fix: handle case with no available worker

If there are no available workers, the HTTP server on the primary
process will now return a HTTP 503 error.

Related: #10

index.js

@@ -3,6 +3,10 @@ const { randomBytes } = require("crypto");
 
 const randomId = () => randomBytes(8).toString("hex");
 
+function destroySocket() {
+  this.destroy();
+}
+
 const setupMaster = (httpServer, opts) => {
   if (!cluster.isMaster) {
     throw new Error("not master");
@@ -69,6 +73,21 @@ const setupMaster = (httpServer, opts) => {
     };
 
     socket.on("data", (buffer) => {
+      if (Object.keys(cluster.workers).length === 0) {
+        if (workerId) {
+          // the socket was already assigned to a worker, so we destroy it directly
+          socket.destroy();
+        } else {
+          socket.on("error", destroySocket);
+          socket.once("finish", destroySocket);
+
+          socket.end(`HTTP/1.1 503 Service Unavailable\r\n
+          Connection: 'close'\r\n
+          Content-Length: 0\r\n
+          \r\n`);
+        }
+        return;
+      }
       const data = buffer.toString();
       if (workerId && connectionId) {
         cluster.workers[workerId].send(

test/index.js

@@ -1,12 +1,27 @@
 const { join } = require("path");
 const { exec } = require("child_process");
+const { createServer } = require("http");
+const { setupMaster } = require("../index");
+const ioc = require("socket.io-client");
 
 const fixture = (filename) => {
   return (
     '"' + process.execPath + '" "' + join(__dirname, "fixtures", filename) + '"'
   );
 };
 
+function waitFor(emitter, event) {
+  return new Promise((resolve) => {
+    emitter.once(event, resolve);
+  });
+}
+
+function success(done, httpServer, socket) {
+  httpServer.close();
+  socket.disconnect();
+  done();
+}
+
 describe("@socket.io/sticky", () => {
   it("should work with least-connection load-balancing", (done) => {
     exec(fixture("connection.js"), done);
@@ -27,4 +42,48 @@ describe("@socket.io/sticky", () => {
   it("should work with HTTP long-polling only", (done) => {
     exec(fixture("connection.js"), { env: { TRANSPORT: "polling" } }, done);
   });
+
+  it("should work with HTTP long-polling only", (done) => {
+    exec(fixture("connection.js"), { env: { TRANSPORT: "polling" } }, done);
+  });
+
+  it("should return a 503 error when no worker is available (polling)", (done) => {
+    const httpServer = createServer();
+
+    setupMaster(httpServer, {
+      loadBalancingMethod: "least-connection",
+    });
+
+    httpServer.listen(async () => {
+      const { port } = httpServer.address();
+
+      const socket = ioc(`http://localhost:${port}`, {
+        transports: ["polling"],
+      });
+
+      await waitFor(socket, "connect_error");
+
+      success(done, httpServer, socket);
+    });
+  });
+
+  it("should return a 503 error when no worker is available (websocket)", (done) => {
+    const httpServer = createServer();
+
+    setupMaster(httpServer, {
+      loadBalancingMethod: "least-connection",
+    });
+
+    httpServer.listen(async () => {
+      const { port } = httpServer.address();
+
+      const socket = ioc(`http://localhost:${port}`, {
+        transports: ["websocket"],
+      });
+
+      await waitFor(socket, "connect_error");
+
+      success(done, httpServer, socket);
+    });
+  });
 });