Fix planner_active_sessions gauge drifting negative

The gauge was maintained by conditional inc on "this sessionId isn't
in the in-memory `docs` Map yet" and dec on "last client for the
sessionId left." Because `docs` caches entries indefinitely — even
after every client disconnects — a reconnect found the doc still
cached, skipped the inc, then decremented again on the next
disconnect. Every reconnect-after-last-disconnect leaked one
decrement. After a day of normal reload / wifi-blip / tab-hibernate
patterns the gauge hit -182.

Replace the transitions with a `set()` derived from the live `conns`
map: count distinct session ids with at least one open WebSocket.
Self-correcting, no drift possible.

Extract `countActiveSessions()` as a pure helper + unit tests
including a regression case that simulates repeated reconnect/
disconnect cycles on one session and asserts the count never goes
negative.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/planner/app/lib/yjs-server.test.ts

@@ -0,0 +1,45 @@
+import { describe, it, expect } from "vitest";
+import { countActiveSessions } from "./yjs-server.ts";
+
+describe("countActiveSessions", () => {
+  it("is 0 for an empty iterable (regression: prod gauge saw -182 drift)", () => {
+    expect(countActiveSessions([])).toBe(0);
+  });
+
+  it("counts a single session with one client as 1", () => {
+    expect(countActiveSessions([{ sessionId: "s1" }])).toBe(1);
+  });
+
+  it("deduplicates multiple clients on the same session", () => {
+    expect(
+      countActiveSessions([
+        { sessionId: "s1" },
+        { sessionId: "s1" },
+        { sessionId: "s1" },
+      ]),
+    ).toBe(1);
+  });
+
+  it("counts distinct sessions across clients", () => {
+    expect(
+      countActiveSessions([
+        { sessionId: "s1" },
+        { sessionId: "s2" },
+        { sessionId: "s1" },
+        { sessionId: "s3" },
+      ]),
+    ).toBe(3);
+  });
+
+  it("doesn't go negative under the reconnect pattern that broke the old inc/dec logic", () => {
+    // Simulate: client connects, client disconnects, reconnects,
+    // disconnects again — five times over. The old code would have
+    // decremented once per cycle (because `docs` stayed cached, so
+    // the re-connect's `isNewSession` check skipped inc). Here we
+    // derive from live state, so each empty snapshot is 0, not -N.
+    for (let i = 0; i < 5; i++) {
+      expect(countActiveSessions([{ sessionId: "s1" }])).toBe(1); // reconnect
+      expect(countActiveSessions([])).toBe(0); // disconnect
+    }
+  });
+});

apps/planner/app/lib/yjs-server.ts

@@ -15,6 +15,29 @@ const docs = new Map<string, Y.Doc>();
 const awarenessMap = new Map<string, awarenessProtocol.Awareness>();
 const conns = new Map<WebSocket, { sessionId: string; clientIds: Set<number> }>();
 
+/**
+ * Count the number of distinct session ids represented in an iterable
+ * of connection metadata. Pure, for testability — the server calls it
+ * with the `conns` map's values.
+ */
+export function countActiveSessions(
+  connsIter: Iterable<{ sessionId: string }>,
+): number {
+  const sessionIds = new Set<string>();
+  for (const { sessionId } of connsIter) sessionIds.add(sessionId);
+  return sessionIds.size;
+}
+
+/**
+ * Set the active-sessions gauge from the actual `conns` state. Derived
+ * each time, so it can't drift negative — the previous inc/dec pattern
+ * leaked decrements whenever a reconnect found the doc cached in
+ * `docs` and skipped the matching increment.
+ */
+function refreshActiveSessions(): void {
+  plannerActiveSessions.set(countActiveSessions(conns.values()));
+}
+
 export function getOrCreateDoc(sessionId: string): Y.Doc {
   let doc = docs.get(sessionId);
   if (!doc) {
@@ -160,13 +183,12 @@ export function setupYjsWebSocket(server: Server): WebSocketServer {
   });
 
   wss.on("connection", async (ws: WebSocket, _request: IncomingMessage, sessionId: string) => {
-    const isNewSession = !docs.has(sessionId);
     const doc = await getOrLoadDoc(sessionId);
     const awareness = getAwareness(sessionId, doc);
 
     conns.set(ws, { sessionId, clientIds: new Set() });
     plannerConnectedClients.inc();
-    if (isNewSession) plannerActiveSessions.inc();
+    refreshActiveSessions();
 
     // Broadcast doc updates to all connections in this session
     const onUpdate = (update: Uint8Array, origin: unknown) => {
@@ -223,12 +245,12 @@ export function setupYjsWebSocket(server: Server): WebSocketServer {
       }
       conns.delete(ws);
       plannerConnectedClients.dec();
+      refreshActiveSessions();
       doc.off("update", onUpdate);
 
       // Save when last client leaves
       const hasClients = Array.from(conns.values()).some((m) => m.sessionId === sessionId);
       if (!hasClients) {
-        plannerActiveSessions.dec();
         saveSessionState(sessionId).catch(() => {});
       }
     });