Preserve write buffer when raw.write() returns None on blocked socket

Add a check in cheroot.makefile.BufferedWriter._flush_unlocked to prevent
clearing of the write buffer when raw.write() returns None due to a blocked
stream. Also limits each write call to SOCK_WRITE_BLOCKSIZE bytes, which
was defined but not previously used in this method.

Author: cbbm142

cheroot/makefile.py

@@ -30,7 +30,9 @@ def _flush_unlocked(self):
             try:
                 # ssl sockets only except 'bytes', not bytearrays
                 # so perhaps we should conditionally wrap this for perf?
-                n = self.raw.write(bytes(self._write_buf[:SOCK_WRITE_BLOCKSIZE]))
+                n = self.raw.write(
+                    bytes(self._write_buf[:SOCK_WRITE_BLOCKSIZE]),
+                )
             except io.BlockingIOError as e:
                 n = e.characters_written
             if n:

cheroot/test/test_makefile.py

@@ -74,7 +74,9 @@ def mock_raw_write(b):
         nonlocal call_count
         call_count += 1
         if call_count == 1:
-            return None  # simulates socket returning None on first blocked write
+            return (
+                None  # simulates socket returning None on first blocked write
+            )
         written.extend(b)
         return len(b)