list_objects_filter_options: plug leak of filter_spec strings

The list_objects_filter_options struct contains a string_list to store
the filter_spec. Because we allow the options struct to be
zero-initialized by callers, the string_list's strdup_strings field is
generally not set.

Because we don't want to depend on the memory lifetimes of any strings
passed in to the list-objects API, everything we add to the string_list
is duplicated (either via xstrdup(), or via strbuf_detach()). So far so
good, but now we have a problem at cleanup time: when we clear the
list, the string_list API doesn't realize that it needs to free all of
those strings, and we leak them.

One option would be to set strdup_strings right before clearing the
list. But this is tricky for two reasons:

  1. There's one spot, in partial_clone_get_default_filter_spec(),
     that fails to duplicate its argument. We could fix that, but...

  2. We clear the list in a surprising number of places. As you might
     expect, we do so in list_objects_filter_release(). But we also
     clear and rewrite it in expand_list_objects_filter_spec(),
     list_objects_filter_spec(), and transform_to_combine_type().
     We'd have to put the same hack in all of those spots.

Instead, let's just set strdup_strings before adding anything. That lets
us drop the extra manual xstrdup() calls, fixes the spot mentioned
in (1) above that _should_ be duplicating, and future-proofs further
calls. We do have to switch the strbuf_detach() calls to use the nodup
form, but that's an easy change, and the resulting code more clearly
shows the expected ownership transfer.

This also resolves a weird inconsistency: when we make a deep copy with
list_objects_filter_copy(), it initializes the copy's filter_spec with
string_list_init_dup(). So the copy frees its string_list memory
correctly, but accidentally leaks the extra manual-xstrdup()'d strings!

There is one hiccup, though. In an ideal world, everyone would allocate
the list_objects_filter_options struct with an initializer which used
STRING_LIST_INIT_DUP under the hood. But there are a bunch of existing
callers which think that zero-initializing is good enough. We can leave
them as-is by noting that the list is always initially populated via
parse_list_objects_filter(). So we can just initialize the
strdup_strings flag there.

This is arguably a band-aid, but it works reliably. And it doesn't make
anything harder if we want to switch all the callers later to a new
LIST_OBJECTS_FILTER_INIT.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter-options.c

@@ -207,7 +207,7 @@ static void filter_spec_append_urlencode(
 	struct strbuf buf = STRBUF_INIT;
 	strbuf_addstr_urlencode(&buf, raw, allow_unencoded);
 	trace_printf("Add to combine filter-spec: %s\n", buf.buf);
-	string_list_append(&filter->filter_spec, strbuf_detach(&buf, NULL));
+	string_list_append_nodup(&filter->filter_spec, strbuf_detach(&buf, NULL));
 }
 
 /*
@@ -226,12 +226,13 @@ static void transform_to_combine_type(
 			xcalloc(initial_sub_alloc, sizeof(*sub_array));
 		sub_array[0] = *filter_options;
 		memset(filter_options, 0, sizeof(*filter_options));
+		string_list_init_dup(&filter_options->filter_spec);
 		filter_options->sub = sub_array;
 		filter_options->sub_alloc = initial_sub_alloc;
 	}
 	filter_options->sub_nr = 1;
 	filter_options->choice = LOFC_COMBINE;
-	string_list_append(&filter_options->filter_spec, xstrdup("combine:"));
+	string_list_append(&filter_options->filter_spec, "combine:");
 	filter_spec_append_urlencode(
 		filter_options,
 		list_objects_filter_spec(&filter_options->sub[0]));
@@ -256,8 +257,14 @@ void parse_list_objects_filter(
 	struct strbuf errbuf = STRBUF_INIT;
 	int parse_error;
 
+	if (!filter_options->filter_spec.strdup_strings) {
+		if (filter_options->filter_spec.nr)
+			BUG("unexpected non-allocated string in filter_spec");
+		filter_options->filter_spec.strdup_strings = 1;
+	}
+
 	if (!filter_options->choice) {
-		string_list_append(&filter_options->filter_spec, xstrdup(arg));
+		string_list_append(&filter_options->filter_spec, arg);
 
 		parse_error = gently_parse_list_objects_filter(
 			filter_options, arg, &errbuf);
@@ -268,7 +275,7 @@ void parse_list_objects_filter(
 		 */
 		transform_to_combine_type(filter_options);
 
-		string_list_append(&filter_options->filter_spec, xstrdup("+"));
+		string_list_append(&filter_options->filter_spec, "+");
 		filter_spec_append_urlencode(filter_options, arg);
 		ALLOC_GROW_BY(filter_options->sub, filter_options->sub_nr, 1,
 			      filter_options->sub_alloc);
@@ -306,7 +313,7 @@ const char *list_objects_filter_spec(struct list_objects_filter_options *filter)
 		strbuf_add_separated_string_list(
 			&concatted, "", &filter->filter_spec);
 		string_list_clear(&filter->filter_spec, /*free_util=*/0);
-		string_list_append(
+		string_list_append_nodup(
 			&filter->filter_spec, strbuf_detach(&concatted, NULL));
 	}
 
@@ -321,7 +328,7 @@ const char *expand_list_objects_filter_spec(
 		strbuf_addf(&expanded_spec, "blob:limit=%lu",
 			    filter->blob_limit_value);
 		string_list_clear(&filter->filter_spec, /*free_util=*/0);
-		string_list_append(
+		string_list_append_nodup(
 			&filter->filter_spec,
 			strbuf_detach(&expanded_spec, NULL));
 	}