fix: debounce WHO on JOIN to prevent flood on netsplit recovery

WHO doesn't support multiple targets (absent from TARGMAX on all
major IRCds). Replace per-nick WHO with a debounced per-channel WHO:
on JOIN, schedule WHO #channel after 2s delay. Subsequent JOINs
within the window reset the timer, so a netsplit producing dozens
of JOINs results in a single WHO.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/derp/bot.py

@@ -87,6 +87,7 @@ class Bot:
         self._admins: list[str] = config.get("bot", {}).get("admins", [])
         self._opers: set[str] = set()  # hostmasks of known IRC operators
         self._caps: set[str] = set()  # negotiated IRCv3 caps
+        self._who_pending: dict[str, asyncio.Task] = {}  # debounced WHO per channel
         self.state = StateStore()
         # Rate limiter: default 2 msg/sec, burst of 5
         rate_cfg = config.get("bot", {})
@@ -265,11 +266,16 @@ class Bot:
             if "*" in flags:
                 self._opers.add(f"{nick}!{user}@{host}")
 
-        # JOIN — WHO the joining user to detect oper status
+        # JOIN — debounced WHO to detect oper status without flooding
         if msg.command == "JOIN" and msg.nick and msg.nick != self.nick:
             channel = msg.params[0] if msg.params else ""
             if channel:
-                await self.conn.send(format_msg("WHO", msg.nick))
+                existing = self._who_pending.get(channel)
+                if existing and not existing.done():
+                    existing.cancel()
+                self._who_pending[channel] = self._spawn(
+                    self._delayed_who(channel), name=f"who:{channel}",
+                )
 
         # QUIT — remove departed nicks from oper set
         if msg.command == "QUIT" and msg.prefix:
@@ -298,6 +304,16 @@ class Bot:
         if msg.command == "PRIVMSG" and msg.text:
             self._dispatch_command(msg)
 
+    async def _delayed_who(self, channel: str, delay: float = 2.0) -> None:
+        """Send WHO after a delay, debouncing rapid JOINs (e.g. netsplit)."""
+        try:
+            await asyncio.sleep(delay)
+            await self.conn.send(format_msg("WHO", channel))
+        except asyncio.CancelledError:
+            pass
+        finally:
+            self._who_pending.pop(channel, None)
+
     async def _handle_ctcp(self, msg: Message) -> None:
         """Respond to CTCP VERSION, TIME, and PING queries."""
         text = msg.text.strip("\x01")

tests/test_integration.py

@@ -293,12 +293,12 @@ class TestAdmin:
         assert any("Opers:" in r for r in replies)
 
     def test_oper_detection_on_join(self):
-        """User joining a channel triggers WHO; oper detected mid-session."""
+        """User joining a channel triggers debounced WHO for oper detection."""
         h = _Harness(channels=["#test"])
         h.inject_registration()
-        # User joins the channel after the bot is already connected
+        # User joins after the bot is connected
         h.conn.inject(":oper!oper@operhost JOIN #test")
-        # Server responds to the WHO triggered by the JOIN
+        # Server responds to the debounced WHO #channel
         h.conn.inject(
             ":server 352 test #test oper operhost irc.server oper H* "
             ":0 Oper Name"
@@ -306,13 +306,28 @@ class TestAdmin:
         h.privmsg("oper", "#test", "!admins", user="oper", host="operhost")
         asyncio.run(h.run())
 
-        # Verify WHO was sent for the joining nick
-        who_sent = [s for s in h.conn.sent if "WHO" in s and "oper" in s]
+        # Verify debounced WHO was sent for the channel (not the nick)
+        who_sent = [s for s in h.conn.sent if "WHO" in s and "#test" in s]
         assert len(who_sent) >= 1
+        # Oper detected from the 352 reply
         replies = h.sent_privmsgs("#test")
         assert not any("Permission denied" in r for r in replies)
         assert any("oper!oper@operhost" in r for r in replies)
 
+    def test_join_who_debounce(self):
+        """Multiple rapid JOINs produce only one WHO per channel."""
+        h = _Harness(channels=["#test"])
+        h.inject_registration()
+        # Simulate netsplit recovery: many users rejoin at once
+        for nick in ("alice", "bob", "carol", "dave", "eve"):
+            h.conn.inject(f":{nick}!{nick}@host JOIN #test")
+        asyncio.run(h.run())
+
+        # Only the initial connect WHO + one debounced WHO, not 5
+        who_lines = [s for s in h.conn.sent
+                     if "WHO" in s and "#test" in s]
+        assert len(who_lines) == 2  # 001 connect + 1 debounced
+
 
 # -- Channel filter ---------------------------------------------------------