line-log: always allocate the output prefix

The returned string by `output_prefix()` is sometimes a string constant
and sometimes an allocated string. This has been fine until now because
we always leak the allocated strings, and thus we never tried to free
the string constant.

Fix the code to always return an allocated string and free the returned
value at all callsites.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

line-log.c

@@ -899,14 +899,12 @@ static void print_line(const char *prefix, char first,
 
 static char *output_prefix(struct diff_options *opt)
 {
-	char *prefix = "";
-
 	if (opt->output_prefix) {
 		struct strbuf *sb = opt->output_prefix(opt, opt->output_prefix_data);
-		prefix = sb->buf;
+		return sb->buf;
+	} else {
+		return xstrdup("");
 	}
-
-	return prefix;
 }
 
 static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *range)
@@ -927,7 +925,7 @@ static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *rang
 	const char *c_context = diff_get_color(opt->use_color, DIFF_CONTEXT);
 
 	if (!pair || !diff)
-		return;
+		goto out;
 
 	if (pair->one->oid_valid)
 		fill_line_ends(rev->diffopt.repo, pair->one, &p_lines, &p_ends);
@@ -1002,8 +1000,10 @@ static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *rang
 				   c_context, c_reset, opt->file);
 	}
 
+out:
 	free(p_ends);
 	free(t_ends);
+	free(prefix);
 }
 
 /*
@@ -1012,7 +1012,11 @@ static void dump_diff_hacky_one(struct rev_info *rev, struct line_log_data *rang
  */
 static void dump_diff_hacky(struct rev_info *rev, struct line_log_data *range)
 {
-	fprintf(rev->diffopt.file, "%s\n", output_prefix(&rev->diffopt));
+	char *prefix = output_prefix(&rev->diffopt);
+
+	fprintf(rev->diffopt.file, "%s\n", prefix);
+	free(prefix);
+
 	while (range) {
 		dump_diff_hacky_one(rev, range);
 		range = range->next;