enable HTTP/1.1 when server has multiple workers

HTTP/1.1 with the base single worker server seems to be unreliable,
sometimes simultaneous requests never complete. When the server is
multithread or multiprocess, and the handler doesn't directly set a
protocol version, enable HTTP/1.1. This enables keep-alive
connections and chunked streaming responses.

CHANGES.rst

@@ -71,6 +71,9 @@ Unreleased
 -   The development server uses ``Transfer-Encoding: chunked`` for
     streaming responses when it is configured for HTTP/1.1.
     :issue:`2090, 1327`, :pr:`2091`
+-   The development server uses HTTP/1.1, which enables keep-alive
+    connections and chunked streaming responses, when ``threaded`` or
+    ``processes`` is enabled. :pr:`2323`
 
 
 Version 2.0.3

src/werkzeug/serving.py

@@ -654,6 +654,14 @@ def __init__(
         if handler is None:
             handler = WSGIRequestHandler
 
+        # If the handler doesn't directly set a protocol version and
+        # thread or process workers are used, then allow chunked
+        # responses and keep-alive connections by enabling HTTP/1.1.
+        if "protocol_version" not in vars(handler) and (
+            self.multithread or self.multiprocess
+        ):
+            handler.protocol_version = "HTTP/1.1"
+
         self.host = host
         self.port = port
         self.app = app

tests/live_apps/run.py

@@ -4,15 +4,9 @@
 
 from werkzeug.serving import generate_adhoc_ssl_context
 from werkzeug.serving import run_simple
-from werkzeug.serving import WSGIRequestHandler
 from werkzeug.wrappers import Request
 from werkzeug.wrappers import Response
 
-
-class WSGI11RequestHandler(WSGIRequestHandler):
-    protocol_version = "HTTP/1.1"
-
-
 name = sys.argv[1]
 mod = import_module(f"{name}_app")
 
@@ -30,9 +24,6 @@ def app(request):
 kwargs.update(json.loads(sys.argv[2]))
 ssl_context = kwargs.get("ssl_context")
 
-if kwargs.get("request_handler") == "HTTP/1.1":
-    kwargs["request_handler"] = WSGI11RequestHandler
-
 if ssl_context == "custom":
     kwargs["ssl_context"] = generate_adhoc_ssl_context()
 elif isinstance(ssl_context, list):

tests/test_serving.py

@@ -259,7 +259,7 @@ def test_streaming_chunked_response(dev_server):
 
     https://tools.ietf.org/html/rfc2616#section-3.6.1
     """
-    r = dev_server("streaming", request_handler="HTTP/1.1").request("/")
+    r = dev_server("streaming", threaded=True).request("/")
     assert r.getheader("transfer-encoding") == "chunked"
     assert r.data == "".join(str(x) + "\n" for x in range(5)).encode()
 
@@ -270,4 +270,4 @@ def test_streaming_chunked_truncation(dev_server):
     content truncated by a prematurely closed connection.
     """
     with pytest.raises(http.client.IncompleteRead):
-        dev_server("streaming", request_handler="HTTP/1.1").request("/crash")
+        dev_server("streaming", threaded=True).request("/crash")