Support async transformWsUrl returning a Promise<string>

[Copilot]

transformWsUrl only supported synchronous functions. Returning a Promise<string> (e.g. for async URL signing) caused the client to connect with a stringified [object Promise] URL, resulting in endless reconnect loops.

Changes

• client.ts: Updated transformWsUrl type to () => string | Promise<string>
• connect/ws.ts (Node.js): When transformWsUrl returns a Promise, a deferred Duplex stream is returned immediately while buffering writes; once the URL resolves the real WebSocket is created and the buffer is flushed
• connect/ws.ts (Browser): createBrowserWebSocket propagates the Promise; browserStreamBuilder creates a BufferedDuplex with a placeholder socket and wires up the real socket post-resolution
• connect/ali.ts / connect/wx.ts: Same pattern — connectSocket is deferred until the URL Promise resolves
• test/node/websocket_client.ts: Added test covering async transformWsUrl

Usage

mqtt.connect({
  protocol: 'ws',
  transformWsUrl: async (url, opts, client) => {
    const signed = await renewSignedUrl(url)
    return signed
  },
})

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• localhost_fake
  • Triggering command: /opt/hostedtoolcache/node/24.14.0/x64/bin/node /opt/hostedtoolcache/node/24.14.0/x64/bin/node --heap-prof-interval=524288 --cpu-prof-interval=1000 --test-coverage-functions=0 --test-concurrency=4 --inspect-publish-uid=stderr,http --inspect-port=127.0.0.1:9229 --report-signal=SIGUSR2 --test-coverage-lines=0 --test-isolation=process --tls-cipher-list=TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA (dns block)
• mqtt
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node -r esbuild-register --test-concurrency 1 test/node/websocket_client.ts (dns block)
  • Triggering command: /opt/hostedtoolcache/node/24.14.0/x64/bin/node /opt/hostedtoolcache/node/24.14.0/x64/bin/node --heap-prof-interval=524288 --cpu-prof-interval=1000 --test-coverage-functions=0 --test-concurrency=4 --inspect-publish-uid=stderr,http --inspect-port=127.0.0.1:9229 --report-signal=SIGUSR2 --test-coverage-lines=0 --test-isolation=process --tls-cipher-list=TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA (dns block)
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node -r esbuild-register --test-concurrency 1 test/node/websocket_client.ts r bin/sh (dns block)
• this_hostname_should_not_exist
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node -r esbuild-register --test-concurrency 1 test/node/websocket_client.ts (dns block)
  • Triggering command: /opt/hostedtoolcache/node/24.14.0/x64/bin/node /opt/hostedtoolcache/node/24.14.0/x64/bin/node --heap-prof-interval=524288 --cpu-prof-interval=1000 --test-coverage-functions=0 --test-concurrency=4 --inspect-publish-uid=stderr,http --inspect-port=127.0.0.1:9229 --report-signal=SIGUSR2 --test-coverage-lines=0 --test-isolation=process --tls-cipher-list=TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA (dns block)
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node -r esbuild-register --test-concurrency 1 test/node/websocket_client.ts r bin/sh (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>TransformUrl function returns a async promise instead of new url string</issue_title>
<issue_description>I'm trying to request for a new signed url from a server that already signs the url, the operation is async and by returning the promise I wasn't able to have the MQTT socket connected (it was never connected, went straight to offline).

mqtt.connect({
      protocal: 'ws',
      transformWsUrl: () => {
        return renewStreamUrl(endpoint, token, conversationId) //this is an async operation
      }
    }
  )

The websocket just starts to reconnect nonstop with a new url.

The expected behavior is: transformUrl callback returns a resolved promise, which in my case is the signed url, but it seems like its only returning a wrapped Promise which causes my mqtt client to reconnect nonstop because it didn't find a valid stream URL</issue_description>

<agent_instructions>Make transformWsUrl option accepting a promise</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@robertsLando PR?

• Fixes TransformUrl function returns a async promise instead of new url string #828

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.