fix(daemon): name the cause when a background start fails before logging (#5332)

`paseo daemon start` runs the supervisor with its output discarded, and the
supervisor opens its log only after config.json loads and the PID lock is
held. A failure before that point went to stderr only, so the launcher
pointed at a daemon.log that was never written and showed no cause.

Startup failures now also go to the log path the launcher reports, so the
error message tails the real cause.

Closes #5327

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>

packages/cli/src/commands/daemon/lifecycle.e2e.test.ts

@@ -591,6 +591,25 @@ test("malformed configuration cannot turn a readiness timeout into launch cancel
   }
 }, 60_000);
 
+test("a background start that fails before the supervisor logs names the cause in a log that exists", async () => {
+  const f = await fixture();
+  const home = f.homes[0]!;
+  const configPath = path.join(home, "config.json");
+  try {
+    await mkdir(home, { recursive: true });
+    await writeFile(configPath, '{"version":1,');
+    const failed = await f.run(["daemon", "start", "--home", home, "--timeout", "30"]);
+    expect(failed.code).toBe(1);
+    expect(failed.stderr).toContain(`Logs: ${path.join(home, "daemon.log")}`);
+    expect(failed.stderr).toContain(`Invalid JSON in ${configPath}`);
+    expect(await readFile(path.join(home, "daemon.log"), "utf8")).toContain(
+      `Invalid JSON in ${configPath}`,
+    );
+  } finally {
+    await f.close();
+  }
+}, 60_000);
+
 test("onboarding preserves an existing MCP choice unless explicitly supplied", async () => {
   const f = await fixture();
   const home = f.homes[0]!;

packages/server/scripts/supervisor-entrypoint.ts

@@ -1,5 +1,5 @@
 import { fileURLToPath } from "url";
-import { existsSync } from "node:fs";
+import { appendFileSync, existsSync, mkdirSync } from "node:fs";
 import path from "node:path";
 import {
   acquirePidLock,
@@ -9,6 +9,8 @@ import {
   updatePidLock,
 } from "../src/server/pid-lock.js";
 import { resolvePaseoHome } from "../src/server/paseo-home.js";
+import { daemonLogPath } from "../src/server/daemon-instance.js";
+import { PRIVATE_FILE_MODE } from "../src/server/private-files.js";
 import { loadPersistedConfig } from "../src/server/persisted-config.js";
 import { runSupervisor } from "./supervisor.js";
 import { resolveSupervisorLogFile } from "./supervisor-log-config.js";
@@ -118,9 +120,7 @@ async function main(): Promise<void> {
     });
   } catch (error) {
     if (error instanceof PidLockError) {
-      process.stderr.write(`${error.message}\n`);
-      process.exit(1);
-      return;
+      failStartup(error.message, error.message);
     }
     throw error;
   }
@@ -181,8 +181,31 @@ async function main(): Promise<void> {
   requestSupervisorShutdown = supervisor.requestShutdown;
 }
 
-void main().catch((error) => {
-  const message = error instanceof Error ? (error.stack ?? error.message) : String(error);
-  process.stderr.write(`${message}\n`);
+// The supervisor opens its log only after config and the PID lock succeed. A background
+// launch discards stderr, so earlier failures also go to the log the launcher points at.
+function failStartup(detail: string, summary: string): never {
+  process.stderr.write(`${detail}\n`);
+  try {
+    const logPath = daemonLogPath(resolvePaseoHome(process.env));
+    mkdirSync(path.dirname(logPath), { recursive: true });
+    appendFileSync(
+      logPath,
+      `${JSON.stringify({
+        level: "fatal",
+        time: new Date().toISOString(),
+        pid: process.pid,
+        name: "DaemonRunner",
+        msg: summary,
+      })}\n`,
+      { mode: PRIVATE_FILE_MODE },
+    );
+  } catch {
+    // stderr already carries the failure.
+  }
   process.exit(1);
+}
+
+void main().catch((error) => {
+  if (error instanceof Error) failStartup(error.stack ?? error.message, error.message);
+  failStartup(String(error), String(error));
 });