wip: add defer_close logic to FileIO

If the file is wrapped by a write buffer, set defer_close to true.  That
ensures the underlying FileIO object is not closed until the buffer is
closed (and can flush the unwritten data).  This avoids data loss, as
demoed by buffer_not_flushed.py.

Author: nascheme

Modules/_io/bufferedio.c

@@ -545,6 +545,8 @@ _io__Buffered_closed_get_impl(buffered *self)
     return PyObject_GetAttr(self->raw, &_Py_ID(closed));
 }
 
+void _PyFileIO_set_defer_close(PyObject *self, bool defer_close);
+
 /*[clinic input]
 @critical_section
 _io._Buffered.close
@@ -588,6 +590,10 @@ _io__Buffered_close_impl(buffered *self)
         exc = PyErr_GetRaisedException();
     }
 
+    if (self->fast_closed_checks) {
+        _PyFileIO_set_defer_close(self->raw, false);
+    }
+
     res = PyObject_CallMethodNoArgs(self->raw, &_Py_ID(close));
 
     if (self->buffer) {
@@ -1972,6 +1978,9 @@ _io_BufferedWriter___init___impl(buffered *self, PyObject *raw,
         Py_IS_TYPE(self, state->PyBufferedWriter_Type) &&
         Py_IS_TYPE(raw, state->PyFileIO_Type)
     );
+    if (self->fast_closed_checks) {
+        _PyFileIO_set_defer_close(self->raw, true);
+    }
 
     self->ok = 1;
     return 0;

Modules/_io/fileio.c

@@ -82,6 +82,8 @@ typedef struct {
     struct _Py_stat_struct *stat_atopen;
     PyObject *weakreflist;
     PyObject *dict;
+    /* if true, we are wrapped by buffer, don't close until buffer is closed */
+    bool defer_close;
 } fileio;
 
 #define PyFileIO_Check(state, op) (PyObject_TypeCheck((op), state->PyFileIO_Type))
@@ -96,6 +98,13 @@ _PyFileIO_closed(PyObject *self)
     return (PyFileIO_CAST(self)->fd < 0);
 }
 
+void
+_PyFileIO_set_defer_close(PyObject *self, bool defer_close)
+{
+    PyFileIO_CAST(self)->defer_close = defer_close;
+}
+
+
 /* Because this can call arbitrary code, it shouldn't be called when
    the refcount is 0 (that is, not directly from tp_dealloc unless
    the refcount has been temporarily re-incremented). */
@@ -128,6 +137,9 @@ _io_FileIO__dealloc_warn_impl(fileio *self, PyObject *source)
 static int
 internal_close(fileio *self)
 {
+    if (self->defer_close) {
+        return 0;
+    }
     int err = 0;
     int save_errno = 0;
     if (self->fd >= 0) {
@@ -221,6 +233,7 @@ fileio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     self->stat_atopen = NULL;
     self->closefd = 1;
     self->weakreflist = NULL;
+    self->defer_close = false;
     return (PyObject *) self;
 }