fix: move safety timeout to connect event (#111)

The ndt7-js client sets a 12s safety timeout when the worker is created
(`runNDT7Worker`, line 206). This timeout covers the entire worker
lifecycle including WebSocket connection setup (TCP + TLS + WS upgrade ≈
3 RTTs). For a 10s test with a 2s margin, clients with RTT > ~667ms have
their tests terminated early by the client before the server's 10s
duration expires.

This is the same bug fixed in msak-js (m-lab/msak-js#23).

BQ data confirms the effect (ndt7 downloads, one week sample Feb 2025):

| RTT bucket | Median elapsed (of 10s) | Median shortfall | % tests < 9s
|
|---|---|---|---|
| <20ms | 9.79s | 0.21s | 38% |
| 200-500ms | 9.20s | 0.80s | 47% |
| 500ms+ | **5.84s** | **4.17s** | **78%** |

The fix splits the single 12s timeout into two:

- **Connection timeout (10s)**: set at worker creation, protects against
WebSocket connections that never open. Cleared when the worker reports
`start` (i.e. `sock.onopen`).
- **Test duration timeout (12s)**: set when the `start` message arrives,
so handshake time does not eat into the test duration.

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Author: robertodauria

src/ndt7.js

@@ -198,23 +198,32 @@
         };
       });
 
-      // If the worker takes 12 seconds, kill it and return an error code.
-      // Most clients take longer than 10 seconds to complete the upload and
-      // finish sending the buffer's content, sometimes hitting the socket's
-      // timeout of 15 seconds. This makes sure uploads terminate on time and
-      // get a chance to send one last measurement after 10s.
-      const workerTimeout = setTimeout(() => worker.resolve(0), 12000);
+      // Connection timeout: if the WebSocket never opens (and never
+      // fires onerror/onclose), resolve after 10s to prevent the
+      // promise from hanging indefinitely.
+      const connTimeout = setTimeout(() => worker.resolve(0), 10000);
+
+      // The test duration timeout is set when the worker reports
+      // 'start' (i.e. sock.onopen), so that WS+TLS handshake time
+      // does not eat into the test duration.
+      let workerTimeout;
 
       // This is how the worker communicates back to the main thread of
       // execution.  The MsgTpe of `ev` determines which callback the message
       // gets forwarded to.
       worker.onmessage = function(ev) {
         if (!ev.data || !ev.data.MsgType || ev.data.MsgType === 'error') {
+          clearTimeout(connTimeout);
           clearTimeout(workerTimeout);
           worker.resolve(1);
           const msg = (!ev.data) ? `${testType} error` : ev.data.Error;
           callbacks.error(msg);
         } else if (ev.data.MsgType === 'start') {
+          clearTimeout(connTimeout);
+          // Safety timeout anchored to connect, not worker creation.
+          // 12s gives the server's 10s duration time to finish plus
+          // margin for the upload buffer to drain.
+          workerTimeout = setTimeout(() => worker.resolve(0), 12000);
           callbacks.start(ev.data.Data);
         } else if (ev.data.MsgType == 'measurement') {
           // For performance reasons, we parse the JSON outside of the thread
@@ -241,7 +250,8 @@
       // We can't start the worker until we know the right server, so we wait
       // here to find that out.
       const urls = await urlPromise.catch((err) => {
-        // Clear timer, terminate the worker and rethrow the error.
+        // Clear timers, terminate the worker and rethrow the error.
+        clearTimeout(connTimeout);
         clearTimeout(workerTimeout);
         worker.resolve(2);
         throw err;