Add Opportunistic TLS implementation

This commit introduces the functionality required to build opportunistic TLS clients and servers with
ReactPHP. It does so by introducing a prefix to `tls://`, namely `opportunistic`, to create
`opportunistic+tls://example.com:5432` for example as the full URL. This will create an
`OpportunisticTlsConnectionInterface` (instead of a `ConnectionInterface`) that extends the
`ConnectionInterface` and exposes the `enableEncryption` method to enable TLS encryption at the
desired moment. Inside this PR is an example of a server and client negotiating when to enable TLS
and enable it when ready.

Opportunistic Security described in RFC7435: https://www.rfc-editor.org/rfc/rfc7435
External PR using the proposed changes in this commit: voryx/PgAsync#52

Author: WyriHaximus

README.md

@@ -23,6 +23,8 @@ handle multiple concurrent connections without blocking.
   * [ConnectionInterface](#connectioninterface)
     * [getRemoteAddress()](#getremoteaddress)
     * [getLocalAddress()](#getlocaladdress)
+  * [OpportunisticTlsConnectionInterface](#opportunistictlsconnectioninterface)
+    * [enableEncryption()](#enableencryption)
 * [Server usage](#server-usage)
   * [ServerInterface](#serverinterface)
     * [connection event](#connection-event)
@@ -193,6 +195,64 @@ If your system has multiple interfaces (e.g. a WAN and a LAN interface),
 you can use this method to find out which interface was actually
 used for this connection.
 
+### OpportunisticTlsConnectionInterface
+
+The `OpportunisticTlsConnectionInterface` extends the 
+[`ConnectionInterface`](#connectioninterface) and adds the ability of 
+enabling the TLS encryption on the connection when desired.
+
+#### enableEncryption
+
+When negotiated with the server when to start encrypting traffic using TLS, you 
+can enable it by calling `enableEncryption()`. This will either return a promise 
+that resolves with a `OpportunisticTlsConnectionInterface` connection or throw a 
+`RuntimeException` if the encryption failed. If successful, all traffic back and 
+forth will be encrypted. In the following example we ask the server if they want 
+to encrypt the connection, and when it responds with `yes` we enable the encryption:
+
+```php
+$connector = new React\Socket\Connector();
+$connector->connect('opportunistic+tls://example.com:5432/')->then(function (React\Socket\OpportunisticTlsConnectionInterface $startTlsConnection) {
+    $connection->write('let\'s encrypt?');
+
+    return React\Promise\Stream\first($connection)->then(function ($data) use ($connection) {
+        if ($data === 'yes') {
+            return $connection->enableEncryption();
+        }
+        
+        return $stream;
+    });
+})->then(function (React\Socket\ConnectionInterface $connection) {
+    $connection->write('Hello!');
+});
+```
+
+The `enableEncryption` function resolves with itself. As such you can't see the data 
+encrypted when you hook into the events before enabling, as shown below:
+
+```php
+$connector = new React\Socket\Connector();
+$connector->connect('opportunistic+tls://example.com:5432/')->then(function (React\Socket\OpportunisticTlsConnectionInterface $startTlsConnection) {   
+    $connection->on('data', function ($data) {
+        echo 'Raw: ', $data, PHP_EOL;
+    });
+    
+    return $connection->enableEncryption();
+})->then(function (React\Socket\ConnectionInterface $connection) {
+    $connection->on('data', function ($data) {
+        echo 'TLS: ', $data, PHP_EOL;
+    });
+});
+```
+
+When the other side sends `Hello World!` over the encrypted connection, the output 
+will be the following:
+
+```
+Raw: Hello World!
+TLS: Hello World!
+```
+
 ## Server usage
 
 ### ServerInterface
@@ -253,10 +313,10 @@ If the address can not be determined or is unknown at this time (such as
 after the socket has been closed), it MAY return a `NULL` value instead.
 
 Otherwise, it will return the full address (URI) as a string value, such
-as `tcp://127.0.0.1:8080`, `tcp://[::1]:80`, `tls://127.0.0.1:443`
-`unix://example.sock` or `unix:///path/to/example.sock`.
-Note that individual URI components are application specific and depend
-on the underlying transport protocol.
+as `tcp://127.0.0.1:8080`, `tcp://[::1]:80`, `tls://127.0.0.1:443`,
+`unix://example.sock`, `unix:///path/to/example.sock`, or 
+`opportunistic+tls://127.0.0.1:443`.  Note that individual URI components 
+are application specific and depend on the underlying transport protocol.
 
 If this is a TCP/IP based server and you only want the local port, you may
 use something like this:
@@ -478,6 +538,22 @@ $socket = new React\Socket\SocketServer('tls://127.0.0.1:8000', array(
 ));
 ```
 
+To start a server with opportunistic TLS support use `opportunistic+tls://` as the scheme instead of `tls://`:
+
+```php
+$socket = new React\Socket\SocketServer('opportunistic+tls://127.0.0.1:8000', array(
+    'tls' => array(
+        'local_cert' => 'server.pem',
+        'crypto_method' => STREAM_CRYPTO_METHOD_TLSv1_2_SERVER
+    )
+));
+$server->on('connection', static function (OpportunisticTlsConnectionInterface $connection) use ($server) {
+    return $connection->enableEncryption();
+});
+```
+
+See also the [examples](examples).
+
 > Note that available [TLS context options](https://www.php.net/manual/en/context.ssl.php),
   their defaults and effects of changing these may vary depending on your system
   and/or PHP version.
@@ -697,6 +773,21 @@ here in order to use the [default loop](https://github.com/reactphp/event-loop#l
 This value SHOULD NOT be given unless you're sure you want to explicitly use a
 given event loop instance.
 
+The `SecureServer` class supports opportunistic TLS  by passing true in as a 4th 
+constructor parameter. This, when a client connects, emits a 
+[`OpportunisticTlsConnectionInterface`](#opportunistictlsconnectioninterface) 
+instead of the default [`ConnectionInterface`](#connectioninterface). It won't be 
+TLS encrypted from the start, but you can enable the TLS encryption on the connection 
+after negotiating with the client.
+
+```php
+$server = new React\Socket\TcpServer(8000);
+$server = new React\Socket\SecureServer($server, null, array(
+    'local_cert' => 'server.pem',
+    'passphrase' => 'secret'
+), true);
+```
+
 > Advanced usage: Despite allowing any `ServerInterface` as first parameter,
 you SHOULD pass a `TcpServer` instance as first parameter, unless you
 know what you're doing.
@@ -1389,6 +1480,22 @@ $secureConnector = new React\Socket\SecureConnector($dnsConnector, null, array(
 ));
 ```
 
+The `SecureConnector` class supports opportunistic TLS  by using 
+`opportunistic-tls://` as scheme instead of `tls://`. This, when connected, 
+returns a [`OpportunisticTlsConnectionInterface`](#opportunistictlsconnectioninterface) 
+instead of the default [`ConnectionInterface`](#connectioninterface). It won't be 
+TLS encrypted from the start, but you can enable the TLS encryption on the connection 
+after negotiating with the server.
+
+```php
+$secureConnector = new React\Socket\SecureConnector($dnsConnector);
+$secureConnector->connect('opportunistic-tls://example.com:5432')->then(function (OpportunisticTlsConnectionInterface $connection) {
+    return $connection->enableEncryption();
+})->then(function (OpportunisticTlsConnectionInterface $connection) {
+    $connection->write('Encrypted hi!');
+});
+```
+
 > Advanced usage: Internally, the `SecureConnector` relies on setting up the
 required *context options* on the underlying stream resource.
 It should therefor be used with a `TcpConnector` somewhere in the connector

examples/31-opportunistic-tls.php

@@ -0,0 +1,68 @@
+<?php
+
+// Opportunistic TLS example showing a basic negotiation before enabling the encryption. It starts out as an
+// unencrypted TCP connection. After both parties agreed to encrypt the connection they both enable the encryption.
+// After which any communication over the line is encrypted.
+//
+// This example is design to show both sides in one go, as such the server stops listening for new connection after
+// the first, this makes sure the loop shuts down after the example connection has closed.
+//
+// $ php examples/31-opportunistic-tls.php
+
+use React\EventLoop\Loop;
+use React\Socket\ConnectionInterface;
+use React\Socket\Connector;
+use React\Socket\OpportunisticTlsConnectionInterface;
+use React\Socket\SocketServer;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+$server = new SocketServer('opportunistic+tls://127.0.0.1:0', array(
+    'tls' => array(
+        'local_cert' => __DIR__ . '/localhost.pem',
+    )
+));
+$server->on('connection', static function (OpportunisticTlsConnectionInterface $connection) use ($server) {
+    $server->close();
+
+    $connection->on('data', function ($data) {
+        echo 'From Client: ', $data, PHP_EOL;
+    });
+    React\Promise\Stream\first($connection)->then(function ($data) use ($connection) {
+        if ($data === 'Let\'s encrypt?') {
+            $connection->write('yes');
+            return $connection->enableEncryption();
+        }
+
+        return $connection;
+    })->then(static function (ConnectionInterface $connection) {
+        $connection->write('Encryption enabled!');
+    })->done();
+});
+
+$client = new Connector(array(
+    'tls' => array(
+        'verify_peer' => false,
+        'verify_peer_name' => false,
+        'allow_self_signed' => true,
+    ),
+));
+$client->connect($server->getAddress())->then(static function (OpportunisticTlsConnectionInterface $connection) {
+    $connection->on('data', function ($data) {
+        echo 'From Server: ', $data, PHP_EOL;
+    });
+    $connection->write('Let\'s encrypt?');
+
+    return React\Promise\Stream\first($connection)->then(function ($data) use ($connection) {
+        if ($data === 'yes') {
+            return $connection->enableEncryption();
+        }
+
+        return $connection;
+    });
+})->then(function (ConnectionInterface $connection) {
+    $connection->write('Encryption enabled!');
+    Loop::addTimer(1, static function () use ($connection) {
+        $connection->end('Cool! Bye!');
+    });
+})->done();

src/Connector.php

@@ -75,6 +75,7 @@ public function __construct($context = array(), $loop = null)
             'dns' => true,
             'timeout' => true,
             'happy_eyeballs' => true,
+            'opportunistic+tls' => true,
         );
 
         if ($context['timeout'] === true) {
@@ -150,6 +151,9 @@ public function __construct($context = array(), $loop = null)
             }
 
             $this->connectors['tls'] = $context['tls'];
+            if ($context['opportunistic+tls'] !== false) {
+                $this->connectors['opportunistic+tls'] = $this->connectors['tls'];
+            }
         }
 
         if ($context['unix'] !== false) {

src/OpportunisticTlsConnection.php

@@ -0,0 +1,109 @@
+<?php
+
+namespace React\Socket;
+
+use Evenement\EventEmitter;
+use React\EventLoop\LoopInterface;
+use React\Promise\PromiseInterface;
+use React\Stream\DuplexResourceStream;
+use React\Stream\Util;
+use React\Stream\WritableResourceStream;
+use React\Stream\WritableStreamInterface;
+
+/**
+ * The actual connection implementation for StartTlsConnectionInterface
+ *
+ * This class should only be used internally, see StartTlsConnectionInterface instead.
+ *
+ * @see OpportunisticTlsConnectionInterface
+ * @internal
+ */
+class OpportunisticTlsConnection extends EventEmitter implements OpportunisticTlsConnectionInterface
+{
+    /** @var Connection */
+    private $connection;
+
+    /** @var StreamEncryption */
+    private $streamEncryption;
+
+    /** @var string */
+    private $uri;
+
+    public function __construct(Connection $connection, StreamEncryption $streamEncryption, $uri)
+    {
+        $this->connection = $connection;
+        $this->streamEncryption = $streamEncryption;
+        $this->uri = $uri;
+
+        Util::forwardEvents($connection, $this, array('data', 'end', 'error', 'close'));
+    }
+
+    public function getRemoteAddress()
+    {
+        return $this->connection->getRemoteAddress();
+    }
+
+    public function getLocalAddress()
+    {
+        return $this->connection->getLocalAddress();
+    }
+
+    public function isReadable()
+    {
+        return $this->connection->isReadable();
+    }
+
+    public function pause()
+    {
+        $this->connection->pause();
+    }
+
+    public function resume()
+    {
+        $this->connection->resume();
+    }
+
+    public function pipe(WritableStreamInterface $dest, array $options = array())
+    {
+        return $this->connection->pipe($dest, $options);
+    }
+
+    public function close()
+    {
+        $this->connection->close();
+    }
+
+    public function enableEncryption()
+    {
+        $that = $this;
+        $connection = $this->connection;
+        $uri = $this->uri;
+
+        return $this->streamEncryption->enable($connection)->then(function () use ($that) {
+            return $that;
+        }, function ($error) use ($connection, $uri) {
+            // establishing encryption failed => close invalid connection and return error
+            $connection->close();
+
+            throw new \RuntimeException(
+                'Connection to ' . $uri . ' failed during TLS handshake: ' . $error->getMessage(),
+                $error->getCode()
+            );
+        });
+    }
+
+    public function isWritable()
+    {
+        return $this->connection->isWritable();
+    }
+
+    public function write($data)
+    {
+        return $this->connection->write($data);
+    }
+
+    public function end($data = null)
+    {
+        $this->connection->end($data);
+    }
+}