camview: tell the client when the camera is unavailable

The server sends "camera unavailable" after 3 s without a frame and
"camera ok" when frames come; the app shows the notice instead of a
stale frame.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

tool/camview_server.py

@@ -4,9 +4,12 @@
 Streams the USB camera's own MJPEG frames, unmodified, to any client on 7710/tcp:
 
   server → client, per frame:  [u32 length][f64 capture time, unix s][u32 seq][JPEG]
-               per reply:      [u32 0][f64 time][u32 text length][UTF-8 text]
+               per text:       [u32 0][f64 time][u32 text length][UTF-8 text]
   client → server, lines:      "exposure auto" | "exposure <100 µs units>" | "gain <0-240>"
-                               → one reply, "ok <state>" or "err <reason>"
+                               → one text reply, "ok <state>" or "err <reason>"
+
+Texts also carry availability: "camera unavailable" when no frame has come for
+UNAVAILABLE_S (none yet, or capture stopped), "camera ok" when frames resume.
 
 Latest-wins: each client gets the newest frame when it is ready for one, never a
 queue of old ones. Sockets are paced (SO_MAX_PACING_RATE, project-yukart decision
@@ -25,6 +28,7 @@ import time
 
 SO_MAX_PACING_RATE = 47
 CONTROLS = {"exposure", "gain"}
+UNAVAILABLE_S = 3.0
 
 
 class Latest:
@@ -39,7 +43,7 @@ class Latest:
             self.frame = (seq, t, jpeg)
             self.cond.notify_all()
 
-    def after(self, seq, timeout=5.0):
+    def after(self, seq, timeout):
         with self.cond:
             self.cond.wait_for(lambda: self.frame and self.frame[0] > seq, timeout)
             return self.frame
@@ -106,7 +110,13 @@ def serve_client(conn, peer, dev, latest, pace):
     conn.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
     alive = threading.Event()
     alive.set()
-    send = threading.Lock()  # frames and replies share the socket; never interleave them
+    send = threading.Lock()  # frames and texts share the socket; never interleave them
+
+    def send_text(text):
+        # Texts go in-band as a frame with length 0 and the text after the header.
+        msg = text.encode()
+        with send:
+            conn.sendall(struct.pack("!IdI", 0, time.time(), len(msg)) + msg)
 
     def reader():
         buf = b""
@@ -120,10 +130,7 @@ def serve_client(conn, peer, dev, latest, pace):
                     line, buf = buf.split(b"\n", 1)
                     reply = control(dev, line.decode(errors="replace").strip())
                     print(f"{peer}: {line.decode(errors='replace').strip()} -> {reply}", flush=True)
-                    # Replies go in-band as a frame with length 0 and the text after the header.
-                    msg = reply.encode()
-                    with send:
-                        conn.sendall(struct.pack("!IdI", 0, time.time(), len(msg)) + msg)
+                    send_text(reply)
         except OSError:
             pass
         alive.clear()
@@ -131,11 +138,18 @@ def serve_client(conn, peer, dev, latest, pace):
     threading.Thread(target=reader, daemon=True).start()
     seq = 0
     sent = 0
+    available = None  # unknown until the first frame or the first timeout
     try:
         while alive.is_set():
-            f = latest.after(seq)
+            f = latest.after(seq, UNAVAILABLE_S)
             if not f or f[0] <= seq:
+                if available is not False:
+                    available = False
+                    send_text("camera unavailable")
                 continue
+            if available is not True:
+                available = True
+                send_text("camera ok")
             seq, t, jpeg = f
             with send:
                 conn.sendall(struct.pack("!IdI", len(jpeg), t, seq) + jpeg)