gh-109523: Reading text from a non-blocking stream with read may now raise a BlockingIOError if the operation cannot immediately return bytes.

Lib/test/test_io.py

@@ -3919,6 +3919,22 @@ def test_issue35928(self):
         f.write(res)
         self.assertEqual(res + f.readline(), 'foo\nbar\n')
 
+    @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
+    def test_read_non_blocking(self):
+        import os
+        r, w = os.pipe()
+        try:
+            os.set_blocking(r, False)
+            with open(r, 'r') as textfile:
+                r = None
+                # Nothing has been written so a non-blocking read raises a BlockingIOError exception.
+                with self.assertRaises(BlockingIOError):
+                    textfile.read()
+        finally:
+            if r is not None:
+                os.close(r)
+            os.close(w)
+
 
 class MemviewBytesIO(io.BytesIO):
     '''A BytesIO object whose read method returns memoryviews

Misc/NEWS.d/next/C_API/2024-08-12-10-15-19.gh-issue-109523.S2c3fi.rst

@@ -0,0 +1 @@
+In _io_TextIOWrapper_read_impl, raise BlockingIOError when read() from a non-blocking pipe does not return bytes.

Modules/_io/textio.c

@@ -1992,6 +1992,12 @@ _io_TextIOWrapper_read_impl(textio *self, Py_ssize_t n)
         if (bytes == NULL)
             goto fail;
 
+        if (bytes == Py_None){
+            PyErr_SetString(PyExc_BlockingIOError, "read() should return bytes");
+            return NULL;
+
+        }
+
         _PyIO_State *state = self->state;
         if (Py_IS_TYPE(self->decoder, state->PyIncrementalNewlineDecoder_Type))
             decoded = _PyIncrementalNewlineDecoder_decode(self->decoder,