Cooler features and many many fixes later

firestore.rules

@@ -22,5 +22,27 @@ service cloud.firestore {
       // nobody can delete messages through the client
       allow delete: if false;
     }
+
+    // --- global "memory counter" (GlobalCountPill) -------------------------
+    // a single shared doc, meta/stats, holding totalMessagesEverPosted.
+    // anyone can read it (it's shown to every visitor); writes are limited to
+    // just that one field, same "only these fields can change" pattern as the
+    // echoCount update rule above, so this doc can't be hijacked to store
+    // arbitrary data.
+    match /meta/stats {
+      allow read: if true;
+
+      // first-ever write: only the counter field, and it must be a number
+      allow create: if
+        request.resource.data.keys().hasOnly(['totalMessagesEverPosted']) &&
+        request.resource.data.totalMessagesEverPosted is number;
+
+      // every later write (increment(1)) may only touch that same field
+      allow update: if
+        request.resource.data.diff(resource.data).affectedKeys().hasOnly(['totalMessagesEverPosted']);
+
+      // nobody can delete the counter through the client
+      allow delete: if false;
+    }
   }
 }