fix: make YAML config persistence atomic and add MJPEG idle keepalive heartbeat

CHANGELOG.md

@@ -17,6 +17,10 @@ All notable changes to the `chicken-counting-sukawarna-det` project are document
   - Configured `pip install -e .` in environment setup to resolve `chicken_counter` imports and enable automatic unit test discovery without manual `PYTHONPATH` prefixes.
 
 ### ⚡ Performance & Optimization
+- **Atomic Config Persistence (`dashboard.py`)**:
+  - Added `_config_write_lock` and atomic temporary file replacement (`os.replace`) in `_persist_cycle_start_date` to prevent race conditions or half-written YAML reads.
+- **Streaming Socket Keepalive (`dashboard.py`)**:
+  - Added an idle keepalive heartbeat (`b"--frame\r\n\r\n"`) every 3 seconds to immediately detect and terminate disconnected MJPEG streaming threads during pipeline idle periods.
 - **Disk Traversal Caching in Live Dashboard (`dashboard.py`)**:
   - Replaced repetitive `Path.rglob()` scans on every `/api/mortality/*` endpoint with a thread-safe 15-second in-memory TTL cache (`_MORTALITY_CACHE_TTL = 15.0s`).
   - Added $O(1)$ memory index lookup for serving annotated images (`/api/mortality/image/<filename>`), eliminating full filesystem traversals across 30GB+ video datasets.

dashboard.py

@@ -6,6 +6,7 @@ from __future__ import annotations
 import argparse
 import json
 import mimetypes
+import os
 import re
 import sqlite3
 import threading
@@ -27,6 +28,7 @@ TEMPLATE_DIR = Path(__file__).resolve().parent / "templates"
 
 _thread_local = threading.local()
 _db_lock = threading.Lock()
+_config_write_lock = threading.Lock()
 _db_path = ""
 _mortality_dirs: list[Path] = []
 _mortality_cache_lock = threading.Lock()
@@ -51,29 +53,35 @@ _cycle_start_date: str = _load_initial_cycle_start_date()
 
 
 def _persist_cycle_start_date(new_date: str) -> bool:
-    """Update in-memory cycle_start_date and save to config YAML files."""
+    """Update in-memory cycle_start_date and save to config YAML files atomically."""
     global _cycle_start_date
-    _cycle_start_date = new_date
     updated_any = False
-    for cfg_name in ("cycle7_batch_optimized.yaml", "cycle7_batch.yaml"):
-        cfg_path = Path(__file__).resolve().parent / "configs" / cfg_name
-        if cfg_path.exists():
-            content = cfg_path.read_text(encoding="utf-8")
-            pattern = r"^([ \t]*cycle_start_date:[ \t]*)(?:['\"]?)([^'\"\r\n#]+)(?:['\"]?)([ \t]*(?:#.*)?)$"
+    with _config_write_lock:
+        _cycle_start_date = new_date
+        for cfg_name in ("cycle7_batch_optimized.yaml", "cycle7_batch.yaml"):
+            cfg_path = Path(__file__).resolve().parent / "configs" / cfg_name
+            if cfg_path.exists():
+                try:
+                    content = cfg_path.read_text(encoding="utf-8")
+                    pattern = r"^([ \t]*cycle_start_date:[ \t]*)(?:['\"]?)([^'\"\r\n#]+)(?:['\"]?)([ \t]*(?:#.*)?)$"
 
-            def replacer(match: re.Match) -> str:
-                prefix = match.group(1)
-                comment = match.group(3) or ""
-                if comment and not comment.startswith(" "):
-                    comment = f" {comment.lstrip()}"
-                    if not comment.startswith(" "):
-                        comment = f" {comment}"
-                return f'{prefix}"{new_date}"{comment}'
+                    def replacer(match: re.Match) -> str:
+                        prefix = match.group(1)
+                        comment = match.group(3) or ""
+                        if comment and not comment.startswith(" "):
+                            comment = f" {comment.lstrip()}"
+                            if not comment.startswith(" "):
+                                comment = f" {comment}"
+                        return f'{prefix}"{new_date}"{comment}'
 
-            new_content, count = re.subn(pattern, replacer, content, count=1, flags=re.MULTILINE)
-            if count > 0:
-                cfg_path.write_text(new_content, encoding="utf-8")
-                updated_any = True
+                    new_content, count = re.subn(pattern, replacer, content, count=1, flags=re.MULTILINE)
+                    if count > 0:
+                        tmp_path = cfg_path.with_suffix(f".tmp.{os.getpid()}")
+                        tmp_path.write_text(new_content, encoding="utf-8")
+                        os.replace(tmp_path, cfg_path)
+                        updated_any = True
+                except Exception as err:
+                    print(f"[dashboard] ⚠️ Error atomically updating {cfg_name}: {err}")
     print(f"[dashboard] 📅 Updated cycle_start_date to: {new_date} (persisted in configs: {updated_any})")
     return updated_any
 
@@ -299,9 +307,14 @@ class DashboardHandler(SimpleHTTPRequestHandler):
         self.send_header("Cache-Control", "no-cache")
         self.end_headers()
 
-        last_mtime = 0
+        last_mtime = 0.0
+        last_write_time = time.time()
+        keepalive_interval = 3.0
+
         try:
             while True:
+                now = time.time()
+                frame_written = False
                 try:
                     mtime = frame_path.stat().st_mtime
                     if mtime != last_mtime:
@@ -314,10 +327,19 @@ class DashboardHandler(SimpleHTTPRequestHandler):
                             data + b"\r\n"
                         )
                         self.wfile.flush()
+                        last_write_time = now
+                        frame_written = True
                 except (FileNotFoundError, OSError):
                     pass
+
+                # If idle (no new frames), send a lightweight keepalive ping to detect disconnected clients
+                if not frame_written and (now - last_write_time) >= keepalive_interval:
+                    self.wfile.write(b"--frame\r\n\r\n")
+                    self.wfile.flush()
+                    last_write_time = now
+
                 time.sleep(0.1)
-        except (BrokenPipeError, ConnectionResetError):
+        except (BrokenPipeError, ConnectionResetError, OSError):
             pass
 
     def _handle_shm(self, path):