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/test/test_io.py

@@ -42,7 +42,10 @@ try:
     import threading
 except ImportError:
     threading = None
-
+try:
+    import fcntl
+except ImportError:
+    fcntl = None
 
 def _default_chunk_size():
     """Get the default TextIOWrapper chunk size"""
@@ -242,9 +245,14 @@ class MockNonBlockWriterIO:
             except ValueError:
                 pass
             else:
-                self._blocker_char = None
-                self._write_stack.append(b[:n])
-                raise self.BlockingIOError(0, "test blocking", n)
+                if n > 0:
+                    # write data up to the first blocker
+                    self._write_stack.append(b[:n])
+                    return n
+                else:
+                    # cancel blocker and indicate would block
+                    self._blocker_char = None
+                    return None
         self._write_stack.append(b)
         return len(b)
 
@@ -2753,6 +2761,70 @@ class MiscIOTest(unittest.TestCase):
                 with self.open(support.TESTFN, **kwargs) as f:
                     self.assertRaises(TypeError, pickle.dumps, f, protocol)
 
+    @unittest.skipUnless(fcntl, 'fcntl required for this test')
+    def test_nonblock_pipe_write_bigbuf(self):
+        self._test_nonblock_pipe_write(16*1024)
+
+    @unittest.skipUnless(fcntl, 'fcntl required for this test')
+    def test_nonblock_pipe_write_smallbuf(self):
+        self._test_nonblock_pipe_write(1024)
+
+    def _set_non_blocking(self, fd):
+        flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+        self.assertNotEqual(flags, -1)
+        res = fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
+        self.assertEqual(res, 0)
+
+    def _test_nonblock_pipe_write(self, bufsize):
+        sent = []
+        received = []
+        r, w = os.pipe()
+        self._set_non_blocking(r)
+        self._set_non_blocking(w)
+
+        # To exercise all code paths in the C implementation we need
+        # to play with buffer sizes.  For instance, if we choose a
+        # buffer size less than or equal to _PIPE_BUF (4096 on Linux)
+        # then we will never get a partial write of the buffer.
+        rf = self.open(r, mode='rb', closefd=True, buffering=bufsize)
+        wf = self.open(w, mode='wb', closefd=True, buffering=bufsize)
+
+        with rf, wf:
+            for N in 9999, 73, 7574:
+                try:
+                    i = 0
+                    while True:
+                        msg = bytes([i % 26 + 97]) * N
+                        sent.append(msg)
+                        wf.write(msg)
+                        i += 1
+
+                except self.BlockingIOError as e:
+                    self.assertEqual(e.args[0], errno.EAGAIN)
+                    self.assertEqual(e.args[2], e.characters_written)
+                    sent[-1] = sent[-1][:e.characters_written]
+                    received.append(rf.read())
+                    msg = b'BLOCKED'
+                    wf.write(msg)
+                    sent.append(msg)
+
+            while True:
+                try:
+                    wf.flush()
+                    break
+                except self.BlockingIOError as e:
+                    self.assertEqual(e.args[0], errno.EAGAIN)
+                    self.assertEqual(e.args[2], e.characters_written)
+                    self.assertEqual(e.characters_written, 0)
+                    received.append(rf.read())
+
+            received += iter(rf.read, None)
+
+        sent, received = b''.join(sent), b''.join(received)
+        self.assertTrue(sent == received)
+        self.assertTrue(wf.closed)
+        self.assertTrue(rf.closed)
+
 class CMiscIOTest(MiscIOTest):
     io = io