bpo-31976: Fix race condition when flushing a file is slow. (#4331)

Lib/_pyio.py

@@ -1188,11 +1188,11 @@ class BufferedWriter(_BufferedIOMixin):
         return self.raw.writable()
 
     def write(self, b):
-        if self.closed:
-            raise ValueError("write to closed file")
         if isinstance(b, str):
             raise TypeError("can't write str to binary stream")
         with self._write_lock:
+            if self.closed:
+                raise ValueError("write to closed file")
             # XXX we can implement some more tricks to try and avoid
             # partial writes
             if len(self._write_buf) > self.buffer_size:
@@ -1253,6 +1253,21 @@ class BufferedWriter(_BufferedIOMixin):
             self._flush_unlocked()
             return _BufferedIOMixin.seek(self, pos, whence)
 
+    def close(self):
+        with self._write_lock:
+            if self.raw is None or self.closed:
+                return
+        # We have to release the lock and call self.flush() (which will
+        # probably just re-take the lock) in case flush has been overridden in
+        # a subclass or the user set self.flush to something. This is the same
+        # behavior as the C implementation.
+        try:
+            # may raise BlockingIOError or BrokenPipeError etc
+            self.flush()
+        finally:
+            with self._write_lock:
+                self.raw.close()
+
 
 class BufferedRWPair(BufferedIOBase):