homepage

Below is a backtrace from the deadlock.
It happens because the logging code is trying to acquire two per-logger locks; and in an order different to the random order used by the fork() handler.
The code in question has a custom class DebugHandler(logging.Handler). The default logging.Handler.handle() method grabs its lock and calls .emit() vis:
 if rv:
 self.acquire()
 try:
 self.emit(record)
 finally:
 self.release()
the custom .emit() then sends the record to a sub-logger stream vis:
 def emit(self, record):
 for stream_handler in self.stream_handlers:
 stream_handler.emit(record)
 if _DEBUG_STREAM:
 _DEBUG_STREAM.emit(record)
and one of these emit() functions calls flush() which tries to acquire a further lock.
Thread 0x00007f976b7fe700 (most recent call first):
 File "/usr/lib64/python3.7/logging/__init__.py", line 854 in acquire
 File "/usr/lib64/python3.7/logging/__init__.py", line 1015 in flush
 def flush(self):
 """
 Flushes the stream.
 """
 self.acquire() <----
 try:
 if self.stream and hasattr(self.stream, "flush"):
 self.stream.flush()
 finally:
 self.release()
 File "/usr/lib64/python3.7/logging/__init__.py", line 1038 in emit
 self.flush() <----
 File "/home/build/libreswan-web/master/testing/utils/fab/logutil.py", line 163 in emit
 def emit(self, record):
 for stream_handler in self.stream_handlers:
 stream_handler.emit(record) <---
 if _DEBUG_STREAM:
 _DEBUG_STREAM.emit(record)
 File "/usr/lib64/python3.7/logging/__init__.py", line 905 in handle
 def handle(self, record):
 """
 Conditionally emit the specified logging record.
 Emission depends on filters which may have been added to the handler.
 Wrap the actual emission of the record with acquisition/release of
 the I/O thread lock. Returns whether the filter passed the record for
 emission.
 """
 rv = self.filter(record)
 if rv:
 self.acquire()
 try:
 self.emit(record) <---
 finally:
 self.release()
 return rv
 File "/usr/lib64/python3.7/logging/__init__.py", line 1591 in callHandlers
 hdlr.handle(record)
 File "/usr/lib64/python3.7/logging/__init__.py", line 1529 in handle
 self.callHandlers(record)
 File "/usr/lib64/python3.7/logging/__init__.py", line 1519 in _log
 self.handle(record)
 File "/usr/lib64/python3.7/logging/__init__.py", line 1449 in log
 self._log(level, msg, args, **kwargs)
 File "/usr/lib64/python3.7/logging/__init__.py", line 1768 in log
 self.logger.log(level, msg, *args, **kwargs)
 File "/usr/lib64/python3.7/logging/__init__.py", line 1724 in debug
 self.log(DEBUG, msg, *args, **kwargs)
 File "/home/build/libreswan-web/master/testing/utils/fab/shell.py", line 110 in write
 self.logger.debug(self.message, ascii(text))