs = sb->revs;
+	struct object_array_entry *found = NULL;
 
-	/*
-	 * There must be one and only one positive commit in the
-	 * revs->pending array.
-	 */
 	for (i = 0; i < revs->pending.nr; i++) {
 		struct object *obj = revs->pending.objects[i].item;
 		if (obj->flags & UNINTERESTING)
@@ -2419,14 +2414,24 @@ static char *prepare_final(struct scoreboard *sb)
 			obj = deref_tag(obj, NULL, 0);
 		if (obj->type != OBJ_COMMIT)
 			die("Non commit %s?", revs->pending.objects[i].name);
-		if (sb->final)
+		if (found)
 			die("More than one commit to dig from %s and %s?",
 			    revs->pending.objects[i].name,
-			    final_commit_name);
-		sb->final = (struct commit *) obj;
-		final_commit_name = revs->pending.objects[i].name;
+			    found->name);
+		found = &(revs->pending.objects[i]);
+	}
+	return found;
+}
+
+static char *prepare_final(struct scoreboard *sb)
+{
+	struct object_array_entry *found = find_single_final(sb->revs);
+	if (found) {
+		sb->final = (struct commit *) found->item;
+		return xstrdup(found->name);
+	} else {
+		return NULL;
 	}
-	return xstrdup_or_null(final_commit_name);
 }
 
 static char *prepare_initial(struct scoreboard *sb)

builtin/blame.c, t/t8009-blame-vs-topicbranches.sh

diff --git a/builtin/blame.c b/builtin/blame.c
index 38f6267..98b1810 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -2507,6 +2507,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	long dashdash_pos, lno;
 	char *final_commit_name = NULL;
 	enum object_type type;
+	struct commit *final_commit = NULL;
 
 	static struct string_list range_list;
 	static int output_option = 0, opt = 0;
@@ -2697,11 +2698,11 @@ 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;
+		if (revs.first_parent_only)
+			revs.children.name = NULL;
 	}
 
 	if (!sb.final) {
@@ -2718,6 +2719,14 @@ parse_done:
 	else if (contents_from)
 		die("Cannot use --contents with final commit object name");
 
+	if (reverse && revs.first_parent_only) {
+		struct object_array_entry *entry = find_single_final(sb.revs);
+		if (!entry)
+			die("--reverse and --first-parent together require specified latest commit");
+		else
+			final_commit = (struct commit*) entry->item;
+	}
+
 	/*
 	 * If we have bottom, this will mark the ancestors of the
 	 * bottom commits we would reach while traversing as
@@ -2726,6 +2735,25 @@ parse_done:
 	if (prepare_revision_walk(&revs))
 		die(_("revision walk setup failed"));
 
+	if (reverse && revs.first_parent_only) {
+		struct commit *c = final_commit;
+
+		sb.revs->children.name = "children";
+		while (c->parents &&
+		       hashcmp(c->object.sha1, sb.final->object.sha1)) {
+			struct commit_list *l = xcalloc(1, sizeof(*l));
+
+			l->item = c;
+			if (add_decoration(&sb.revs->children,
+					   &c->parents->item->object, l))
+				die("BUG: not unique item in first-parent chain");
+			c = c->parents->item;
+		}
+
+		if (hashcmp(c->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-vs-topicbranches.sh b/t/t8009-blame-vs-topicbranches.sh
index 175ad37..72596e3 100755
--- a/t/t8009-blame-vs-topicbranches.sh
+++ b/t/t8009-blame-vs-topicbranches.sh
@@ -26,7 +26,7 @@ test_expect_success setup '
 	test_merge A3 C1
 	'
 
-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 -n 1 <actual_full | sed -e "s/ .*//" >actual &&
 	git rev-parse A1 >expect &&