enact provided ResumePolicy (incl. cancelling handshake)

ircrobots/__init__.py

@@ -1,5 +1,5 @@
 from .bot    import Bot
 from .server import Server
 from .params import (ConnectionParams, SASLUserPass, SASLExternal, SASLSCRAM,
-    STSPolicy)
+    STSPolicy, ResumePolicy)
 from .ircv3  import Capability

ircrobots/ircv3.py

@@ -88,6 +88,9 @@ async def resume_transmute(params: ConnectionParams):
     if params.resume is not None:
         params.host = params.resume.address
 
+class HandshakeCancel(Exception):
+    pass
+
 class CAPContext(ServerContext):
     async def on_ls(self, tokens: Dict[str, str]):
         await self._sts(tokens)
@@ -125,14 +128,28 @@ class CAPContext(ServerContext):
         line = await self.server.wait_for(Response("RESUME", ["TOKEN", ANY]))
         token = line.params[1]
         address, port = self.server.server_address()
-
         resume_policy = ResumePolicy(address, token)
+
+        previous_policy = self.server.params.resume
         self.server.params.resume = resume_policy
         await self.server.resume_policy(resume_policy)
 
+        if previous_policy is not None and not self.server.registered:
+            await self.server.send(build("RESUME", [previous_policy.token]))
+            line = await self.server.wait_for(ResponseOr(
+                Response("RESUME", ["SUCCESS"]),
+                Response("FAIL",   ["RESUME"])
+            ))
+            if line.command == "RESUME":
+                raise HandshakeCancel()
+
     async def handshake(self):
-        await self.on_ls(self.server.available_caps)
-        await self.server.send(build("CAP", ["END"]))
+        try:
+            await self.on_ls(self.server.available_caps)
+        except HandshakeCancel:
+            return
+        else:
+            await self.server.send(build("CAP", ["END"]))
 
     async def _sts(self, tokens: Dict[str, str]):
         cap_sts = CAP_STS.available(tokens)