Android device video never starts when the server runs without XDG_RUNTIME_DIR #12313

What happened

"I've noticed for the past few days that device hub doesn't always work. When I try to connect a simulator it will just hang and never actually show. Sometimes it works. It specifically hangs on connecting video."

The user also saw [apple-utils] Failed to run \xcrun simctl list devices --json`` at the top of the T3 Code window on Linux and reasonably assumed that was the cause.

Diagnosis

The xcrun message is a red herring. expo-device-hub enumerates iOS simulators unconditionally on every platform, and on Linux the spawn xcrun ENOENT is collected into the errors[] array of /api/devices and never touches the Android path. Worth suppressing on non-darwin for noise reasons, but unrelated to this bug.

The real cause is bearer-token discovery for the emulator's gRPC endpoint.

The Android emulator secures its gRPC endpoint and publishes the token in a per-run file at $XDG_RUNTIME_DIR/avd/running/pid_<pid>.ini (grpc.port, grpc.token). In expo-device-hub@0.9.0, vendor/serve-emu/dist/emulator-grpc.js:20, discoveryDirs() builds its candidate list like this:

const dirs = [join(home, "Library", "Caches", "TemporaryItems", "avd", "running")];
if (process.env.XDG_RUNTIME_DIR) dirs.push(join(process.env.XDG_RUNTIME_DIR, "avd", "running"));
if (process.env.LOCALAPPDATA)    dirs.push(join(process.env.LOCALAPPDATA, "Temp", "avd", "running"));
dirs.push(join(home, ".android", "avd", "running"));

On Linux the only unconditional candidate is ~/.android/avd/running, which the emulator does not write to. So if XDG_RUNTIME_DIR is absent from the hub process environment, discovery finds nothing. It falls back to adb -s <serial> emu grpc <port>, retried 5 times with 40 probes each (this is the visible hang), and at probe === 39 returns { port: activePort, token: null }. useEndpoint() only warns when the token is null, and the authorization header is only set when a token exists, so every subsequent call fails grpc-status 16 UNAUTHENTICATED.

T3 Code's contribution is that it does not pass XDG_RUNTIME_DIR down. apps/server/src/device/LocalDeviceHost.ts:232 (v0.0.42; :245 on main, unchanged as of 2026-09-17):

const hubEnvironment = (): NodeJS.ProcessEnv => ({
  ...hostEnvironment,
  FORCE_COLOR: "0",
  NO_COLOR: "1",
});

hostEnvironment is inherited from the server process. deviceHostEnvironment() adds only ANDROID_HOME and PATH entries. So the hub inherits whatever the server had. A server started from a desktop session has XDG_RUNTIME_DIR; one started over SSH typically does not.

This explains "sometimes it works." On this machine two servers were running: one from the desktop app (has XDG_RUNTIME_DIR) and one t3 serve launched over SSH (does not). The device hub is shared and reaped through device/agent-device/hub.json, so whichever server spawns the hub first silently decides whether device video works for the whole machine.

Three secondary problems make this hard to see:

1. DeviceHubProxy.proxyWebSocket in apps/server/src/device/DeviceHubProxy.ts ends with .pipe(Effect.catchCause(() => Effect.void)), so the upstream failure is swallowed. Every one of 199 logged proxy requests exits Success in server.trace.ndjson while the panel is visibly broken. Nothing surfaces in the UI or the trace.
2. serve-emu treats a null token as a warning rather than an error, so the failure only appears later as a generic grpc-status 16 from getScreenshot.
3. More generally, the device path drops causes on the way to the user. Separately from this bug, the same session hit an unreachable SSH device host, and the server had the exact reason in hand (ssh: connect to host <host> port 22: No route to host, later Operation timed out) while the user was shown only The device command failed (exit code 255). and then Could not communicate with device support. Try refreshing devices. The real stderr was sitting in server.trace.ndjson the whole time. Surfacing the underlying SshCommandError message would have made that self-service.

Steps to reproduce

1. On Linux, start an Android emulator.
2. Start the server from an environment with no XDG_RUNTIME_DIR (an SSH session does this by default): env -u XDG_RUNTIME_DIR npx t3 serve --port 3775.
3. Make sure that server is the one that spawns the device hub (delete ~/.t3/userdata/device/agent-device/hub.json first if a hub is already running).
4. Open the device panel and select the Android device.
5. It hangs on "Connecting video" indefinitely. Confirm with curl 'http://127.0.0.1:<hubPort>/vendor/serve-emu/health?device=emulator-5554', which returns getScreenshot: grpc-status 16.

Starting the same server with XDG_RUNTIME_DIR set makes it work.

Version

0.0.42 (runtime 0.0.43-nightly.20260917.1866), expo-device-hub 0.9.0, agent-device 0.20.10

Environment

Linux x64, Nobara 7.2.3 (7.2.3-200.nobara.fc44.x86_64), Node v26.8.2, emulator 37.1.11.0, opencode as the agent CLI. Desktop app on macOS connecting to this Linux server, which also has a remote SSH device host configured.

Evidence

# hub health while broken
{"ok":false,"error":"getScreenshot: grpc-status 16 (Missing the 'authorization' header with security credentials.)"}
{"ok":false,"error":"serve-emu start for emulator-5554 is cooling down after a failure"}

# proven on the wire, token from the discovery ini, values redacted
POST /android.emulation.control.EmulatorController/getStatus
  no authorization header           -> grpc-status: 16 "Missing the 'authorization' header with security credentials."
  Bearer <token from pid_<pid>.ini> -> grpc-status: 0

# discovery file the hub never reads without XDG_RUNTIME_DIR
$XDG_RUNTIME_DIR/avd/running/pid_<pid>.ini
  grpc.port=8554
  grpc.token=<REDACTED>
  avd.id=Medium_Phone

# hub process environment
XDG_RUNTIME_DIR absent; SSH_CLIENT and SSH_CONNECTION present

# server.trace.ndjson: 199 proxy requests, every one Success, while the panel is broken
GET /api/device-hub/vendor/serve-emu/ws?device=emulator-5554&frame-meta=1&wsTicket=<REDACTED>&hostId=local
  exit._tag: "Success"   durationMs: 27..2519

# point 3: cause present in the trace, absent from the UI
DeviceService.fetchDevices  Failure
  DeviceOperationError: Device list failed: The device command failed (exit code 255).
ssh/command.runSshCommand   Failure
  SshCommandError: ssh: connect to host <host> port 22: No route to host

Related issues

#12229, same user-visible symptom ("Connecting video") but a different bug. There the device streams and is interactive and the failure is a keyframe/decoder restart deadlock. Here getScreenshot fails outright and serve-emu never starts. #10810 (Honor XDG Base Directory paths on Linux) is adjacent but about config paths, not process environment inheritance.

Fix applied or workaround

Symlinked the unconditional discovery directory to the real one, which works regardless of how the server was launched and needs no restart:

ln -s "$XDG_RUNTIME_DIR/avd/running" ~/.android/avd/running

Both devices went to status: "streaming" immediately.

Suggested real fixes, in the order I'd rank them:

1. hubEnvironment() should pass XDG_RUNTIME_DIR through explicitly, and fall back to /run/user/$(id -u) on Linux when the server's own environment lacks it.
2. serve-emu should treat a null token on Linux as a hard error rather than a warning, so the failure names itself.
3. Surface the underlying cause in the device path rather than discarding it: log the swallowed cause in proxyWebSocket, and include the SshCommandError message instead of a bare exit code.
4. Skip the xcrun probe on non-darwin.

Filed by

Claude Opus 5 via Claude Code, through t3 triage