Replace owned handlers when logging is configured again

agent-notes/improve-repeated-setup.md

@@ -0,0 +1,19 @@
+# Goal
+
+Replace only this package's handlers on repeated logging setup.
+
+# State
+
+Rebuilt on master after #2, #3, #4, #6 and #7 merged. Keep the root logger and existing helper API. After the staged replacement succeeds, detach and close the handlers this package installed previously.
+
+# Decisions and why
+
+Builds on #4's staging: a failed setup closes its staged handlers and leaves installed handlers, root level and exception hook untouched, so this change needs no rollback code of its own. Fault handling uses #3's crashlogger state: a call that does not enable crash logging disables it before old streams close; a call that enables it has already retargeted it to the new stream. Track only owned handlers, preserve unrelated ones, and restore the previous exception hook when a repeat call or shutdown turns it off. Setup and shutdown run on the main thread.
+
+# Surprises
+
+The earlier version of this PR duplicated #3's fault-handler tracking and #4's rollback; both were removed when rebuilding on merged master. Opening a file may still truncate its contents before a later failure.
+
+# Next step
+
+Read the PR's package checks and merge. No release.

logger_setup/logger_setup.py

@@ -16,6 +16,9 @@
 except NameError:
     logger = logging.getLogger()
 
+_handlers = []
+_previous_excepthook = None
+
 
 def create_handler(
     filename=None,
@@ -56,6 +59,7 @@ def setup_logging(
     remote_address=None,
     remote_level=logging.ERROR,
 ):
+    global _handlers, _previous_excepthook
     formatter = logging.Formatter(message_format)
     handlers = []
     try:
@@ -96,13 +100,33 @@ def setup_logging(
 
     for handler in handlers:
         logger.addHandler(handler)
+    # The fault handler must stop using an old stream before that stream closes.
+    if not (error_log and log_crashes):
+        crashlogger.disable_crashlogger()
+    for handler in _handlers:
+        logger.removeHandler(handler)
+        handler.close()
+    _handlers = handlers
     logger.setLevel(logging.DEBUG)
     if log_unhandled_exceptions:
+        if sys.excepthook is not custom_excepthook:
+            _previous_excepthook = sys.excepthook
         sys.excepthook = custom_excepthook
+    elif sys.excepthook is custom_excepthook:
+        sys.excepthook = _previous_excepthook
+        _previous_excepthook = None
     logger.info("Logging initialized. PID: %d", os.getpid())
     return logger
 
 
 def shutdown():
+    global _handlers, _previous_excepthook
     crashlogger.disable_crashlogger()
+    for handler in _handlers:
+        logger.removeHandler(handler)
+        handler.close()
+    _handlers = []
+    if sys.excepthook is custom_excepthook:
+        sys.excepthook = _previous_excepthook
+        _previous_excepthook = None
     logging.shutdown()