Make builtin-commit.c more careful about parenthood

When creating the commit object, be a whole lot more careful about making
sure that the parent lines really are valid parent lines. Check things
like MERGE_HEAD having proper SHA1 lines in it, and double-check that all
the parents exist and are actually commits.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -756,6 +756,17 @@ static const char commit_utf8_warn[] =
 "You may want to amend it after fixing the message, or set the config\n"
 "variable i18n.commitencoding to the encoding your project uses.\n";
 
+static void add_parent(struct strbuf *sb, const unsigned char *sha1)
+{
+	struct object *obj = parse_object(sha1);
+	const char *parent = sha1_to_hex(sha1);
+	if (!obj)
+		die("Unable to find commit parent %s", parent);
+	if (obj->type != OBJ_COMMIT)
+		die("Parent %s isn't a proper commit", parent);
+	strbuf_addf(sb, "parent %s\n", parent);
+}
+
 int cmd_commit(int argc, const char **argv, const char *prefix)
 {
 	int header_len;
@@ -818,21 +829,24 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 			die("could not parse HEAD commit");
 
 		for (c = commit->parents; c; c = c->next)
-			strbuf_addf(&sb, "parent %s\n",
-				      sha1_to_hex(c->item->object.sha1));
+			add_parent(&sb, c->item->object.sha1);
 	} else if (in_merge) {
 		struct strbuf m;
 		FILE *fp;
 
 		reflog_msg = "commit (merge)";
-		strbuf_addf(&sb, "parent %s\n", sha1_to_hex(head_sha1));
+		add_parent(&sb, head_sha1);
 		strbuf_init(&m, 0);
 		fp = fopen(git_path("MERGE_HEAD"), "r");
 		if (fp == NULL)
 			die("could not open %s for reading: %s",
 			    git_path("MERGE_HEAD"), strerror(errno));
-		while (strbuf_getline(&m, fp, '\n') != EOF)
-			strbuf_addf(&sb, "parent %s\n", m.buf);
+		while (strbuf_getline(&m, fp, '\n') != EOF) {
+			unsigned char sha1[20];
+			if (get_sha1_hex(m.buf, sha1) < 0)
+				die("Corrupt MERGE_HEAD file (%s)", m.buf);
+			add_parent(&sb, sha1);
+		}
 		fclose(fp);
 		strbuf_release(&m);
 	} else {