trigger: Fix target for QUIT events

This was fun to debug! Basically, Soepl encountered an exception
when removing unknown users when they QUIT. While this shouldn't
happen, it should still be handled gracefully.

Since it was an exception, Sopel's response was to try and send
the exception line to the channel (sender) the message came from, but
QUIT events don't come from channels (or users by PRIVMSG)!

Since QUIT was not special-cased, the naive assumption that
the first argument is the "sender" was used, and when Sopel
tried to send the exception line to the "sender", and the sender
had a space in it, this would lead to spam if a user exists with a nick
that is identical to the first word in the QUIT message. Ouch.

The fix special-cases QUIT in pretrigger to never have a "sender".
I also added a test to make sure we parse QUIT correctly.

This solves issue #1026.

sopel/trigger.py

@@ -64,7 +64,8 @@ def __init__(self, own_nick, line):
         self.nick = sopel.tools.Identifier(self.nick)
 
         # If we have arguments, the first one is the sender
-        if self.args:
+        # Unless it's a QUIT event
+        if self.args and self.event != 'QUIT':
             target = sopel.tools.Identifier(self.args[0])
         else:
             target = None

test/test_trigger.py

@@ -44,6 +44,20 @@ def test_pm_pretrigger(nick):
     assert pretrigger.sender == Identifier('Foo')
 
 
+def test_quit_pretrigger(nick):
+    line = ':Foo!foo@example.com QUIT :quit message text'
+    pretrigger = PreTrigger(nick, line)
+    assert pretrigger.tags == {}
+    assert pretrigger.hostmask == 'Foo!foo@example.com'
+    assert pretrigger.line == line
+    assert pretrigger.args == ['quit message text']
+    assert pretrigger.event == 'QUIT'
+    assert pretrigger.nick == Identifier('Foo')
+    assert pretrigger.user == 'foo'
+    assert pretrigger.host == 'example.com'
+    assert pretrigger.sender is None
+
+
 def test_tags_pretrigger(nick):
     line = '@foo=bar;baz;sopel.chat/special=value :Foo!foo@example.com PRIVMSG #Sopel :Hello, world'
     pretrigger = PreTrigger(nick, line)