Fix agent crash on missing DISPLAY and relay disconnect error

- Auto-detect DISPLAY on Linux by scanning /tmp/.X11-unix/ sockets,
  falling back to 'w' output, then :0 — runs before mss/pynput import
- ScreenCapture no longer raises on init failure; agent stays connected
  and notifies the viewer with an error message if capture unavailable
- stream_frames skips None frames instead of crashing the WebSocket
- Relay: check for websocket.disconnect message type to avoid
  'Cannot call receive once a disconnect message has been received'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent/agent.py

@@ -27,6 +27,46 @@ from typing import Optional
 import argparse
 import signal
 
+# ── Display detection (Linux) — must run before mss/pynput imports ────────────
+
+def _ensure_display():
+    """Auto-detect and set $DISPLAY on Linux if not already set."""
+    if platform.system() != "Linux":
+        return
+    if os.environ.get("DISPLAY"):
+        return
+
+    import glob
+
+    # 1. Look for X11 Unix sockets in /tmp/.X11-unix/
+    sockets = sorted(glob.glob("/tmp/.X11-unix/X*"))
+    if sockets:
+        num = sockets[0].replace("/tmp/.X11-unix/X", "")
+        os.environ["DISPLAY"] = f":{num}"
+        log.info(f"Auto-detected DISPLAY=:{num}")
+        return
+
+    # 2. Ask 'w' which displays logged-in users are on
+    try:
+        out = subprocess.check_output(
+            ["bash", "-c", "w -h 2>/dev/null | awk '{print $3}' | grep '^:' | head -1"],
+            timeout=2, text=True
+        ).strip()
+        if out.startswith(":"):
+            os.environ["DISPLAY"] = out
+            log.info(f"Auto-detected DISPLAY={out} from w")
+            return
+    except Exception:
+        pass
+
+    # 3. Fallback — :0 is correct on most single-user desktop systems
+    os.environ["DISPLAY"] = ":0"
+    log.info("No display found via socket scan — falling back to DISPLAY=:0")
+
+
+# Run before importing mss / pynput (they read DISPLAY at import time on Linux)
+_ensure_display()
+
 # Third-party — installed via requirements.txt / bundled by PyInstaller
 import httpx
 import websockets
@@ -128,20 +168,34 @@ class ScreenCapture:
         self._sct = None
 
     def __enter__(self):
+        try:
             self._sct = mss()
+        except Exception as e:
+            log.warning(f"Screen capture unavailable: {e}")
+            self._sct = None
         return self
 
     def __exit__(self, *args):
         if self._sct:
             self._sct.close()
 
-    def capture(self) -> bytes:
+    @property
+    def available(self) -> bool:
+        return self._sct is not None
+
+    def capture(self) -> Optional[bytes]:
+        if not self._sct:
+            return None
+        try:
             monitor = self._sct.monitors[1]  # Primary monitor
             img = self._sct.grab(monitor)
             pil = Image.frombytes("RGB", img.size, img.bgra, "raw", "BGRX")
             buf = BytesIO()
             pil.save(buf, format="JPEG", quality=self.quality, optimize=False)
             return buf.getvalue()
+        except Exception as e:
+            log.warning(f"Frame capture failed: {e}")
+            return None
 
     @property
     def frame_delay(self) -> float:
@@ -292,13 +346,20 @@ class Agent:
 
     async def _message_loop(self, ws):
         with ScreenCapture(fps=15, quality=60) as screen:
+            if not screen.available:
+                log.warning(
+                    "Screen capture unavailable — agent will stay connected "
+                    "but cannot stream. Check that $DISPLAY is set."
+                )
+
             stream_task: Optional[asyncio.Task] = None
 
             async def stream_frames():
                 while self._streaming and not self._stop_event.is_set():
                     t0 = time.monotonic()
-                    try:
                     frame = screen.capture()
+                    if frame:
+                        try:
                             await ws.send(frame)
                         except Exception:
                             break
@@ -323,6 +384,12 @@ class Agent:
                         log.info(f"Viewer connected — session {session_id}")
                         self._streaming = True
                         self._active_session = session_id
+                        if not screen.available:
+                            # Tell viewer why they won't see frames
+                            await ws.send(json.dumps({
+                                "type": "error",
+                                "message": "Screen capture unavailable on this machine (no display). Set $DISPLAY and restart the agent.",
+                            }))
                         if stream_task and not stream_task.done():
                             stream_task.cancel()
                         stream_task = asyncio.create_task(stream_frames())

relay/main.py

@@ -113,6 +113,10 @@ async def agent_endpoint(
         while True:
             msg = await websocket.receive()
 
+            # Client sent a close frame — exit cleanly
+            if msg.get("type") == "websocket.disconnect":
+                break
+
             if "bytes" in msg and msg["bytes"]:
                 # Binary = JPEG frame → forward to all viewers watching this machine
                 frame_data = msg["bytes"]