respondd: add provider "respondd-providers" to list registrations

Add an internal respondd provider "respondd-providers" which allows a
respondd querier to query which providers are registered on a node.
Example output:

{
	"respondd-providers": [
		"neighbours",
		"nodeinfo",
		"statistics",
		"statistics-extended"
	]
}

This can be useful for providers which are known to be bulky, where the
actual data is better queried via HTTP/TCP instead. But where such a
provider is only present on a few nodes, so you'd want to avoid
trying each node via HTTP.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

net/respondd/src/respondd.c

@@ -78,6 +78,11 @@ struct provider_list {
 	respondd_provider provider;
 };
 
+struct key_list {
+	struct key_list *next;
+	char *key;
+};
+
 struct request_type {
 	struct provider_list *providers;
 
@@ -101,6 +106,7 @@ struct request_schedule {
 
 static int64_t now;
 static struct hsearch_data htab;
+static struct key_list *keys;
 
 
 static struct json_object * merge_json(struct json_object *a, struct json_object *b);
@@ -272,6 +278,23 @@ static void load_cache_time(struct request_type *r, const char *name) {
 
 }
 
+static void add_key(const char *key) {
+	struct key_list **pos, *pentry;
+	int i = 0;
+
+	for (pos = &keys; *pos; pos = &(*pos)->next) {
+		if (strcmp(key, (*pos)->key) == 0)
+			return;
+		if (strcmp(key, (*pos)->key) < 0)
+			break;
+	}
+
+	pentry = malloc(sizeof(*pentry));
+	pentry->key = strdup(key);
+	pentry->next = *pos;
+	*pos = pentry;
+}
+
 static void add_provider(const char *name, const struct respondd_provider_info *provider) {
 	ENTRY key = {
 		.key = (char *)provider->request,
@@ -306,6 +329,31 @@ static void add_provider(const char *name, const struct respondd_provider_info *
 	*pos = pentry;
 }
 
+static struct json_object *respondd_provider_providers(void) {
+	struct key_list **pos;
+	struct json_object *provider;
+	struct json_object *ret = json_object_new_array();
+
+	if (!ret)
+		return NULL;
+
+	for (pos = &keys; *pos; pos = &(*pos)->next) {
+		provider = json_object_new_string((*pos)->key);
+		json_object_array_add(ret, provider);
+	}
+
+	return ret;
+}
+
+static void add_providers_provider(void) {
+	const struct respondd_provider_info providers[] = {
+		{"respondd-providers", respondd_provider_providers},
+		{}
+	};
+
+	add_provider("respondd", providers);
+}
+
 static void load_providers(const char *path) {
 	update_time();
 
@@ -339,10 +387,14 @@ static void load_providers(const char *path) {
 		if (!providers)
 			continue;
 
-		for (; providers->request; providers++)
+		for (; providers->request; providers++) {
 			add_provider(ent->d_name, providers);
+			add_key(providers->request);
+		}
 	}
 
+	add_providers_provider();
+
 	closedir(dir);
 
 out: