MONITOR command and online-state notifications

Adds the IRCv3 MONITOR command (+/-/C/L/S subcommands), advertised via
the new ISUPPORT MONITOR=100 token. Per-client subscriptions live in
:monitor-targets (set of lowercased nicks); a global :nick-monitors
atom keyed by lowercased nick maps to the set of watching client atoms
for fast reverse lookup.

RPL_MONONLINE (730) is emitted to subscribed watchers when:
- a client completes registration (welcome path),
- a registered client changes nick to a different lowercased value.

RPL_MONOFFLINE (731) is emitted when:
- the canonical session for a nick disconnects (only if no sibling
  multi-session entry remains),
- a registered client changes nick to a different lowercased value
  (the old nick).

Case-only nick changes do not emit either, matching the spec.

MONITOR + filters out invalid nicks (which would include hostmasks)
and reports them as offline rather than rejecting the whole batch, so
testMonitorForbidsMasks's RPL_MONOFFLINE branch is satisfied.

Disconnect cleanup also drops the leaving session from every
:nick-monitors entry it was watching, so we don't deliver to closed
writers.

1 files changed, 151 insertions, 2 deletions

diff --git a/src/papod.clj b/src/papod.clj
index d155b27..9f148c3 100644
--- a/src/papod.clj
+++ b/src/papod.clj
@@ -1058,6 +1058,7 @@
     true))
 
 (declare join-one!)
+(declare notify-monitors!)
 
 (defn- persistent-channels
   "Channel handles where `nick` has stored membership."
@@ -1190,7 +1191,8 @@
                                 " MAXTARGETS=4"
                                 " :are supported by this server"))
                          (numeric-reply client "005"
-                           (str "MODES=4 NETWORK=papod"
+                           (str "MODES=4 MONITOR=100"
+                                " NETWORK=papod"
                                 " NICKLEN=30 PREFIX=(ov)@+"
                                 " STATUSMSG=@+"
                                 " TARGMAX=KICK:1,LIST:,WHOIS:1,PRIVMSG:4,NAMES:"
@@ -1206,6 +1208,7 @@
                                   " unread memo(s)."
                                   " Use /msg MemoServ LIST")])
                 rejoins (auto-rejoin-replies client components)]
+            (notify-monitors! components nick "730")
             (vec (concat welcome memo-line rejoins)))))))
 
 (defn- valid-nick?
@@ -1323,6 +1326,13 @@
                                         @notified mn)))]
                 (deliver-to-client! (:w m) line)
                 (swap! notified conj mn))))
+          ;; MONITOR notifications: only on real (non-case)
+          ;; nick changes — old goes offline, new comes
+          ;; online.
+          (when (not= (string/lower-case old-nick)
+                      (string/lower-case new-nick))
+            (notify-monitors! components old-nick "731")
+            (notify-monitors! components new-nick "730"))
           [line])
         ;; Pre-registration: just set the nick
         (do (swap! client assoc :nick new-nick)
@@ -3778,6 +3788,130 @@
           [(numeric-reply client "306"
              ":You have been marked as being away")]))))
 
+(def- +monitor-limit+ 100)
+
+(defn- nick-online?
+  [clients nick]
+  (let [low (string/lower-case nick)]
+    (boolean
+     (some (fn [[k _]] (= (string/lower-case k) low))
+           @clients))))
+
+(defn- mononline-line
+  [client target]
+  (numeric-reply client "730" (str ":" target)))
+
+(defn- monoffline-line
+  [client target]
+  (numeric-reply client "731" (str ":" target)))
+
+(defn- handle-monitor
+  [params client components]
+  (let [{:keys [clients nick-monitors]} components
+        sub  (some-> (first params) string/upper-case)
+        arg  (second params)
+        targets (when arg (string/split arg #","))
+        valid? (fn [t] (and (seq t) (valid-nick? t)))]
+    (case sub
+      "+"
+      (let [valid-ts (filter valid? targets)
+            invalid-ts (remove valid? targets)
+            current  (or (:monitor-targets @client) #{})
+            new-lows (->> valid-ts
+                          (map string/lower-case)
+                          (remove current))
+            next-set (into current new-lows)]
+        (cond
+          (> (count next-set) +monitor-limit+)
+          [(numeric-reply client "734"
+             (str +monitor-limit+ " "
+                  (string/join "," targets)
+                  " :Monitor list is full."))]
+          :else
+          (do (swap! client assoc :monitor-targets next-set)
+              (when nick-monitors
+                (doseq [low new-lows]
+                  (swap! nick-monitors update low
+                         (fnil conj #{}) client)))
+              (let [online (filter
+                            #(nick-online? clients %)
+                            valid-ts)
+                    offline (concat
+                             invalid-ts
+                             (remove
+                              #(nick-online? clients %)
+                              valid-ts))]
+                (cond-> []
+                  (seq online)
+                  (conj (mononline-line client
+                          (string/join "," online)))
+                  (seq offline)
+                  (conj (monoffline-line client
+                          (string/join "," offline))))))))
+
+      "-"
+      (let [removed-lows (->> targets
+                              (map string/lower-case)
+                              set)
+            current (or (:monitor-targets @client) #{})
+            next-set (into #{}
+                           (remove removed-lows current))]
+        (swap! client assoc :monitor-targets next-set)
+        (when nick-monitors
+          (doseq [low removed-lows]
+            (swap! nick-monitors update low
+                   (fnil disj #{}) client)))
+        [])
+
+      "C"
+      (let [current (or (:monitor-targets @client) #{})]
+        (swap! client assoc :monitor-targets #{})
+        (when nick-monitors
+          (doseq [low current]
+            (swap! nick-monitors update low
+                   (fnil disj #{}) client)))
+        [])
+
+      "L"
+      (let [current (or (:monitor-targets @client) #{})
+            list-line (when (seq current)
+                        [(numeric-reply client "732"
+                           (str ":"
+                                (string/join "," current)))])]
+        (vec
+         (concat list-line
+                 [(numeric-reply client "733"
+                    ":End of MONITOR list")])))
+
+      "S"
+      (let [current (or (:monitor-targets @client) #{})
+            online (filter #(nick-online? clients %)
+                           current)
+            offline (remove (set online) current)]
+        (cond-> []
+          (seq online)
+          (conj (mononline-line client
+                  (string/join "," online)))
+          (seq offline)
+          (conj (monoffline-line client
+                  (string/join "," offline)))))
+
+      [(numeric-reply client "461"
+         "MONITOR :Not enough parameters")])))
+
+(defn- notify-monitors!
+  [components nick numeric]
+  (when-let [nm (:nick-monitors components)]
+    (let [low (string/lower-case nick)
+          watchers (get @nm low #{})]
+      (doseq [watcher watchers
+              :let [w (:w @watcher)
+                    wn (or (:nick @watcher) "*")]
+              :when w]
+        (deliver-to-client! w
+          (str ":" +server-name+ " " numeric " "
+               wn " :" nick))))))
+
 (defn- handle-setname
   [params client components]
   (let [{:keys [clients channels]} components
@@ -4174,6 +4308,9 @@
         (= command "SETNAME")
         (handle-setname params client components)
 
+        (= command "MONITOR")
+        (handle-monitor params client components)
+
         (= command "NOTICE")
         (handle-notice params client components)
 
@@ -5334,6 +5471,7 @@
      :process-id  process-id
      :clients     (atom {})
      :account-clients (atom {})
+     :nick-monitors (atom {})
      :channels    (atom {})
      :ops         (atom {})
      :voiced      (atom {})
@@ -5422,7 +5560,18 @@
             (let [entry (get @(:clients components) nick)]
               (when (or (nil? entry)
                         (= client (:client-atom entry)))
-                (swap! (:clients components) dissoc nick))))
+                (swap! (:clients components) dissoc nick)
+                ;; Notify monitors only when the nick is
+                ;; actually going offline (no other session
+                ;; remains for it).
+                (when (:registered? @client)
+                  (notify-monitors! components nick
+                    "731")))))
+          ;; Drop our own monitor subscriptions.
+          (when-let [nm (:nick-monitors components)]
+            (doseq [low (or (:monitor-targets @client)
+                            #{})]
+              (swap! nm update low (fnil disj #{}) client)))
           (when (:channels components)
             (doseq [[ch _] @(:channels components)]
               (swap! (:channels components) update ch disj nick)))