feat(pi-ext): self-healing respawn + scoped init timeout for mempalace-mcp

A stall-kill (or any crash) of mempalace-mcp was a permanent latch:
available flipped off and stayed off until pi restart. Now the next tool
call transparently respawns the server and retries.

- ensureAlive(): bounded respawn with capped exponential backoff
  (MEMPALACE_MCP_MAX_RESPAWNS, default 2; MEMPALACE_MCP_RESPAWN_BACKOFF_MS,
  default 1000). Respawn budget resets on any successful JSON-RPC response,
  so a recovered server regains full patience while a persistently-broken
  one hits the cap and stays down (no hot-loop).
- Init timeout default raised 120000 -> 300000 (scoped to init only): a
  genuine virtiofs cold-open shouldn't be killed mid-progress only to
  respawn and re-pay the same cost. Per-call timeout stays 60000.
- Concurrency hardening: generation counter so a late exit from a killed
  old process can't tear down a fresh respawn; explicit healthy flag
  replaces racy proc!=null liveness check.
- README: document self-heal, new env vars, and why generous-init +
  bounded-respawn compose rather than overlap.

extensions/pi/README.md

@@ -60,15 +60,44 @@ wedged server (classically: an OrbStack/virtiofs cold-open of a large
 *forever*, which freezes the pi TUI — ESC cancels the LLM stream, not a
 pending tool `execute()`. On timeout the extension rejects the request
 **and** kills the stalled child (SIGTERM→SIGKILL), so pi gets a clear
-error instead of hanging and later calls fail fast (`available` flips off;
+error instead of hanging. This is a per-REQUEST timeout, not a process-lifetime
-restart pi to retry). This is a per-REQUEST timeout, not a process-lifetime
 one — the long-lived server is only killed when a request genuinely stalls.
 
 - `MEMPALACE_MCP_TIMEOUT_MS` — tool-call/request timeout. Default `60000`.
+  Kept short on purpose: a *query* taking this long is genuinely wedged.
 - `MEMPALACE_MCP_INIT_TIMEOUT_MS` — `initialize` + `tools/list` handshake
-  timeout (cold-open is expected to be slower here). Default `120000`.
+  timeout. Default `300000`. Deliberately generous: a genuine first
+  cold-open over virtiofs can legitimately take minutes, and killing a
+  still-progressing init only to respawn and re-pay the same cold cost is
+  strictly worse than waiting.
 - Set either to `0` to disable (legacy unbounded behavior).
 
+### Self-heal (respawn instead of a permanent latch)
+
+A stall-kill (or any crash) used to be a **permanent** latch: `available`
+flipped off and stayed off until you restarted pi. It is now self-healing —
+the next tool call transparently respawns `mempalace-mcp` and retries.
+
+- Respawns use **capped exponential backoff** so a persistently-broken
+  server can't hot-loop: `MEMPALACE_MCP_MAX_RESPAWNS` attempts (default
+  `2`; set `0` to disable self-heal and keep the old fail-fast latch),
+  with `MEMPALACE_MCP_RESPAWN_BACKOFF_MS` (default `1000`) doubled per
+  attempt.
+- The budget **resets on any successful JSON-RPC response** — proof the
+  server is actually live — so a server that recovers regains full
+  patience, while one that keeps dying hits the cap and stays down (then
+  restart pi).
+- Why the long init timeout and bounded respawn compose rather than
+  overlap: once a server has opened the palace once, the OS page cache is
+  warm, so respawn cold-opens are fast. The long init timeout prevents
+  killing a healthy *first* cold-open; the respawn handles a genuinely
+  dead server cheaply afterwards. (Note the HNSW deserialize is CPU work
+  that isn't page-cacheable across spawns, which is exactly why we can't
+  rely on respawn-warming alone and keep the generous init budget.)
+- The initial startup is tolerant too: if the very first `start()` fails,
+  the extension runs the same bounded respawn before falling back to
+  fail-soft (pi keeps working without palace tools).
+
 ## Debugging
 
 - `MEMPALACE_EXT_DEBUG=1` — surface `mempalace-mcp` stderr into pi's

extensions/pi/mempalace.ts

@@ -30,9 +30,24 @@
  * This is a per-REQUEST timeout, not a process-lifetime one — the
  * long-lived server is only killed when a request genuinely stalls.
  *   - MEMPALACE_MCP_TIMEOUT_MS       tool-call/request timeout (default 60000)
- *   - MEMPALACE_MCP_INIT_TIMEOUT_MS  initialize+tools/list timeout (default 120000)
+ *   - MEMPALACE_MCP_INIT_TIMEOUT_MS  initialize+tools/list timeout (default 300000)
  *   Set either to 0 to disable (legacy unbounded behavior).
  *
+ * Self-heal (respawn): a stall-kill (or any crash) is no longer a permanent
+ * latch. The next tool call transparently respawns `mempalace-mcp` and
+ * retries, with capped exponential backoff so a persistently-broken server
+ * can't hot-loop. The respawn budget resets on ANY successful JSON-RPC
+ * response (proof the server is actually live), so a recovered server
+ * regains full patience. The two timeouts above are deliberately split:
+ * the long INIT timeout lets a genuine first cold-open finish without being
+ * killed (the original incident), while the short per-call timeout still
+ * aggressively kills a stuck query. After a server has opened the palace
+ * once, the OS page cache is warm, so respawn cold-opens are fast — which
+ * is exactly why a generous INIT timeout + bounded respawn compose well
+ * instead of overlapping.
+ *   - MEMPALACE_MCP_MAX_RESPAWNS       respawn attempts before giving up (default 2; 0 disables self-heal)
+ *   - MEMPALACE_MCP_RESPAWN_BACKOFF_MS base backoff, doubled per attempt (default 1000)
+ *
  * Debug: set MEMPALACE_EXT_DEBUG=1 to surface mempalace-mcp stderr.
  */
 
