More debug on watching

cista/watching.py

@@ -1,4 +1,5 @@
 import asyncio
+import os
 import shutil
 import sys
 import threading
@@ -7,6 +8,7 @@ from contextlib import suppress
 from os import stat_result
 from pathlib import Path, PurePosixPath
 from stat import S_ISDIR, S_ISREG
+import signal
 
 import msgspec
 from natsort import humansorted, natsort_keygen, ns
@@ -174,6 +176,9 @@ state = State()
 rootpath: Path = None  # type: ignore
 quit = threading.Event()
 
+# Keep a reference so the file stays open for faulthandler outputs
+_faulthandler_file = None  # type: ignore
+
 ## Filesystem scanning
 
 
@@ -574,6 +579,29 @@ async def start(app, loop):
     global rootpath
     config.load_config()
     rootpath = config.config.path
+    # Optional: enable SIGUSR1 stack dumps in production for debugging hangs
+    # Control with env CISTA_STACK_DUMP (default: enabled). Sends all thread
+    # stacks to a per-process log in /tmp when receiving SIGUSR1.
+    if os.environ.get("CISTA_STACK_DUMP", "1") == "1":
+        try:
+            import faulthandler
+
+            global _faulthandler_file
+            if _faulthandler_file is None:
+                log_path = f"/tmp/cista-stacks-{os.getpid()}.log"
+                # Line-buffered text file so writes appear promptly
+                _faulthandler_file = open(log_path, "a", buffering=1)
+                faulthandler.enable(file=_faulthandler_file)
+                faulthandler.register(
+                    signal.SIGUSR1, file=_faulthandler_file, all_threads=True, chain=True
+                )
+                logger.info(
+                    "Stack dump enabled: send SIGUSR1 to PID %s to write all thread stacks to %s",
+                    os.getpid(),
+                    log_path,
+                )
+        except Exception:
+            logger.exception("Failed to enable SIGUSR1 stack dump handler")
     use_inotify = sys.platform == "linux"
     app.ctx.watcher = threading.Thread(
         target=watcher_inotify if use_inotify else watcher_poll,