diff: refactor code to clarify memory ownership of prefixes

The source and destination prefixes are tracked in a `const char *`
array, but may at times contain allocated strings. The result is that
those strings may be leaking because we never free them.

Refactor the code to always store allocated strings in those variables,
freeing them as required. This requires us to handle the default values
a bit different compared to before. But given that there is only a
single callsite where we use the variables to `struct diff_options` it's
easy to handle the defaults there.

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

Author: pks-t

diff.c

@@ -62,8 +62,8 @@ static char *diff_order_file_cfg;
 int diff_auto_refresh_index = 1;
 static int diff_mnemonic_prefix;
 static int diff_no_prefix;
-static const char *diff_src_prefix = "a/";
-static const char *diff_dst_prefix = "b/";
+static char *diff_src_prefix;
+static char *diff_dst_prefix;
 static int diff_relative;
 static int diff_stat_name_width;
 static int diff_stat_graph_width;
@@ -411,10 +411,12 @@ int git_diff_ui_config(const char *var, const char *value,
 		return 0;
 	}
 	if (!strcmp(var, "diff.srcprefix")) {
-		return git_config_string(&diff_src_prefix, var, value);
+		FREE_AND_NULL(diff_src_prefix);
+		return git_config_string((const char **) &diff_src_prefix, var, value);
 	}
 	if (!strcmp(var, "diff.dstprefix")) {
-		return git_config_string(&diff_dst_prefix, var, value);
+		FREE_AND_NULL(diff_dst_prefix);
+		return git_config_string((const char **) &diff_dst_prefix, var, value);
 	}
 	if (!strcmp(var, "diff.relative")) {
 		diff_relative = git_config_bool(var, value);
@@ -3433,8 +3435,8 @@ void diff_set_noprefix(struct diff_options *options)
 
 void diff_set_default_prefix(struct diff_options *options)
 {
-	options->a_prefix = diff_src_prefix;
-	options->b_prefix = diff_dst_prefix;
+	options->a_prefix = diff_src_prefix ? diff_src_prefix : "a/";
+	options->b_prefix = diff_dst_prefix ? diff_dst_prefix : "b/";
 }
 
 struct userdiff_driver *get_textconv(struct repository *r,
@@ -5371,8 +5373,8 @@ static int diff_opt_default_prefix(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 	BUG_ON_OPT_ARG(optarg);
-	diff_src_prefix = "a/";
-	diff_dst_prefix = "b/";
+	FREE_AND_NULL(diff_src_prefix);
+	FREE_AND_NULL(diff_dst_prefix);
 	diff_set_default_prefix(options);
 	return 0;
 }