@@ -58,6 +73,12 @@ const num = (envVal: string | undefined, fallback: number): number => {
 	return Number.isFinite(n) && n >= 0 ? n : fallback;
 };
 
+const sleep = (ms: number): Promise<void> =>
+	new Promise((res) => {
+		const t = setTimeout(res, ms);
+		if (typeof t.unref === "function") t.unref();
+	});
+
 class McpClient {
 	private proc: ChildProcessWithoutNullStreams | null = null;
 	private nextId = 1;
@@ -68,18 +89,42 @@ class McpClient {
 
 	// Per-request timeouts (ms). 0 = disabled (unbounded, legacy behavior).
 	private requestTimeoutMs = num(process.env.MEMPALACE_MCP_TIMEOUT_MS, 60_000);
-	private initTimeoutMs = num(process.env.MEMPALACE_MCP_INIT_TIMEOUT_MS, 120_000);
+	private initTimeoutMs = num(process.env.MEMPALACE_MCP_INIT_TIMEOUT_MS, 300_000);
+	// Self-heal (respawn) controls.
+	private maxRespawns = num(process.env.MEMPALACE_MCP_MAX_RESPAWNS, 2);
+	private respawnBackoffMs = num(process.env.MEMPALACE_MCP_RESPAWN_BACKOFF_MS, 1_000);
+	private respawns = 0; // consecutive respawn attempts; reset on any success
+	private reviving: Promise<boolean> | null = null;
+	// Liveness: true only between a completed init and the matching death.
+	// Inferring from `proc` is racy (non-null during the SIGTERM→exit window).
+	private healthy = false;
+	// Spawn generation. Each (re)spawn bumps it; death handlers carry the gen
+	// they were attached for and no-op if a newer server has since taken over
+	// (prevents a stale OLD-proc 'exit' from clobbering a freshly respawned one).
+	private gen = 0;
+	// Spawn args remembered so a respawn can reuse them.
+	private command = "mempalace-mcp";
+	private args: string[] = [];
 	// Fired when the child process dies (exit or stall-kill). Lets the
 	// extension flip `available` so later tool calls fail fast.
 	public onExit: (() => void) | null = null;
 
+	/** True only when a server has completed init and not since died. */
+	get alive(): boolean {
+		return this.healthy;
+	}
+
 	async start(command: string, args: string[] = []): Promise<void> {
 		if (this.ready) return this.ready;
+		this.command = command;
+		this.args = args;
 		this.ready = (async () => {
-			this.proc = spawn(command, args, { stdio: ["pipe", "pipe", "pipe"] });
+			const myGen = ++this.gen;
-			this.proc.on("error", (err) => this.handleDeath(err));
+			const child = spawn(command, args, { stdio: ["pipe", "pipe", "pipe"] });
-			this.proc.on("exit", (code) =>
+			this.proc = child;
-				this.handleDeath(new Error(`mempalace-mcp exited (code=${code})`)),
+			child.on("error", (err) => this.handleDeath(myGen, err));
+			child.on("exit", (code) =>
+				this.handleDeath(myGen, new Error(`mempalace-mcp exited (code=${code})`)),
 			);
 			this.proc.stdout.setEncoding("utf8");
 			this.proc.stdout.on("data", (chunk: string) => this.onStdout(chunk));
@@ -108,6 +153,9 @@ class McpClient {
 
 			const listed = await this.request("tools/list", {}, this.initTimeoutMs);
 			this.tools = (listed?.tools as McpTool[]) ?? [];
+			// Init complete and the process is still ours — mark live so ensureAlive()
+			// reports true. Guard against a death that raced in during init.
+			if (myGen === this.gen) this.healthy = true;
 		})();
 		return this.ready;
 	}
@@ -129,7 +177,12 @@ class McpClient {
 				const p = this.pending.get(msg.id)!;
 				this.settle(msg.id);
 				if (msg.error) p.reject(new Error(msg.error.message ?? "MCP error"));
-				else p.resolve(msg.result);
+				else {
+					// A successful response proves the server is live — restore the
+					// full respawn budget for any future (unrelated) stall.
+					this.respawns = 0;
+					p.resolve(msg.result);
+				}
 			}
 			// notifications (no id) ignored for now.
 		}
@@ -150,15 +203,60 @@ class McpClient {
 		}
 	}
 
