blame: fix object casting regression

Commit 1b0d400 refactored the prepare_final() function so
that it could be reused in multiple places. Originally, the
loop had two outputs: a commit to stuff into sb->final, and
the name of the commit from the rev->pending array.

After the refactor, that loop is put in its own function
with a single return value: the object_array_entry from the
rev->pending array. This contains both the name and the object,
but with one important difference: the object is the
_original_ object found by the revision parser, not the
dereferenced commit. If one feeds a tag to "git blame", we
end up casting the tag object to a "struct commit", which
causes a segfault.

Instead, let's return the commit (properly casted) directly
from the function, and take the "name" as an optional
out-parameter. This does the right thing, and actually
simplifies the callers, who no longer need to cast or
dereference the object_array_entry themselves.

[test case by Max Kirillov <max@max630.net>]

Signed-off-by: Jeff King <peff@peff.net>

builtin/blame.c

@@ -2396,10 +2396,12 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	return commit;
 }
 
-static struct object_array_entry *find_single_final(struct rev_info *revs)
+static struct commit *find_single_final(struct rev_info *revs,
+					const char **name_p)
 {
 	int i;
-	struct object_array_entry *found = NULL;
+	struct commit *found = NULL;
+	const char *name = NULL;
 
 	for (i = 0; i < revs->pending.nr; i++) {
 		struct object *obj = revs->pending.objects[i].item;
@@ -2411,22 +2413,20 @@ static struct object_array_entry *find_single_final(struct rev_info *revs)
 			die("Non commit %s?", revs->pending.objects[i].name);
 		if (found)
 			die("More than one commit to dig from %s and %s?",
-			    revs->pending.objects[i].name,
-			    found->name);
-		found = &(revs->pending.objects[i]);
+			    revs->pending.objects[i].name, name);
+		found = (struct commit *)obj;
+		name = revs->pending.objects[i].name;
 	}
+	if (name_p)
+		*name_p = name;
 	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;
-	}
+	const char *name;
+	sb->final = find_single_final(sb->revs, &name);
+	return xstrdup_or_null(name);
 }
 
 static char *prepare_initial(struct scoreboard *sb)
@@ -2712,11 +2712,9 @@ parse_done:
 		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)
+		final_commit = find_single_final(sb.revs, NULL);
+		if (!final_commit)
 			die("--reverse and --first-parent together require specified latest commit");
-		else
-			final_commit = (struct commit*) entry->item;
 	}
 
 	/*

t/annotate-tests.sh

@@ -68,6 +68,13 @@ test_expect_success 'blame 1 author' '
 	check_count A 2
 '
 
+test_expect_success 'blame by tag objects' '
+	git tag -m "test tag" testTag &&
+	git tag -m "test tag #2" testTag2 testTag &&
+	check_count -h testTag A 2 &&
+	check_count -h testTag2 A 2
+'
+
 test_expect_success 'setup B lines' '
 	echo "2A quick brown fox jumps over the" >>file &&
 	echo "lazy dog" >>file &&