Issue #24291: Avoid WSGIRequestHandler doing partial writes

If the underlying send() method indicates a partial write, such as when the
call is interrupted to handle a signal, the server would silently drop the
remaining data.

Also add deprecated support for SimpleHandler.stdout.write() doing partial
writes.

Lib/test/test_wsgiref.py

@@ -1,18 +1,22 @@
 from unittest import mock
+from test import support
+from test.test_httpservers import NoLogRequestHandler
 from unittest import TestCase
 from wsgiref.util import setup_testing_defaults
 from wsgiref.headers import Headers
-from wsgiref.handlers import BaseHandler, BaseCGIHandler
+from wsgiref.handlers import BaseHandler, BaseCGIHandler, SimpleHandler
 from wsgiref import util
 from wsgiref.validate import validator
 from wsgiref.simple_server import WSGIServer, WSGIRequestHandler
 from wsgiref.simple_server import make_server
+from http.client import HTTPConnection
 from io import StringIO, BytesIO, BufferedReader
 from socketserver import BaseServer
 from platform import python_implementation
 
 import os
 import re
+import signal
 import sys
 import unittest
 
@@ -245,6 +249,56 @@ class IntegrationTests(TestCase):
             ],
             out.splitlines())
 
+    def test_interrupted_write(self):
+        # BaseHandler._write() and _flush() have to write all data, even if
+        # it takes multiple send() calls.  Test this by interrupting a send()
+        # call with a Unix signal.
+        threading = support.import_module("threading")
+        pthread_kill = support.get_attribute(signal, "pthread_kill")
+
+        def app(environ, start_response):
+            start_response("200 OK", [])
+            return [bytes(support.SOCK_MAX_SIZE)]
+
+        class WsgiHandler(NoLogRequestHandler, WSGIRequestHandler):
+            pass
+
+        server = make_server(support.HOST, 0, app, handler_class=WsgiHandler)
+        self.addCleanup(server.server_close)
+        interrupted = threading.Event()
+
+        def signal_handler(signum, frame):
+            interrupted.set()
+
+        original = signal.signal(signal.SIGUSR1, signal_handler)
+        self.addCleanup(signal.signal, signal.SIGUSR1, original)
+        received = None
+        main_thread = threading.get_ident()
+
+        def run_client():
+            http = HTTPConnection(*server.server_address)
+            http.request("GET", "/")
+            with http.getresponse() as response:
+                response.read(100)
+                # The main thread should now be blocking in a send() system
+                # call.  But in theory, it could get interrupted by other
+                # signals, and then retried.  So keep sending the signal in a
+                # loop, in case an earlier signal happens to be delivered at
+                # an inconvenient moment.
+                while True:
+                    pthread_kill(main_thread, signal.SIGUSR1)
+                    if interrupted.wait(timeout=float(1)):
+                        break
+                nonlocal received
+                received = len(response.read())
+            http.close()
+
+        background = threading.Thread(target=run_client)
+        background.start()
+        server.handle_request()
+        background.join()
+        self.assertEqual(received, support.SOCK_MAX_SIZE - 100)
+
 
 class UtilityTests(TestCase):
 
@@ -701,6 +755,31 @@ class HandlerTests(TestCase):
         h.run(error_app)
         self.assertEqual(side_effects['close_called'], True)
 
+    def testPartialWrite(self):
+        written = bytearray()
+
+        class PartialWriter:
+            def write(self, b):
+                partial = b[:7]
+                written.extend(partial)
+                return len(partial)
+
+            def flush(self):
+                pass
+
+        environ = {"SERVER_PROTOCOL": "HTTP/1.0"}
+        h = SimpleHandler(BytesIO(), PartialWriter(), sys.stderr, environ)
+        msg = "should not do partial writes"
+        with self.assertWarnsRegex(DeprecationWarning, msg):
+            h.run(hello_app)
+        self.assertEqual(b"HTTP/1.0 200 OK\r\n"
+            b"Content-Type: text/plain\r\n"
+            b"Date: Mon, 05 Jun 2006 18:49:54 GMT\r\n"
+            b"Content-Length: 13\r\n"
+            b"\r\n"
+            b"Hello, world!",
+            written)
+
 
 if __name__ == "__main__":
     unittest.main()