Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is

raised when the wrapped raw file is non-blocking and the write would block.
Previous code assumed that the raw write() would raise BlockingIOError, but
RawIOBase.write() is defined to returned None when the call would block.
Patch by sbt.

Lib/_pyio.py

@@ -6,6 +6,7 @@ import os
 import abc
 import codecs
 import warnings
+import errno
 # Import _thread instead of threading to reduce startup cost
 try:
     from _thread import allocate_lock as Lock
@@ -720,8 +721,11 @@ class _BufferedIOMixin(BufferedIOBase):
 
     def close(self):
         if self.raw is not None and not self.closed:
-            self.flush()
-            self.raw.close()
+            try:
+                # may raise BlockingIOError or BrokenPipeError etc
+                self.flush()
+            finally:
+                self.raw.close()
 
     def detach(self):
         if self.raw is None:
@@ -1080,13 +1084,9 @@ class BufferedWriter(_BufferedIOMixin):
             # XXX we can implement some more tricks to try and avoid
             # partial writes
             if len(self._write_buf) > self.buffer_size:
-                # We're full, so let's pre-flush the buffer
-                try:
-                    self._flush_unlocked()
-                except BlockingIOError as e:
-                    # We can't accept anything else.
-                    # XXX Why not just let the exception pass through?
-                    raise BlockingIOError(e.errno, e.strerror, 0)
+                # We're full, so let's pre-flush the buffer.  (This may
+                # raise BlockingIOError with characters_written == 0.)
+                self._flush_unlocked()
             before = len(self._write_buf)
             self._write_buf.extend(b)
             written = len(self._write_buf) - before
@@ -1117,24 +1117,23 @@ class BufferedWriter(_BufferedIOMixin):
     def _flush_unlocked(self):
         if self.closed:
             raise ValueError("flush of closed file")
-        written = 0
-        try:
-            while self._write_buf:
-                try:
-                    n = self.raw.write(self._write_buf)
-                except IOError as e:
-                    if e.errno != EINTR:
-                        raise
-                    continue
-                if n > len(self._write_buf) or n < 0:
-                    raise IOError("write() returned incorrect number of bytes")
-                del self._write_buf[:n]
-                written += n
-        except BlockingIOError as e:
-            n = e.characters_written
+        while self._write_buf:
+            try:
+                n = self.raw.write(self._write_buf)
+            except BlockingIOError:
+                raise RuntimeError("self.raw should implement RawIOBase: it "
+                                   "should not raise BlockingIOError")
+            except IOError as e:
+                if e.errno != EINTR:
+                    raise
+                continue
+            if n is None:
+                raise BlockingIOError(
+                    errno.EAGAIN,
+                    "write could not complete without blocking", 0)
+            if n > len(self._write_buf) or n < 0:
+                raise IOError("write() returned incorrect number of bytes")
             del self._write_buf[:n]
-            written += n
-            raise BlockingIOError(e.errno, e.strerror, written)
 
     def tell(self):
         return _BufferedIOMixin.tell(self) + len(self._write_buf)