Add participantChannel to isolate keeping count of participants

participantChannel is a special channel used only to keep track of participants. No data goes through it. This prevents bugs associated with having non-participating subscribers on the channels that have data going through them.

lib/index.ts

@@ -66,6 +66,7 @@ export class RedisAdapter extends Adapter {
   public readonly requestsTimeout: number;
 
   private readonly channel: string;
+  private readonly participantChannel: string;
   private readonly requestChannel: string;
   private readonly responseChannel: string;
   private requests: Map<string, Request> = new Map();
@@ -94,6 +95,7 @@ export class RedisAdapter extends Adapter {
     const prefix = opts.key || "socket.io";
 
     this.channel = prefix + "#" + nsp.name + "#";
+    this.participantChannel = prefix + "-participant#" + this.nsp.name + "#";
     this.requestChannel = prefix + "-request#" + this.nsp.name + "#";
     this.responseChannel = prefix + "-response#" + this.nsp.name + "#";
 
@@ -107,7 +109,7 @@ export class RedisAdapter extends Adapter {
     this.subClient.on("pmessageBuffer", this.onmessage.bind(this));
 
     this.subClient.subscribe(
-      [this.requestChannel, this.responseChannel],
+      [this.participantChannel, this.requestChannel, this.responseChannel],
       onError
     );
     this.subClient.on("messageBuffer", this.onrequest.bind(this));
@@ -855,12 +857,17 @@ export class RedisAdapter extends Adapter {
       const nodes = this.pubClient.nodes();
       return Promise.all(
         nodes.map((node) =>
-          node.send_command("pubsub", ["numsub", this.requestChannel])
+          node.send_command("pubsub", [
+            "numsub",
+            this.requestChannel,
+            this.participantChannel,
+          ])
         )
       ).then((values) => {
         let numSub = 0;
         values.map((value) => {
-          numSub += parseInt(value[1], 10);
+          // Fall back to requestChannel for backwards compatibility
+          numSub += parseInt(value[3] || value[1], 10);
         });
         return numSub;
       });
@@ -869,10 +876,11 @@ export class RedisAdapter extends Adapter {
       return new Promise((resolve, reject) => {
         this.pubClient.send_command(
           "pubsub",
-          ["numsub", this.requestChannel],
+          ["numsub", this.requestChannel, this.participantChannel],
           (err, numSub) => {
             if (err) return reject(err);
-            resolve(parseInt(numSub[1], 10));
+            // Fall back to requestChannel for backwards compatibility
+            resolve(parseInt(numSub[3] || numSub[1], 10));
           }
         );
       });