dgram: add support for UDP connected sockets

Added the `dgram.connect()` and `dgram.disconnect()` methods that
associate/disassociate a udp socket to/from a remote address.
It optimizes for cases where lots of packets are sent to the same
address.
Also added the `dgram.remoteAddress()` method to retrieve the associated
remote address.

Backport-PR-URL: #30480
PR-URL: #26871
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

(cherry picked from commit 9e96017)

doc/api/dgram.md

@@ -49,6 +49,14 @@ added: v0.1.99
 The `'close'` event is emitted after a socket is closed with [`close()`][].
 Once triggered, no new `'message'` events will be emitted on this socket.
 
+### Event: 'connect'
+<!-- YAML
+added: REPLACEME
+-->
+
+The `'connect'` event is emitted after a socket is associated to a remote
+address as a result of a successful [`connect()`][] call.
+
 ### Event: 'error'
 <!-- YAML
 added: v0.1.99
@@ -231,6 +239,34 @@ added: v0.1.99
 Close the underlying socket and stop listening for data on it. If a callback is
 provided, it is added as a listener for the [`'close'`][] event.
 
+### socket.connect(port[, address][, callback])
+<!-- YAML
+added: REPLACEME
+-->
+
+* `port` {integer}
+* `address` {string}
+* `callback` {Function} Called when the connection is completed or on error.
+
+Associates the `dgram.Socket` to a remote address and port. Every
+message sent by this handle is automatically sent to that destination. Also,
+the socket will only receive messages from that remote peer.
+Trying to call `connect()` on an already connected socket will result
+in an [`ERR_SOCKET_DGRAM_IS_CONNECTED`][] exception. If `address` is not
+provided, `'127.0.0.1'` (for `udp4` sockets) or `'::1'` (for `udp6` sockets)
+will be used by default. Once the connection is complete, a `'connect'` event
+is emitted and the optional `callback` function is called. In case of failure,
+the `callback` is called or, failing this, an `'error'` event is emitted.
+
+### socket.disconnect()
+<!-- YAML
+added: REPLACEME
+-->
+
+A synchronous function that disassociates a connected `dgram.Socket` from
+its remote address. Trying to call `disconnect()` on an already disconnected
+socket will result in an [`ERR_SOCKET_DGRAM_NOT_CONNECTED`][] exception.
+
 ### socket.dropMembership(multicastAddress[, multicastInterface])
 <!-- YAML
 added: v0.6.9
@@ -277,7 +313,18 @@ Calling `socket.ref()` multiples times will have no additional effect.
 The `socket.ref()` method returns a reference to the socket so calls can be
 chained.
 
-### socket.send(msg[, offset, length], port[, address][, callback])
+### socket.remoteAddress()
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {Object}
+
+Returns an object containing the `address`, `family`, and `port` of the remote
+endpoint. It throws an [`ERR_SOCKET_DGRAM_NOT_CONNECTED`][] exception if the
+socket is not connected.
+
+### socket.send(msg[, offset, length][, port][, address][, callback])
 <!-- YAML
 added: v0.1.99
 changes:
@@ -295,6 +342,9 @@ changes:
     pr-url: https://github.com/nodejs/node/pull/4374
     description: The `msg` parameter can be an array now. Also, the `offset`
                  and `length` parameters are optional now.
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/26871
+    description: Added support for sending data on connected sockets.
 -->
 
 * `msg` {Buffer|Uint8Array|string|Array} Message to be sent.
@@ -304,8 +354,10 @@ changes:
 * `address` {string} Destination hostname or IP address.
 * `callback` {Function} Called when the message has been sent.
 
-Broadcasts a datagram on the socket. The destination `port` and `address` must
-be specified.
+Broadcasts a datagram on the socket.
+For connectionless sockets, the destination `port` and `address` must be
+specified. Connected sockets, on the other hand, will use their associated
+remote endpoint, so the `port` and `address` arguments must not be set.
 
 The `msg` argument contains the message to be sent.
 Depending on its type, different behavior can apply. If `msg` is a `Buffer`
@@ -369,6 +421,20 @@ application and operating system. It is important to run benchmarks to
 determine the optimal strategy on a case-by-case basis. Generally speaking,
 however, sending multiple buffers is faster.
 
+Example of sending a UDP packet using a socket connected to a port on
+`localhost`:
+
+```js
+const dgram = require('dgram');
+const message = Buffer.from('Some bytes');
+const client = dgram.createSocket('udp4');
+client.connect(41234, 'localhost', (err) => {
+  client.send(message, (err) => {
+    client.close();
+  });
+});
+```
+
 **A Note about UDP datagram size**
 
 The maximum size of an `IPv4/v6` datagram depends on the `MTU`
@@ -639,10 +705,13 @@ and `udp6` sockets). The bound address and port can be retrieved using
 
 [`'close'`]: #dgram_event_close
 [`Error`]: errors.html#errors_class_error
+[`ERR_SOCKET_DGRAM_IS_CONNECTED`]: errors.html#errors_err_socket_dgram_is_connected
+[`ERR_SOCKET_DGRAM_NOT_CONNECTED`]: errors.html#errors_err_socket_dgram_not_connected
 [`EventEmitter`]: events.html
 [`System Error`]: errors.html#errors_class_systemerror
 [`close()`]: #dgram_socket_close_callback
 [`cluster`]: cluster.html
+[`connect()`]: #dgram_socket_connect_port_address_callback
 [`dgram.Socket#bind()`]: #dgram_socket_bind_options_callback
 [`dgram.createSocket()`]: #dgram_dgram_createsocket_options_callback
 [`dns.lookup()`]: dns.html#dns_dns_lookup_hostname_options_callback

doc/api/errors.md

@@ -1571,6 +1571,17 @@ Data could be sent on a socket.
 
 An attempt was made to operate on an already closed socket.
 
+<a id="ERR_SOCKET_DGRAM_IS_CONNECTED"></a>
+### ERR_SOCKET_DGRAM_IS_CONNECTED
+
+A [`dgram.connect()`][] call was made on an already connected socket.
+
+<a id="ERR_SOCKET_DGRAM_NOT_CONNECTED"></a>
+### ERR_SOCKET_DGRAM_NOT_CONNECTED
+
+A [`dgram.disconnect()`][] or [`dgram.remoteAddress()`][] call was made on a
+disconnected socket.
+
 <a id="ERR_SOCKET_DGRAM_NOT_RUNNING"></a>
 ### ERR_SOCKET_DGRAM_NOT_RUNNING
 
@@ -2140,7 +2151,10 @@ size.
 [`crypto.scrypt()`]: crypto.html#crypto_crypto_scrypt_password_salt_keylen_options_callback
 [`crypto.scryptSync()`]: crypto.html#crypto_crypto_scryptsync_password_salt_keylen_options
 [`crypto.timingSafeEqual()`]: crypto.html#crypto_crypto_timingsafeequal_a_b
+[`dgram.connect()`]: dgram.html#dgram_socket_connect_port_address_callback
 [`dgram.createSocket()`]: dgram.html#dgram_dgram_createsocket_options_callback
+[`dgram.disconnect()`]: dgram.html#dgram_socket_disconnect
+[`dgram.remoteAddress()`]: dgram.html#dgram_socket_remoteaddress
 [`errno`(3) man page]: http://man7.org/linux/man-pages/man3/errno.3.html
 [`fs.readFileSync`]: fs.html#fs_fs_readfilesync_path_options
 [`fs.readdir`]: fs.html#fs_fs_readdir_path_options_callback