websocket server only needs to reply with a single subprotocol

lib/web/fetch/util.js

@@ -1476,5 +1476,6 @@ module.exports = {
   buildContentRange,
   parseMetadata,
   createInflate,
-  extractMimeType
+  extractMimeType,
+  getDecodeSplit
 }

lib/web/websocket/connection.js

@@ -13,6 +13,7 @@ const { CloseEvent } = require('./events')
 const { makeRequest } = require('../fetch/request')
 const { fetching } = require('../fetch/index')
 const { Headers } = require('../fetch/headers')
+const { getDecodeSplit } = require('../fetch/util')
 const { kHeadersList } = require('../../core/symbols')
 
 /** @type {import('crypto')} */
@@ -176,9 +177,18 @@ function establishWebSocketConnection (url, protocols, ws, onEstablish, options)
       //    the WebSocket Connection_.
       const secProtocol = response.headersList.get('Sec-WebSocket-Protocol')
 
-      if (secProtocol !== null && secProtocol !== request.headersList.get('Sec-WebSocket-Protocol')) {
-        failWebsocketConnection(ws, 'Protocol was not set in the opening handshake.')
-        return
+      if (secProtocol !== null) {
+        const requestProtocols = getDecodeSplit('sec-websocket-protocol', request.headersList)
+
+        // The client can request that the server use a specific subprotocol by
+        // including the |Sec-WebSocket-Protocol| field in its handshake.  If it
+        // is specified, the server needs to include the same field and one of
+        // the selected subprotocol values in its response for the connection to
+        // be established.
+        if (!requestProtocols.includes(secProtocol)) {
+          failWebsocketConnection(ws, 'Protocol was not set in the opening handshake.')
+          return
+        }
       }
 
       response.socket.on('data', onSocketData)

test/websocket/issue-2844.js

@@ -0,0 +1,41 @@
+'use strict'
+
+const { test } = require('node:test')
+const { once } = require('node:events')
+const { WebSocketServer } = require('ws')
+const { WebSocket } = require('../..')
+const { tspl } = require('@matteo.collina/tspl')
+
+test('The server must reply with at least one subprotocol the client sends', async (t) => {
+  const { completed, deepStrictEqual, fail } = tspl(t, { plan: 2 })
+
+  const wss = new WebSocketServer({
+    handleProtocols: (protocols) => {
+      deepStrictEqual(protocols, new Set(['msgpack', 'json']))
+
+      return protocols.values().next().value
+    },
+    port: 0
+  })
+
+  wss.on('connection', (ws) => {
+    ws.on('error', fail)
+    ws.send('something')
+  })
+
+  await once(wss, 'listening')
+
+  const ws = new WebSocket(`ws://localhost:${wss.address().port}`, {
+    protocols: ['msgpack', 'json']
+  })
+
+  ws.onerror = fail
+  ws.onopen = () => deepStrictEqual(ws.protocol, 'msgpack')
+
+  t.after(() => {
+    wss.close()
+    ws.close()
+  })
+
+  await completed
+})