Detect split-brains caused by parallel calls

This is another KPI for PostHog.

src/room/checkForParallelCalls.ts

@@ -0,0 +1,65 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { EventType } from "matrix-js-sdk/src/@types/event";
+import { RoomState } from "matrix-js-sdk/src/models/room-state";
+
+import { PosthogAnalytics } from "../analytics/PosthogAnalytics";
+
+function isObject(x: unknown): x is Record<string, unknown> {
+  return typeof x === "object" && x !== null;
+}
+
+/**
+ * Checks the state of a room for multiple calls happening in parallel, sending
+ * the details to PostHog if that is indeed what's happening. (This is unwanted
+ * as it indicates a split-brain scenario.)
+ */
+export function checkForParallelCalls(state: RoomState): void {
+  const now = Date.now();
+  const participantsPerCall = new Map<string, number>();
+
+  // For each participant in each call, increment the participant count
+  for (const e of state.getStateEvents(EventType.GroupCallMemberPrefix)) {
+    const content = e.getContent<Record<string, unknown>>();
+    const calls: unknown[] = Array.isArray(content["m.calls"])
+      ? content["m.calls"]
+      : [];
+
+    for (const call of calls) {
+      if (isObject(call) && typeof call["m.call_id"] === "string") {
+        const devices: unknown[] = Array.isArray(call["m.devices"])
+          ? call["m.devices"]
+          : [];
+
+        for (const device of devices) {
+          if (isObject(device) && (device["expires_ts"] as number) > now) {
+            const participantCount =
+              participantsPerCall.get(call["m.call_id"]) ?? 0;
+            participantsPerCall.set(call["m.call_id"], participantCount + 1);
+          }
+        }
+      }
+    }
+  }
+
+  if (participantsPerCall.size > 1) {
+    PosthogAnalytics.instance.trackEvent({
+      eventName: "ParallelCalls",
+      participantsPerCall: Object.fromEntries(participantsPerCall),
+    });
+  }
+}