Allow fork in limited cases (#2624)

sanic/application/ext.py

@@ -8,11 +8,6 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from sanic import Sanic
 
-    try:
-        from sanic_ext import Extend  # type: ignore
-    except ImportError:
-        ...
-
 
 def setup_ext(app: Sanic, *, fail: bool = False, **kwargs):
     if not app.config.AUTO_EXTEND:
@@ -33,7 +28,7 @@ def setup_ext(app: Sanic, *, fail: bool = False, **kwargs):
         return
 
     if not getattr(app, "_ext", None):
-        Ext: Extend = getattr(sanic_ext, "Extend")
+        Ext = getattr(sanic_ext, "Extend")
         app._ext = Ext(app, **kwargs)
 
         return app.ext

sanic/compat.py

@@ -3,10 +3,22 @@ import os
 import signal
 import sys
 
-from typing import Awaitable
+from contextlib import contextmanager
+from typing import Awaitable, Union
 
 from multidict import CIMultiDict  # type: ignore
 
+from sanic.helpers import Default
+
+
+if sys.version_info < (3, 8):  # no cov
+    StartMethod = Union[Default, str]
+else:  # no cov
+    from typing import Literal
+
+    StartMethod = Union[
+        Default, Literal["fork"], Literal["forkserver"], Literal["spawn"]
+    ]
 
 OS_IS_WINDOWS = os.name == "nt"
 UVLOOP_INSTALLED = False
@@ -19,6 +31,16 @@ except ImportError:
     pass
 
 
+@contextmanager
+def use_context(method: StartMethod):
+    from sanic import Sanic
+
+    orig = Sanic.start_method
+    Sanic.start_method = method
+    yield
+    Sanic.start_method = orig
+
+
 def enable_windows_color_support():
     import ctypes
 

sanic/mixins/startup.py

@@ -40,9 +40,9 @@ from sanic.application.logo import get_logo
 from sanic.application.motd import MOTD
 from sanic.application.state import ApplicationServerInfo, Mode, ServerStage
 from sanic.base.meta import SanicMeta
-from sanic.compat import OS_IS_WINDOWS, is_atty
+from sanic.compat import OS_IS_WINDOWS, StartMethod, is_atty
 from sanic.exceptions import ServerKilled
-from sanic.helpers import Default
+from sanic.helpers import Default, _default
 from sanic.http.constants import HTTP
 from sanic.http.tls import get_ssl_context, process_to_context
 from sanic.http.tls.context import SanicSSLContext
@@ -88,6 +88,7 @@ class StartupMixin(metaclass=SanicMeta):
     state: ApplicationState
     websocket_enabled: bool
     multiplexer: WorkerMultiplexer
+    start_method: StartMethod = _default
 
     def setup_loop(self):
         if not self.asgi:
@@ -692,12 +693,17 @@ class StartupMixin(metaclass=SanicMeta):
         return any(app.state.auto_reload for app in cls._app_registry.values())
 
     @classmethod
-    def _get_context(cls) -> BaseContext:
-        method = (
-            "spawn"
-            if "linux" not in sys.platform or cls.should_auto_reload()
-            else "fork"
+    def _get_startup_method(cls) -> str:
+        return (
+            cls.start_method
+            if not isinstance(cls.start_method, Default)
+            else "spawn"
         )
+
+    @classmethod
+    def _get_context(cls) -> BaseContext:
+        method = cls._get_startup_method()
+        logger.debug("Creating multiprocessing context using '%s'", method)
         return get_context(method)
 
     @classmethod