Fix static _handler pickling error.

Moves the subfunction _handler out to a module-level function, and parameterizes it with functools.partial().
Fixes the case when picking a sanic app which has a registered static route handler. This is usually encountered when attempting to use multiprocessing or auto_reload on OSX or Windows.
Fixes #1774

sanic/static.py

@@ -1,3 +1,4 @@
+from functools import partial, wraps
 from mimetypes import guess_type
 from os import path
 from re import sub
@@ -15,48 +16,15 @@ from sanic.handlers import ContentRangeHandler
 from sanic.response import HTTPResponse, file, file_stream
 
 
-def register(
-    app,
-    uri,
+async def _static_request_handler(
     file_or_directory,
-    pattern,
     use_modified_since,
     use_content_range,
     stream_large_files,
-    name="static",
-    host=None,
-    strict_slashes=None,
+    request,
     content_type=None,
+    file_uri=None,
 ):
-    # TODO: Though sanic is not a file server, I feel like we should at least
-    #       make a good effort here.  Modified-since is nice, but we could
-    #       also look into etags, expires, and caching
-    """
-    Register a static directory handler with Sanic by adding a route to the
-    router and registering a handler.
-
-    :param app: Sanic
-    :param file_or_directory: File or directory path to serve from
-    :param uri: URL to serve from
-    :param pattern: regular expression used to match files in the URL
-    :param use_modified_since: If true, send file modified time, and return
-                               not modified if the browser's matches the
-                               server's
-    :param use_content_range: If true, process header for range requests
-                              and sends the file part that is requested
-    :param stream_large_files: If true, use the file_stream() handler rather
-                              than the file() handler to send the file
-                              If this is an integer, this represents the
-                              threshold size to switch to file_stream()
-    :param name: user defined name used for url_for
-    :param content_type: user defined content type for header
-    """
-    # If we're not trying to match a file directly,
-    # serve from the folder
-    if not path.isfile(file_or_directory):
-        uri += "<file_uri:" + pattern + ">"
-
-    async def _handler(request, file_uri=None):
     # Using this to determine if the URL is trying to break out of the path
     # served.  os.path.realpath seems to be very slow
     if file_uri and "../" in file_uri:
@@ -66,9 +34,7 @@ def register(
     # from herping a derp and treating the uri as an absolute path
     root_path = file_path = file_or_directory
     if file_uri:
-            file_path = path.join(
-                file_or_directory, sub("^[/]*", "", file_uri)
-            )
+        file_path = path.join(file_or_directory, sub("^[/]*", "", file_uri))
 
     # URL decode the path sent by the browser otherwise we won't be able to
     # match filenames which got encoded (filenames with spaces etc)
@@ -132,10 +98,63 @@ def register(
             "File not found", path=file_or_directory, relative_url=file_uri
         )
 
+
+def register(
+    app,
+    uri,
+    file_or_directory,
+    pattern,
+    use_modified_since,
+    use_content_range,
+    stream_large_files,
+    name="static",
+    host=None,
+    strict_slashes=None,
+    content_type=None,
+):
+    # TODO: Though sanic is not a file server, I feel like we should at least
+    #       make a good effort here.  Modified-since is nice, but we could
+    #       also look into etags, expires, and caching
+    """
+    Register a static directory handler with Sanic by adding a route to the
+    router and registering a handler.
+
+    :param app: Sanic
+    :param file_or_directory: File or directory path to serve from
+    :param uri: URL to serve from
+    :param pattern: regular expression used to match files in the URL
+    :param use_modified_since: If true, send file modified time, and return
+                               not modified if the browser's matches the
+                               server's
+    :param use_content_range: If true, process header for range requests
+                              and sends the file part that is requested
+    :param stream_large_files: If true, use the file_stream() handler rather
+                              than the file() handler to send the file
+                              If this is an integer, this represents the
+                              threshold size to switch to file_stream()
+    :param name: user defined name used for url_for
+    :param content_type: user defined content type for header
+    """
+    # If we're not trying to match a file directly,
+    # serve from the folder
+    if not path.isfile(file_or_directory):
+        uri += "<file_uri:" + pattern + ">"
+
     # special prefix for static files
     if not name.startswith("_static_"):
         name = f"_static_{name}"
 
+    _handler = wraps(_static_request_handler)(
+        partial(
+            _static_request_handler,
+            file_or_directory,
+            use_modified_since,
+            use_content_range,
+            stream_large_files,
+            content_type=content_type,
+        )
+    )
+
     app.route(
         uri,
         methods=["GET", "HEAD"],

tests/test_multiprocessing.py

@@ -87,3 +87,14 @@ def test_pickle_app_with_bp(app, protocol):
     request, response = up_p_app.test_client.get("/")
     assert up_p_app.is_request_stream is False
     assert response.text == "Hello"
+
+@pytest.mark.parametrize("protocol", [3, 4])
+def test_pickle_app_with_static(app, protocol):
+    app.route("/")(handler)
+    app.static('/static', "/tmp/static")
+    p_app = pickle.dumps(app, protocol=protocol)
+    del app
+    up_p_app = pickle.loads(p_app)
+    assert up_p_app
+    request, response = up_p_app.test_client.get("/static/missing.txt")
+    assert response.status == 404