Issue #9854: The default read() implementation in io.RawIOBase now

handles non-blocking readinto() returning None correctly.

Doc/library/io.rst

@@ -361,8 +361,9 @@ I/O Base Classes
 
    .. method:: readinto(b)
 
-      Read up to len(b) bytes into bytearray *b* and return the number of bytes
-      read.
+      Read up to len(b) bytes into bytearray *b* and return the number ofbytes
+      read.  If the object is in non-blocking mode and no bytes are available,
+      ``None`` is returned.
 
    .. method:: write(b)
 

Lib/_pyio.py

@@ -544,6 +544,8 @@ class RawIOBase(IOBase):
             return self.readall()
         b = bytearray(n.__index__())
         n = self.readinto(b)
+        if n is None:
+            return None
         del b[n:]
         return bytes(b)
 
@@ -561,7 +563,7 @@ class RawIOBase(IOBase):
         """Read up to len(b) bytes into b.
 
         Returns number of bytes read (0 for EOF), or None if the object
-        is set not to block as has no data to read.
+        is set not to block and has no data to read.
         """
         self._unsupported("readinto")
 

Lib/test/test_io.py

@@ -48,7 +48,9 @@ def _default_chunk_size():
         return f._CHUNK_SIZE
 
 
-class MockRawIO:
+class MockRawIOWithoutRead:
+    """A RawIO implementation without read(), so as to exercise the default
+    RawIO.read() which calls readinto()."""
 
     def __init__(self, read_stack=()):
         self._read_stack = list(read_stack)
@@ -56,14 +58,6 @@ class MockRawIO:
         self._reads = 0
         self._extraneous_reads = 0
 
-    def read(self, n=None):
-        self._reads += 1
-        try:
-            return self._read_stack.pop(0)
-        except:
-            self._extraneous_reads += 1
-            return b""
-
     def write(self, b):
         self._write_stack.append(bytes(b))
         return len(b)
@@ -110,6 +104,23 @@ class MockRawIO:
     def truncate(self, pos=None):
         return pos
 
+class CMockRawIOWithoutRead(MockRawIOWithoutRead, io.RawIOBase):
+    pass
+
+class PyMockRawIOWithoutRead(MockRawIOWithoutRead, pyio.RawIOBase):
+    pass
+
+
+class MockRawIO(MockRawIOWithoutRead):
+
+    def read(self, n=None):
+        self._reads += 1
+        try:
+            return self._read_stack.pop(0)
+        except:
+            self._extraneous_reads += 1
+            return b""
+
 class CMockRawIO(MockRawIO, io.RawIOBase):
     pass
 
@@ -582,6 +593,19 @@ class IOTest(unittest.TestCase):
         f.close()
         self.assertRaises(ValueError, f.flush)
 
+    def test_RawIOBase_read(self):
+        # Exercise the default RawIOBase.read() implementation (which calls
+        # readinto() internally).
+        rawio = self.MockRawIOWithoutRead((b"abc", b"d", None, b"efg", None))
+        self.assertEqual(rawio.read(2), b"ab")
+        self.assertEqual(rawio.read(2), b"c")
+        self.assertEqual(rawio.read(2), b"d")
+        self.assertEqual(rawio.read(2), None)
+        self.assertEqual(rawio.read(2), b"ef")
+        self.assertEqual(rawio.read(2), b"g")
+        self.assertEqual(rawio.read(2), None)
+        self.assertEqual(rawio.read(2), b"")
+
 class CIOTest(IOTest):
     pass
 
@@ -2590,7 +2614,7 @@ def test_main():
     # Put the namespaces of the IO module we are testing and some useful mock
     # classes in the __dict__ of each test.
     mocks = (MockRawIO, MisbehavedRawIO, MockFileIO, CloseFailureIO,
-             MockNonBlockWriterIO, MockUnseekableIO)
+             MockNonBlockWriterIO, MockUnseekableIO, MockRawIOWithoutRead)
     all_members = io.__all__ + ["IncrementalNewlineDecoder"]
     c_io_ns = {name : getattr(io, name) for name in all_members}
     py_io_ns = {name : getattr(pyio, name) for name in all_members}

Misc/NEWS

@@ -52,6 +52,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #9854: The default read() implementation in io.RawIOBase now
+  handles non-blocking readinto() returning None correctly.
+
 - Issue #1552: socket.socketpair() now returns regular socket.socket
   objects supporting the whole socket API (rather than the "raw"
   _socket.socket objects).

Modules/_io/iobase.c

@@ -777,9 +777,9 @@ rawiobase_read(PyObject *self, PyObject *args)
         return NULL;
 
     res = PyObject_CallMethodObjArgs(self, _PyIO_str_readinto, b, NULL);
-    if (res == NULL) {
+    if (res == NULL || res == Py_None) {
         Py_DECREF(b);
-        return NULL;
+        return res;
     }
 
     n = PyNumber_AsSsize_t(res, PyExc_ValueError);