fix: keep track of active sockets

When a given socket was disconnected, either by the server-side or by the client-side, the manager was closed too, regardless of the other connected sockets.

```js
const socket1 = io({
  autoConnect: false
});
const socket2 = io("/test");

socket1.disconnect(); // also disconnect socket2
```

This bug was introduced in [1].

[1]: b60e909

lib/manager.ts

@@ -292,7 +292,6 @@ export class Manager extends Emitter {
   private _reconnectionDelayMax: number;
   private _timeout: any;
 
-  private connecting: Array<Socket> = [];
   private encoder: Encoder;
   private decoder: Decoder;
   private skipReconnect: boolean;
@@ -601,19 +600,6 @@ export class Manager extends Emitter {
     if (!socket) {
       socket = new Socket(this, nsp, opts);
       this.nsps[nsp] = socket;
-      var self = this;
-      socket.on("connecting", onConnecting);
-
-      if (this._autoConnect) {
-        // manually call here since connecting event is fired before listening
-        onConnecting();
-      }
-    }
-
-    function onConnecting() {
-      if (!~self.connecting.indexOf(socket)) {
-        self.connecting.push(socket);
-      }
     }
 
     return socket;
@@ -626,9 +612,16 @@ export class Manager extends Emitter {
    * @private
    */
   _destroy(socket: Socket) {
-    const index = this.connecting.indexOf(socket);
-    if (~index) this.connecting.splice(index, 1);
-    if (this.connecting.length) return;
+    const nsps = Object.keys(this.nsps);
+
+    for (const nsp of nsps) {
+      const socket = this.nsps[nsp];
+
+      if (socket.active) {
+        debug("socket %s is still active, skipping close", nsp);
+        return;
+      }
+    }
 
     this._close();
   }

lib/socket.ts

@@ -88,6 +88,13 @@ export class Socket extends Emitter {
     ];
   }
 
+  /**
+   * Whether the Socket will try to reconnect when its Manager connects or reconnects
+   */
+  public get active(): boolean {
+    return !!this.subs;
+  }
+
   /**
    * "Opens" the socket.
    *
@@ -343,6 +350,7 @@ export class Socket extends Emitter {
    * @private
    */
   private onconnect(id: string) {
+    debug("socket connected with id %s", id);
     this.id = id;
     this.connected = true;
     this.disconnected = false;

test/connection.ts

@@ -427,6 +427,33 @@ describe("connection", function () {
     });
   });
 
+  it("should not close the connection when disconnecting a single socket", (done) => {
+    const manager = new Manager({
+      autoConnect: false,
+    });
+    const socket1 = manager.socket("/foo");
+    const socket2 = manager.socket("/asd");
+
+    socket1.connect();
+    socket1.on("connect", () => {
+      socket2.connect();
+    });
+
+    socket2.on("connect", () => {
+      socket2.on("disconnect", () => {
+        done(new Error("should not happen for now"));
+      });
+      socket1.disconnect();
+      setTimeout(() => {
+        socket2.off("disconnect");
+        manager.on("close", () => {
+          done();
+        });
+        socket2.disconnect();
+      }, 200);
+    });
+  });
+
   // Ignore incorrect connection test for old IE due to no support for
   // `script.onerror` (see: http://requirejs.org/docs/api.html#ieloadfail)
   if (!global.document || hasCORS) {