net: support sync connect for BoundSocket

When a net.Socket is constructed from a net.BoundSocket, support
synchronous connect().

Signed-off-by: Guy Bedford <guybedford@gmail.com>

Author: guybedford

doc/api/net.md

@@ -1657,6 +1657,10 @@ Adoption transfers ownership of the socket; afterwards `address()` and `close()`
 throw [`ERR_SOCKET_HANDLE_ADOPTED`][]. A handle that is never adopted must be
 closed to avoid leaking the socket.
 
+When an adopted `BoundSocket` connects to a numeric IP literal, `connect(2)` is
+issued synchronously: [`socket.localAddress`][] is resolved once
+[`socket.connect()`][] returns, and failures throw synchronously.
+
 ```mjs
 import net from 'node:net';
 
@@ -2241,6 +2245,7 @@ net.isIPv6('fhqwhgads'); // returns false
 [`socket.connecting`]: #socketconnecting
 [`socket.destroy()`]: #socketdestroyerror
 [`socket.end()`]: #socketenddata-encoding-callback
+[`socket.localAddress`]: #socketlocaladdress
 [`socket.pause()`]: #socketpause
 [`socket.resume()`]: #socketresume
 [`socket.setEncoding()`]: #socketsetencodingencoding

lib/net.js

@@ -1249,7 +1249,7 @@ function checkBindError(err, port, handle) {
 
 
 function internalConnect(
-  self, address, port, addressType, localAddress, localPort, flags) {
+  self, address, port, addressType, localAddress, localPort, flags, boundSocket) {
   // TODO return promise from Socket.prototype.connect which
   // wraps _connectReq.
 
@@ -1313,6 +1313,12 @@ function internalConnect(
     }
 
     const ex = new ExceptionWithHostPort(err, 'connect', address, port, details);
+    // Synchronous connect(2): throw in-tick instead of a deferred 'error'.
+    if (boundSocket) {
+      self.connecting = false;
+      self.destroy();
+      throw ex;
+    }
     self.destroy(ex);
   } else if ((addressType === 6 || addressType === 4) && hasObserver('net')) {
     startPerf(self, kPerfHooksNetConnectContext, { type: 'net', name: 'connect', detail: { host: address, port } });
@@ -1567,6 +1573,16 @@ function lookupAndConnect(self, options) {
   // If host is an IP, skip performing a lookup
   const addressType = isIP(host);
   if (addressType) {
+    // An adopted BoundSocket needs no lookup: issue connect(2) in-tick so the
+    // concrete source address resolves and failures throw synchronously.
+    if (self[kBoundSource]) {
+      defaultTriggerAsyncIdScope(
+        self[async_id_symbol],
+        internalConnect,
+        self, host, port, addressType, localAddress, localPort, 0, true,
+      );
+      return;
+    }
     defaultTriggerAsyncIdScope(self[async_id_symbol], process.nextTick, () => {
       if (self.connecting)
         defaultTriggerAsyncIdScope(

test/parallel/test-net-boundsocket.js

@@ -178,6 +178,42 @@ if (!common.isWindows && process.getuid() !== 0) {
   client.destroy();
 }
 
+// Adopted BoundSocket: connect(2) is synchronous, so localAddress/localPort
+// resolve to the concrete source in-tick, before the 'connect' event.
+{
+  const server = net.createServer();
+  server.listen(0, '127.0.0.1', common.mustCall(() => {
+    const serverPort = server.address().port;
+    const bound = new net.BoundSocket({ host: '127.0.0.1', port: 0 });
+    const localPort = bound.address().port;
+    const client = new net.Socket({ handle: bound });
+
+    client.connect({ host: '127.0.0.1', port: serverPort });
+
+    assert.strictEqual(client.localAddress, '127.0.0.1');
+    assert.strictEqual(client.localPort, localPort);
+
+    client.on('connect', common.mustCall(() => {
+      assert.strictEqual(client.localAddress, '127.0.0.1');
+      assert.strictEqual(client.localPort, localPort);
+      client.destroy();
+      server.close();
+    }));
+  }));
+}
+
+// A synchronous connect(2) failure throws in-tick, without an 'error' event.
+// An IPv4-bound handle connecting to an IPv6 literal fails on family mismatch.
+{
+  const bound = new net.BoundSocket({ host: '127.0.0.1', port: 0 });
+  const client = new net.Socket({ handle: bound });
+  client.on('error', common.mustNotCall());
+  assert.throws(() => {
+    client.connect({ host: '::1', port: 1 });
+  }, { syscall: 'connect' });
+  assert.strictEqual(client.destroyed, true);
+}
+
 // reusePort: SO_REUSEPORT permits multiple listeners on the same port. Support
 // is platform-dependent, so probe first.
 {