Output thread IDs depending on process pid

Signed-off-by: Vadim Markovtsev <vadim@poolside.ai>

Author: vmarkovtsev

flogging/flogging.py

@@ -95,9 +95,17 @@ def format_datetime(dt: datetime.datetime):
     return dt.strftime("%Y-%m-%dT%k:%M:%S.%f000") + tzstr
 
 
+_pid: int = os.getpid()
+
+
+def _change_pid():
+    global _pid
+    _pid = os.getpid()
+
+
 def reduce_thread_id(thread_id: int) -> str:
     """Make a shorter thread identifier by hashing the original."""
-    return xxhash.xxh32(thread_id.to_bytes(8, "little")).hexdigest()[:4]
+    return xxhash.xxh32((thread_id ^ _pid).to_bytes(8, "little")).hexdigest()[:4]
 
 
 def repr_array(arr: ndarray) -> str:
@@ -324,6 +332,8 @@ def ensure_utf8_stream(stream):
     if not allow_trailing_dot:
         handler.emit = check_trailing_dot(handler.emit)
 
+    os.register_at_fork(after_in_child=_change_pid)
+
 
 def set_context(context):
     """Assign the logging context - an abstract object - to the current thread."""