bpo-35950: Raise UnsupportedOperation in BufferedReader.truncate()

Lib/_pyio.py

@@ -792,6 +792,9 @@ def tell(self):
         return pos
 
     def truncate(self, pos=None):
+        self._checkClosed()
+        self._checkWritable()
+
         # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
         # and a flush may be necessary to synch both views of the current
         # file state.

Lib/test/test_io.py

@@ -1528,6 +1528,13 @@ def test_read_on_closed(self):
         self.assertRaises(ValueError, b.peek)
         self.assertRaises(ValueError, b.read1, 1)
 
+    def test_truncate_on_read_only(self):
+        rawio = self.MockFileIO(b"abc")
+        bufio = self.tp(rawio)
+        self.assertFalse(bufio.writable())
+        self.assertRaises(self.UnsupportedOperation, bufio.truncate)
+        self.assertRaises(self.UnsupportedOperation, bufio.truncate, 0)
+
 
 class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     tp = io.BufferedReader
@@ -2372,6 +2379,10 @@ def test_interleaved_readline_write(self):
     # You can't construct a BufferedRandom over a non-seekable stream.
     test_unseekable = None
 
+    # writable() returns True, so there's no point to test it over
+    # a writable stream.
+    test_truncate_on_read_only = None
+
 
 class CBufferedRandomTest(BufferedRandomTest, SizeofTest):
     tp = io.BufferedRandom

Misc/NEWS.d/next/Library/2020-02-21-02-42-41.bpo-35950.9G3-wl.rst

@@ -0,0 +1,2 @@
+Raise :exc:`io.UnsupportedOperation` in :meth:`io.BufferedReader.truncate`
+when it is called on a read-only :class:`io.BufferedReader` instance.

Modules/_io/bufferedio.c

@@ -1315,15 +1315,19 @@ _io__Buffered_truncate_impl(buffered *self, PyObject *pos)
     PyObject *res = NULL;
 
     CHECK_INITIALIZED(self)
+    CHECK_CLOSED(self, "truncate of closed file")
+    if (!self->writable) {
+        return bufferediobase_unsupported("truncate");
+    }
     if (!ENTER_BUFFERED(self))
         return NULL;
 
-    if (self->writable) {
-        res = buffered_flush_and_rewind_unlocked(self);
-        if (res == NULL)
-            goto end;
-        Py_CLEAR(res);
+    res = buffered_flush_and_rewind_unlocked(self);
+    if (res == NULL) {
+        goto end;
     }
+    Py_CLEAR(res);
+
     res = PyObject_CallMethodOneArg(self->raw, _PyIO_str_truncate, pos);
     if (res == NULL)
         goto end;