Call os._exit() for faster shut down if using --fast-exit (#5569)

This can speed up self checking by about a second.

Author: JukkaL

mypy/main.py

@@ -107,8 +107,15 @@ def flush_errors(new_messages: List[str], serious: bool) -> None:
         from mypy.memprofile import print_memory_profile
         print_memory_profile()
 
+    code = 0
     if messages:
         code = 2 if blockers else 1
+    if options.fast_exit:
+        # Exit without freeing objects -- it's faster.
+        #
+        # NOTE: We don't flush all open files on exit (or run other destructors)!
+        util.hard_exit(code)
+    elif code:
         sys.exit(code)
 
 
@@ -613,6 +620,8 @@ def add_invertible_flag(flag: str,
         dest='shadow_file', action='append',
         help="When encountering SOURCE_FILE, read and type check "
              "the contents of SHADOW_FILE instead.")
+    add_invertible_flag('--fast-exit', default=False, help=argparse.SUPPRESS,
+                        group=internals_group)
 
     error_group = parser.add_argument_group(
         title='Error reporting',

mypy/options.py

@@ -211,6 +211,8 @@ def __init__(self) -> None:
         # if they don't have __init__.py.
         self.package_root = []  # type: List[str]
         self.cache_map = {}  # type: Dict[str, Tuple[str, str]]
+        # Don't properly free objects on exit, just kill the current process.
+        self.fast_exit = False
 
     def snapshot(self) -> object:
         """Produce a comparable snapshot of this Option"""

mypy/util.py

@@ -238,3 +238,13 @@ def replace_object_state(new: object, old: object, copy_dict: bool=False) -> Non
 def is_sub_path(path1: str, path2: str) -> bool:
     """Given two paths, return if path1 is a sub-path of path2."""
     return pathlib.Path(path2) in pathlib.Path(path1).parents
+
+
+def hard_exit(status: int = 0) -> None:
+    """Kill the current process without fully cleaning up.
+
+    This can be quite a bit faster than a normal exit() since objects are not freed.
+    """
+    sys.stdout.flush()
+    sys.stderr.flush()
+    os._exit(status)