-	/** Child died (exit, spawn error, or stall-kill): reject everything. */
+	/**
-	private handleDeath(err: Error) {
+	 * Child died (exit, spawn error, or stall-kill): reject everything.
+	 * Ignored if a newer generation has already taken over — a late 'exit'
+	 * from a killed old process must not tear down a fresh respawn.
+	 */
+	private handleDeath(gen: number, err: Error) {
+		if (gen !== this.gen) return; // stale handler from a superseded process
 		this.proc = null;
+		this.healthy = false;
+		// Clear the memoized start() promise so a subsequent start()/ensureAlive()
+		// spawns a fresh server instead of returning the dead one's promise.
+		this.ready = null;
 		this.failAll(err);
 		try {
 			this.onExit?.();
 		} catch {}
 	}
 
+	/**
+	 * Ensure a live, initialized server — respawning a dead one with capped
+	 * exponential backoff. Returns whether the server is alive afterwards.
+	 * Concurrent callers share a single in-flight revive. The attempt counter
+	 * is reset by `onStdout` on any successful response, so a server that comes
+	 * back and works regains its full budget; a server that keeps dying hits
+	 * `maxRespawns` and stays down (restart pi) rather than hot-looping.
+	 */
+	async ensureAlive(): Promise<boolean> {
+		if (this.alive) return true;
+		if (this.reviving) return this.reviving;
+		this.reviving = (async () => {
+			while (!this.alive && this.respawns < this.maxRespawns) {
+				this.respawns++;
+				await sleep(this.respawnBackoffMs * 2 ** (this.respawns - 1));
+				// Force a fresh spawn: drop any settled (rejected/dead) start()
+				// promise so start() doesn't short-circuit on a stale memo. Safe
+				// because ensureAlive is serialized (single `reviving`) and only
+				// runs while not healthy — no useful in-flight start() can exist.
+				this.ready = null;
+				try {
+					await this.start(this.command, this.args);
+				} catch {
+					// start() rejected (e.g. respawn cold-open also stalled and was
+					// killed). Loop until the budget is exhausted.
+				}
+			}
+			return this.alive;
+		})();
+		try {
+			return await this.reviving;
+		} finally {
+			this.reviving = null;
+		}
+	}
+
 	private write(obj: unknown) {
 		if (!this.proc) throw new Error("MCP process not started");
 		this.proc.stdin.write(`${JSON.stringify(obj)}\n`);
@@ -180,7 +278,7 @@ class McpClient {
 						new Error(
 							`mempalace-mcp request '${method}' timed out after ${timeoutMs}ms ` +
 								`(server wedged — likely cold storage open). Terminating the ` +
-								`stalled server; restart pi to retry palace tools.`,
+								`stalled server; it will be respawned on the next call.`,
 						),
 					);
 					// Kill the wedged child so subsequent calls fail fast instead of
@@ -241,17 +339,26 @@ export default async function mempalaceExtension(pi: ExtensionAPI) {
 
 	try {
 		client.onExit = () => {
-			// Child died (stall-kill or crash): later tool calls return a clear
+			// Child died (stall-kill or crash): mark unavailable. The next tool
-			// error instead of trying to talk to a dead/respawn-less server.
+			// call will attempt a bounded respawn via client.ensureAlive().
 			available = false;
 		};
 		await client.start("mempalace-mcp");
 		available = true;
 	} catch (err) {
+		// First cold-open stalled/crashed. Give the bounded self-heal a chance
+		// before giving up entirely — a respawn often succeeds against a now-warm
+		// page cache. Only fail-soft if even that is exhausted.
 		process.stderr.write(
-			`[mempalace ext] failed to start mempalace-mcp: ${(err as Error).message}\n`,
+			`[mempalace ext] mempalace-mcp start failed: ${(err as Error).message} — attempting respawn\n`,
 		);
-		return; // fail-soft: pi keeps working without palace tools
+		available = await client.ensureAlive();
+		if (!available) {
+			process.stderr.write(
+				"[mempalace ext] mempalace-mcp unavailable after retries; continuing without palace tools\n",
+			);
+			return; // fail-soft: pi keeps working without palace tools
+		}
 	}
 
 	// Register MCP tools as pi tools. Pass the MCP `inputSchema` through as
@@ -270,9 +377,18 @@ export default async function mempalaceExtension(pi: ExtensionAPI) {
 			description: tool.description ?? `MemPalace tool: ${tool.name}`,
 			parameters: schema,
 			async execute(_toolCallId, params) {
+				if (!available) {
+					// Stall-kill/crash is not a permanent latch: try a bounded respawn.
+					available = await client.ensureAlive();
+				}
 				if (!available) {
 					return {
-						content: [{ type: "text", text: "mempalace-mcp not available" }],
+						content: [
+							{
+								type: "text",
+								text: "mempalace-mcp not available (respawn budget exhausted) — restart pi to retry palace tools",
+							},
+						],
 						details: {},
 						isError: true,
 					};