fix(profile): resolve replaced binaries and surface real launch failures

Two failures made a profile launch report the wrong cause.

Linux appends " (deleted)" to /proc/<pid>/exe once the running binary is
unlinked, which is what a google-chrome package upgrade does while the
browser stays open. detectRunningBrowser passed that literal string on as
the executable path, so every launch tried to spawn
"/opt/google/chrome/chrome (deleted)" and hit ENOENT. Executable paths are
now validated against disk, with the marker stripped and argv[0] used as a
fallback; a binary genuinely named "… (deleted)" still wins.

spawn() reports a missing or non-executable binary asynchronously via an
'error' event rather than throwing, and that event was discarded — so
openProfileWindow returned success, no window opened, and the caller
blamed the browser 15s later with a sentinel timeout. The launch now
awaits 'spawn'/'error', returns the real cause, and removes its handshake
page on the failure path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2 files changed, 82 insertions, 14 deletions

diff --git a/src/profile/browser-process.ts b/src/profile/browser-process.ts
index c0f6178..77bc05c 100644
--- a/src/profile/browser-process.ts
+++ b/src/profile/browser-process.ts
@@ -92,6 +92,47 @@ export type BrowserCandidate = {
   readonly explicitUserDataDir?: string;
 };
 
+/**
+ * Linux marks a replaced binary in `/proc/<pid>/exe`: once the file behind a
+ * running process is unlinked — which is exactly what a `google-chrome` package
+ * upgrade does while the browser keeps running — the symlink reads
+ * "/opt/google/chrome/chrome (deleted)". Spawning that string fails with
+ * ENOENT, so the marker has to come off.
+ *
+ * It is stripped only as a fallback, and only when the stripped path exists: a
+ * binary genuinely named "… (deleted)" must still win.
+ */
+const DELETED_SUFFIX = " (deleted)";
+
+/** The path without Linux's deleted-binary marker, or undefined if unmarked. */
+export const stripDeletedSuffix = (path: string): string | undefined =>
+  path.endsWith(DELETED_SUFFIX) && path.length > DELETED_SUFFIX.length
+    ? path.slice(0, -DELETED_SUFFIX.length)
+    : undefined;
+
+const exists = async (path: string): Promise<boolean> => {
+  try {
+    await access(path);
+    return true;
+  } catch {
+    return false;
+  }
+};
+
+/**
+ * A spawnable executable path for a raw readlink/argv value: the value itself
+ * when it exists, else the same value with the deleted-binary marker removed.
+ * Undefined when neither form is on disk — a path we cannot spawn must never be
+ * handed on as if it were usable.
+ */
+export const spawnableExePath = async (raw: string): Promise<string | undefined> => {
+  if (raw.length === 0) return undefined;
+  if (await exists(raw)) return raw;
+  const stripped = stripDeletedSuffix(raw);
+  if (stripped && (await exists(stripped))) return stripped;
+  return undefined;
+};
+
 /** Path comparison that tolerates separator and case differences per platform. */
 const samePath = (a: string, b: string): boolean => {
   const norm = (p: string): string => {
@@ -163,14 +204,19 @@ const collectLinux = async (): Promise<ReadonlyArray<BrowserCandidate>> => {
       continue;
     }
     if (!cmdline || hasChildProcessType(cmdline)) continue;
-    let exePath: string | undefined;
+    let link: string | undefined;
     try {
-      exePath = await readlink(`/proc/${pid}/exe`);
+      link = await readlink(`/proc/${pid}/exe`);
     } catch {
-      // /proc/<pid>/exe is unreadable for processes we don't own; argv[0] is
-      // the next best thing.
-      exePath = cmdline.split(" ")[0];
+      // /proc/<pid>/exe is unreadable for processes we don't own.
     }
+    // argv[0] backs up the symlink in both directions: unreadable for a process
+    // we don't own, and pointing at an unlinked binary after an in-place
+    // upgrade. Chrome's browser process rewrites its argv to the bare
+    // executable path, so on Linux argv[0] is the live install location.
+    const exePath =
+      (link ? await spawnableExePath(link) : undefined) ??
+      (await spawnableExePath(cmdline.split(" ")[0] ?? ""));
     const explicit = parseUserDataDirFlag(cmdline);
     candidates.push({
       ...(exePath ? { exePath } : {}),
@@ -354,7 +400,11 @@ export const isBrowserRunning = async (): Promise<boolean> => (await detectRunni
  */
 export const resolveBrowserExecutable = async (running?: RunningBrowser): Promise<string | undefined> => {
   const detected = running ?? (await detectRunningBrowser());
-  if (detected.exePath) return detected.exePath;
+  // Re-check spawnability here too: this function is the single gate every
+  // launch passes through, and a caller may hand us a RunningBrowser collected
+  // before the binary was replaced on disk.
+  const usable = detected.exePath ? await spawnableExePath(detected.exePath) : undefined;
+  if (usable) return usable;
   if (process.platform !== "win32") return undefined;
 
   const fromRegistry = await winRegistryExecutable();
diff --git a/src/profile/launch.ts b/src/profile/launch.ts
index fd93d3d..0088b6d 100644
--- a/src/profile/launch.ts
+++ b/src/profile/launch.ts
@@ -69,29 +69,47 @@ export const openProfileWindow = async (
   if (req.explicitUserDataDir) args.push(`--user-data-dir=${req.explicitUserDataDir}`);
   args.push(`--profile-directory=${req.profileDir}`, "--new-window", sentinelUrl);
 
-  let child: ChildProcess | null = null;
+  let child: ChildProcess;
   try {
     // No shell: arguments are passed as an array, so profile names and paths
     // containing spaces survive intact on every platform.
     child = spawn(req.exePath, args, { detached: true, stdio: "ignore" });
-    child.unref();
-    // The launcher process exits as soon as the running browser takes over the
-    // command line; an error here means the binary itself could not be started.
-    child.on("error", () => {});
   } catch (e) {
     await rm(file, { force: true }).catch(() => {});
     return err(`could not launch ${req.exePath}: ${e instanceof Error ? e.message : String(e)}`);
   }
 
+  // spawn() reports a missing or non-executable binary ASYNCHRONOUSLY, as an
+  // 'error' event — it does not throw. Ignoring that event made an ENOENT
+  // indistinguishable from a browser that simply never opened the window, so
+  // the caller's 15s sentinel poll reported a timeout for what was really a bad
+  // executable path. Node guarantees exactly one of 'spawn' or 'error', so
+  // waiting for it costs nothing and turns the failure into a named cause.
+  const spawnFailure = await new Promise<string | undefined>((resolve) => {
+    child.once("spawn", () => resolve(undefined));
+    child.once("error", (e: Error) => resolve(e.message));
+  });
+  // A later 'error' event with no listener would throw; keep one attached for
+  // the lifetime of the detached launcher.
+  child.on("error", () => {});
+  child.unref();
+
+  if (spawnFailure) {
+    await rm(file, { force: true }).catch(() => {});
+    return err(`could not launch ${req.exePath}: ${spawnFailure}`);
+  }
+
+  let live: ChildProcess | null = child;
+
   return ok({
     sentinelUrl,
     cleanup: async () => {
       await rm(file, { force: true }).catch(() => {});
     },
     kill: () => {
-      if (child) {
-        try { child.kill("SIGTERM"); } catch {}
-        child = null;
+      if (live) {
+        try { live.kill("SIGTERM"); } catch {}
+        live = null;
       }
     },
   });