Fix Buffered.read().

Author: serhiy-storchaka

Lib/test/test_io/test_bufferedio.py

@@ -439,6 +439,13 @@ def test_large_read_from_small_file(self):
             bufio = self.tp(rawio)
             self.assertEqual(bufio.read(size), b'abc')
 
+    def test_large_read_from_large_file(self):
+        data = b'abc' * ((5 << 20) + 54321)
+        for size in (len(data), sys.maxsize):
+            rawio = self.MockFileIO(data)
+            bufio = self.tp(rawio)
+            self.assertEqual(bufio.read(size), data)
+
     def test_large_read1_from_small_file(self):
         for size in support.itersize(1 << 20, sys.maxsize):
             rawio = self.MockRawIO((b'abc',))

Modules/_io/bufferedio.c

@@ -1849,6 +1849,7 @@ _bufferedreader_read_generic(buffered *self, Py_ssize_t size)
                 PyBytesWriter_Discard(writer);
                 goto error;
             }
+            out = PyBytesWriter_GetData(writer);
         }
         Py_ssize_t r = MINUS_LAST_BLOCK(self, allocated - written);
         if (r == 0) {
@@ -1877,6 +1878,7 @@ _bufferedreader_read_generic(buffered *self, Py_ssize_t size)
             PyBytesWriter_Discard(writer);
             goto error;
         }
+        out = PyBytesWriter_GetData(writer);
     }
     /* NOTE: when the read is satisfied, we avoid issuing any additional
        reads, which could block indefinitely (e.g. on a socket).