Add GETINFO hs/service/desc/id/ control command

Adds a control command to fetch a local service descriptor from the
service descriptor cache. The local service descriptor cache is
referenced by the onion address of the service.

This control command is documented in the control spec.

src/or/control.c

@@ -1927,6 +1927,22 @@ getinfo_helper_dir(control_connection_t *control_conn,
       *errmsg = "Not found in cache";
       return -1;
     }
+  } else if (!strcmpstart(question, "hs/service/desc/id/")) {
+    rend_cache_entry_t *e = NULL;
+
+    question += strlen("hs/service/desc/id/");
+    if (strlen(question) != REND_SERVICE_ID_LEN_BASE32) {
+      *errmsg = "Invalid address";
+      return -1;
+    }
+
+    if (!rend_cache_lookup_entry(question, -1, &e, 1)) {
+      /* Descriptor found in cache */
+      *answer = tor_strdup(e->desc);
+    } else {
+      *errmsg = "Not found in cache";
+      return -1;
+    }
   } else if (!strcmpstart(question, "md/id/")) {
     const node_t *node = node_get_by_hex_id(question+strlen("md/id/"));
     const microdesc_t *md = NULL;
@@ -2481,6 +2497,8 @@ static const getinfo_item_t getinfo_items[] = {
   PREFIX("extra-info/digest/", dir, "Extra-info documents by digest."),
   PREFIX("hs/client/desc/id", dir,
          "Hidden Service descriptor in client's cache by onion."),
+  PREFIX("hs/service/desc/id/", dir,
+         "Hidden Service descriptor in services's cache by onion."),
   PREFIX("net/listeners/", listeners, "Bound addresses by type"),
   ITEM("ns/all", networkstatus,
        "Brief summary of router status (v2 directory format)"),