enable QUIC PMTUD

Differential Revision: https://phabricator.services.mozilla.com/D223575

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1909910
gecko-commit: 0da897321dc6c369fe095040ca5b2fef50e7ce3d
gecko-reviewers: necko-reviewers, kershaw

Author: mxinden

webtransport/datagrams.https.any.js

@@ -199,13 +199,25 @@ promise_test(async t => {
   const writer = wt.datagrams.writable.getWriter();
   const reader = wt.datagrams.readable.getReader();
 
-  // Write and read max-size datagram.
-  await writer.write(new Uint8Array(wt.datagrams.maxDatagramSize+1));
-  // This should resolve with no datagram sent, which is hard to test for.
-  // Wait for incoming datagrams to arrive, and if they do, fail.
-  const result = await Promise.race([reader.read(), wait(500)]);
-  assert_equals(result, undefined);
-}, 'Fail to transfer max-size+1 datagram');
+  let maxDatagramSize = wt.datagrams.maxDatagramSize;
+
+  while (true) {
+    // Write and read max-size datagram.
+    await writer.write(new Uint8Array(maxDatagramSize + 1));
+    // This should resolve with no datagram sent, which is hard to test for.
+    // Wait for incoming datagrams to arrive, and if they do, fail.
+    const result = await Promise.race([reader.read(), wait(500)]);
+    if (result === undefined) {
+      return; // Success, no datagram received, exit the test early.
+    }
+
+    // Maybe QUIC's PMTUD increased the max-size datagram in the meantime. If
+    // so, try again.
+    const currentMaxDatagramSize = wt.datagrams.maxDatagramSize;
+    assert_greater_than(currentMaxDatagramSize, maxDatagramSize);
+    maxDatagramSize = currentMaxDatagramSize;
+  }
+}, 'Fail to transfer max-size+1 datagram, handle PMTUD increases');
 
 promise_test(async t => {
   // Make a WebTransport connection, but session is not necessarily established.