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/wsgiref/simple_server.py

@@ -11,6 +11,7 @@ module.  See also the BaseHTTPServer module docs for other API information.
 """
 
 from http.server import BaseHTTPRequestHandler, HTTPServer
+from io import BufferedWriter
 import sys
 import urllib.parse
 from wsgiref.handlers import SimpleHandler
@@ -126,11 +127,17 @@ class WSGIRequestHandler(BaseHTTPRequestHandler):
         if not self.parse_request(): # An error code has been sent, just exit
             return
 
-        handler = ServerHandler(
-            self.rfile, self.wfile, self.get_stderr(), self.get_environ()
-        )
-        handler.request_handler = self      # backpointer for logging
-        handler.run(self.server.get_app())
+        # Avoid passing the raw file object wfile, which can do partial
+        # writes (Issue 24291)
+        stdout = BufferedWriter(self.wfile)
+        try:
+            handler = ServerHandler(
+                self.rfile, stdout, self.get_stderr(), self.get_environ()
+            )
+            handler.request_handler = self      # backpointer for logging
+            handler.run(self.server.get_app())
+        finally:
+            stdout.detach()