fix(net): set correct max size for Datagram (#21611)

ext/net/01_net.js

@@ -28,6 +28,8 @@ import {
   op_set_keepalive,
   op_set_nodelay,
 } from "ext:core/ops";
+const UDP_DGRAM_MAXSIZE = 65507;
+
 const {
   Error,
   Number,
@@ -378,7 +380,7 @@ class DatagramConn {
   #unref = false;
   #promise = null;
 
-  constructor(rid, addr, bufSize = 1024) {
+  constructor(rid, addr, bufSize = UDP_DGRAM_MAXSIZE) {
     this.#rid = rid;
     this.#addr = addr;
     this.bufSize = bufSize;

tests/unit/net_test.ts

@@ -385,6 +385,70 @@ Deno.test(
   },
 );
 
+Deno.test(
+  { permissions: { net: true } },
+  async function netUdpSendReceiveTestSizeLimits() {
+    // Ensure payload being sent is within UDP limit, which seems to be 65507
+    // bytes
+    const alice = Deno.listenDatagram({
+      port: listenPort,
+      transport: "udp",
+      hostname: "127.0.0.1",
+    });
+    // wrap this in a try/catch so other tests can continue if we fail
+    // if we don't close here then listening on future tests fails
+    try {
+      assert(alice.addr.transport === "udp");
+      assertEquals(alice.addr.port, listenPort);
+      assertEquals(alice.addr.hostname, "127.0.0.1");
+    } catch (err) {
+      alice.close();
+      throw err;
+    }
+
+    const bob = Deno.listenDatagram({
+      port: listenPort2,
+      transport: "udp",
+      hostname: "127.0.0.1",
+    });
+    try {
+      assert(bob.addr.transport === "udp");
+      assertEquals(bob.addr.port, listenPort2);
+      assertEquals(bob.addr.hostname, "127.0.0.1");
+    } catch (err) {
+      bob.close();
+      throw err;
+    }
+
+    const sizes = [0, 1, 2, 256, 1024, 4096, 16384, 65506, 65507, 65508, 65536];
+    const rx = /.+ \(os error \d+\)/;
+
+    for (const size of sizes) {
+      const tosend = new Uint8Array(size);
+      let byteLength = 0;
+      try {
+        byteLength = await alice.send(tosend, bob.addr);
+      } catch (err) {
+        // Note: we have to do the test this way as different OS's have
+        // different UDP size limits enabled, so we will just ensure if
+        // an error is thrown it is the one we are expecting.
+        assert(err.message.match(rx));
+        alice.close();
+        bob.close();
+        return;
+      }
+      assertEquals(byteLength, size);
+      const [recvd, remote] = await bob.receive();
+      assert(remote.transport === "udp");
+      assertEquals(remote.port, listenPort);
+      assertEquals(recvd.length, size);
+    }
+
+    alice.close();
+    bob.close();
+  },
+);
+
 Deno.test(
   { permissions: { net: true }, ignore: true },
   async function netUdpSendReceiveBroadcast() {