Tell people how many people are currently in the room. Fixes #165

html/demo.js

@@ -86,6 +86,33 @@ function joinBucket(i) {
   }
 }
 
+function server_api_path() {
+  return new URL(serverPath.value, document.location).href;
+}
+
+function updateCurrentUsers() {
+  var xhr = new XMLHttpRequest();
+  xhr.open('POST', server_api_path() + "?action=status", true);
+  xhr.onreadystatechange = function () {
+    if (this.readyState === XMLHttpRequest.DONE && this.status === 200) {
+      const x_audio_metadata = JSON.parse(this.getResponseHeader(
+        "X-Audio-Metadata"));
+      let roomText = "The room is currently empty.";
+      if (x_audio_metadata.n_connected_users) {
+        if (x_audio_metadata.n_connected_users == 1) {
+          roomText = "There is currently 1 person in the room.";
+        } else {
+          roomText = "There are currently " + x_audio_metadata.n_connected_users +
+            " people in the room.";
+        }
+      }
+      window.currentUsers.innerText = roomText;
+    }
+  };
+  xhr.send();
+}
+updateCurrentUsers();
+
 const user_bucket_index = {};  // userid -> bucket index (-1 means unbucketed)
 const bucket_divs = [];  // bucket index -> bucket div
 
@@ -1337,7 +1364,7 @@ function connect_twilio() {
 }
 
 async function start_singing() {
-  var final_url = new URL(serverPath.value, document.location).href;
+  var final_url = server_api_path();
 
   singer_client = new bb.SingerClient({
     context: bucket_ctx,

html/index.html

@@ -648,7 +648,7 @@ <h3>Settings that affect everyone</h3>
 <div id=tutorial>
   <div id=tutorial_questions>
     Welcome!  This is a program for singing with people over the
-    Internet. <span id=eventWelcome></span>
+    Internet. <span id=eventWelcome></span> <span id=currentUsers></span>
 
     <p>
     <br>

server.py

@@ -36,6 +36,12 @@
 
 SUPPORT_SERVER_CONTROL = False
 
+# The maximum number of users to allow to join.  This is enforced on a
+# best-effort basis by the client.  If many people are calibrating at
+# the same time this will be exceeded, because we only check before
+# calibration.
+MAX_USERS = 22  # XXX needs tuning
+
 try:
     # Grab these on startup, when they are very very likely to be the actual
     #   running version.
@@ -881,6 +887,13 @@ def handle_post(in_data, query_string, print_status, client_address=None) -> Tup
         raw_params = urllib.parse.parse_qs(query_string, strict_parsing=True)
     query_params = clean_query_params(raw_params)
 
+    action = query_params.get("action", None)
+    if action == "status":
+        return np.zeros(0, np.uint8), json.dumps({
+            "n_connected_users": len(active_users()),
+            "max_users": MAX_USERS,
+        })
+
     userid = query_params.get("userid", None)
     if userid is not None:
         if int(userid) < 0 or int(userid) > 0xffff_ffff_ffff_ffff: