Add support for SASL PLAIN authentication

This implements a minimal state machine to do the SASL PLAIN
authentication dance.

It could have been done even simpler by sending all the commands and not
checking for server capabilities, but I don't think it's being a good
citizen.

It changes the ServerConnection api by adding a sasl_login argument, and
reusing the old password argument, since I couldn't think of a usecase
where both were needed. The SimpleIRCClient and SingleServerIRCBot can
also pass this new argument.

A new "login_failed" generated event is added and is sent in some of the
cases where the SASL login can fail. Note that there is no timeout on
the state machine, so if the server does not send any of the expected
commands, it will just stay active forever, potentially with the login
failing.

It was tested on libera.chat with the cobe bot and seems to work
reasonably well.

Fixes #195

irc/client.py

@@ -47,6 +47,7 @@
 .. [IRC specifications] http://www.irchelp.org/irchelp/rfc/
 """
 
+import base64
 import bisect
 import re
 import select
@@ -144,6 +145,7 @@ def connect(
         username=None,
         ircname=None,
         connect_factory=connection.Factory(),
+        sasl_login=None,
     ):
         """Connect/reconnect to a server.
 
@@ -158,6 +160,8 @@ def connect(
         * server_address - The remote host/port of the server
         * connect_factory - A callable that takes the server address and
           returns a connection (with a socket interface)
+        * sasl_login - A string used to toggle sasl plain login. Password needs
+        to be set as well, and will be used for SASL, not PASS login.
 
         This function can be called to reconnect a closed connection.
 
@@ -182,6 +186,7 @@ def connect(
         self.ircname = ircname or nickname
         self.password = password
         self.connect_factory = connect_factory
+        self.sasl_login = sasl_login
         try:
             self.socket = self.connect_factory(self.server_address)
         except socket.error as ex:
@@ -190,12 +195,69 @@ def connect(
         self.reactor._on_connect(self.socket)
 
         # Log on...
+        if self.sasl_login and self.password:
+            for i in ["cap", "authenticate", "saslsuccess", "saslfail"]:
+                self.add_global_handler(i, self._sasl_state_machine, -42)
+            self.cap("LS")
+            self.nick(self.nickname)
+            self.user(self.username, self.ircname)
+            self._sasl_step = "ls"
+            return self
         if self.password:
             self.pass_(self.password)
         self.nick(self.nickname)
         self.user(self.username, self.ircname)
         return self
 
+    def _sasl_state_machine(self, connection, event):
+        if self._sasl_step == "ls":
+            if (
+                event.type == "cap"
+                and len(event.arguments) > 1
+                and event.arguments[0] == "LS"
+            ):
+                if 'sasl' in event.arguments[1].split():
+                    self.cap("REQ", "sasl")
+                    self._sasl_step = "req"
+                else:
+                    event = Event(
+                        "login_failed", event.target, ["server does not support sasl"]
+                    )
+                    self._handle_event(event)
+                    self._sasl_end()
+        elif self._sasl_step == "req":
+            if event.type == "cap" and len(event.arguments) > 1:
+                if event.arguments[0] == "ACK" and 'sasl' in event.arguments:
+                    self.send_items('AUTHENTICATE', 'PLAIN')
+                    self._sasl_step = "auth-plain"
+                elif event.arguments[0] == "NAK":
+                    event = Event(
+                        "login_failed", event.target, ["server refused sasl protocol"]
+                    )
+                    self._handle_event(event)
+                    self._sasl_end()
+        elif self._sasl_step == "auth-plain":
+            if event.type == "authenticate" and event.target == "+":
+                auth_string = base64.b64encode(
+                    self.encode("\x00%s\x00%s" % (self.sasl_login, self.password))
+                ).decode()
+                self.send_items('AUTHENTICATE', auth_string)
+                self._sasl_step = "auth-sent"
+        elif self._sasl_step == "auth-sent":
+            if event.type == "saslsuccess":
+                self._sasl_end()
+            elif event.type == "saslfail":
+                event = Event("login_failed", event.target, event.arguments)
+                self._handle_event(event)
+                self._sasl_end()
+
+    def _sasl_end(self):
+        self._sasl_step = ""
+        self.cap("END")
+        # SASL done, de-register handlers
+        for i in ["cap", "authenticate", "saslsuccess", "saslfail"]:
+            self.remove_global_handler(i, self._sasl_state_machine)
+
     def reconnect(self):
         """
         Reconnect with the last arguments passed to self.connect()

irc/events.py

@@ -188,6 +188,7 @@
     "disconnect",
     "ctcp",
     "ctcpreply",
+    "login_failed",
 ]
 
 protocol = [