fix: reap finalized runtimes before session start

backend/src/routes/sessions.ts

@@ -35,6 +35,10 @@ export function sessionRoutes(
     string,
     ReturnType<PiProcessManager["createFor"]>
   >();
+  const runtimeManifestReaders = new WeakMap<
+    ReturnType<PiProcessManager["createFor"]>,
+    () => Promise<any>
+  >();
   const failurePersistenceClaimed = new WeakSet<
     ReturnType<PiProcessManager["createFor"]>
   >();
@@ -82,11 +86,33 @@ export function sessionRoutes(
 
   const storageFailure = (reply: any) => reply.code(503).send({ error: "session storage is unavailable" });
 
+  const releaseIfFinalized = async (
+    id: string, rt: ReturnType<PiProcessManager["createFor"]>,
+  ): Promise<boolean> => {
+    if (d.mgr.get(id) !== rt || boundRuntimes.get(id) !== rt) return false;
+    const readManifest = runtimeManifestReaders.get(rt);
+    if (!readManifest || (await readManifest())?.status !== "finalized") return false;
+    if (!d.mgr.teardownIfCurrent(id, rt)) return false;
+    if (boundRuntimes.get(id) === rt) boundRuntimes.delete(id);
+    return true;
+  };
+
+  const releaseFinalizedRuntimes = async (): Promise<void> => {
+    await Promise.all([...boundRuntimes.entries()].map(([id, rt]) =>
+      withSessionLifecycle(id, () => releaseIfFinalized(id, rt)).catch((error: unknown) => {
+        console.error(`[session:${id}] stale runtime cleanup failed:`, error);
+      }),
+    ));
+  };
+
   const bindRuntime = (
     id: string, rt: ReturnType<PiProcessManager["createFor"]>, runner: any, workspace?: string,
   ) => {
     const previous = boundRuntimes.get(id);
     boundRuntimes.set(id, rt);
+    if (typeof runner.sessionShow === "function") {
+      runtimeManifestReaders.set(rt, () => runner.sessionShow(id, workspace));
+    }
     try {
       rt.bridge.onClientEvent((e) => {
         // Child termination is asynchronous. Ignore queued events from a runtime once a newer
@@ -115,14 +141,7 @@ export function sessionRoutes(
           if (d.mgr.get(id) !== rt && boundRuntimes.get(id) === rt) {
             boundRuntimes.delete(id);
           } else if (typeof runner.sessionShow === "function") {
-            void withSessionLifecycle(id, async () => {
-              if (d.mgr.get(id) !== rt || boundRuntimes.get(id) !== rt) return;
-              const manifest = await runner.sessionShow(id, workspace);
-              if (manifest?.status !== "finalized") return;
-              if (d.mgr.teardownIfCurrent(id, rt) && boundRuntimes.get(id) === rt) {
-                boundRuntimes.delete(id);
-              }
-            }).catch((error: unknown) => {
+            void withSessionLifecycle(id, () => releaseIfFinalized(id, rt)).catch((error: unknown) => {
               console.error(`[session:${id}] terminal runtime cleanup failed:`, error);
             });
           }
@@ -189,6 +208,7 @@ export function sessionRoutes(
     let s: Settings;
     try { s = await d.getSettings(principal); } catch { return storageFailure(reply); }
     const runner = runnerFor(principal);
+    await releaseFinalizedRuntimes();
     const ensure = await d.readiness.ensure(s.workspace ?? "", principal);
     if (!ensure.ok) return reply.code(503).send({ error: READINESS_FAILURE_MESSAGE });
     // Local-only: verify the DWH is reachable BEFORE creating the session, so a dropped

backend/test/routes-sessions.test.ts

@@ -181,6 +181,64 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
   unlinkSync(modelKey);
 });
 
+test("POST /sessions reclaims a finalized runtime before enforcing the Pi limit", async () => {
+  const runtimes = new Map<string, any>();
+  const statuses = new Map<string, string>();
+  const tornDown: string[] = [];
+  const order: string[] = [];
+  let nextId = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: (id: string) => runtimes.get(id),
+      createFor: (id: string) => {
+        order.push(`create:${id}`);
+        if (runtimes.size >= 1) throw new Error("max Pi processes reached");
+        const runtime = { bridge: { onClientEvent: () => {} } };
+        runtimes.set(id, runtime);
+        return runtime;
+      },
+      configure: async () => {},
+      start: () => {},
+      teardownIfCurrent: (id: string, expected: any) => {
+        if (runtimes.get(id) !== expected) return false;
+        runtimes.delete(id);
+        tornDown.push(id);
+        order.push(`teardown:${id}`);
+        return true;
+      },
+    } as any,
+    thtRunner: {
+      sessionNew: async () => {
+        const id = `s${++nextId}`;
+        order.push(`new:${id}`);
+        statuses.set(id, "open");
+        return { id };
+      },
+      sessionShow: async (id: string) => {
+        order.push(`show:${id}`);
+        return { status: statuses.get(id) };
+      },
+      searchPack: async () => {},
+      failSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  expect((await app.inject({ method: "POST", url: "/sessions", payload: { question: "one" } })).statusCode)
+    .toBe(200);
+  statuses.set("s1", "finalized");
+  order.length = 0;
+
+  const second = await app.inject({ method: "POST", url: "/sessions", payload: { question: "two" } });
+
+  expect(second.statusCode).toBe(200);
+  expect(second.json()).toEqual({ id: "s2" });
+  expect(tornDown).toEqual(["s1"]);
+  expect(order).toEqual(["show:s1", "teardown:s1", "new:s2", "create:s2"]);
+  expect(runtimes.has("s2")).toBe(true);
+});
+
 test("POST /sessions refuses to create a session when the local DWH precheck fails", async () => {
   let sessionNewCalls = 0;
   let pinged = 0;