bpo-17852: Revert incorrect fix based on misunderstanding of _Py_PyAt…

…Exit() semantics (python#4826)

Lib/_pyio.py

@@ -1182,7 +1182,6 @@ def __init__(self, raw, buffer_size=DEFAULT_BUFFER_SIZE):
         self.buffer_size = buffer_size
         self._write_buf = bytearray()
         self._write_lock = Lock()
-        _register_writer(self)
 
     def writable(self):
         return self.raw.writable()
@@ -2587,26 +2586,3 @@ def encoding(self):
     def detach(self):
         # This doesn't make sense on StringIO.
         self._unsupported("detach")
-
-
-# ____________________________________________________________
-
-import atexit, weakref
-
-_all_writers = weakref.WeakSet()
-
-def _register_writer(w):
-    # keep weak-ref to buffered writer
-    _all_writers.add(w)
-
-def _flush_all_writers():
-    # Ensure all buffered writers are flushed before proceeding with
-    # normal shutdown.  Otherwise, if the underlying file objects get
-    # finalized before the buffered writer wrapping it then any buffered
-    # data will be lost.
-    for w in _all_writers:
-        try:
-            w.flush()
-        except:
-            pass
-atexit.register(_flush_all_writers)

Misc/NEWS.d/next/Library/2017-12-13-00-00-37.bpo-17852.Q8BP8N.rst

@@ -0,0 +1 @@
+Revert incorrect fix based on misunderstanding of _Py_PyAtExit() semantics.

Modules/_io/_iomodule.c

@@ -766,8 +766,6 @@ PyInit__io(void)
         !(_PyIO_empty_bytes = PyBytes_FromStringAndSize(NULL, 0)))
         goto fail;
 
-    _Py_PyAtExit(_PyIO_atexit_flush);
-
     state->initialized = 1;
 
     return m;

Modules/_io/_iomodule.h

@@ -183,5 +183,3 @@ extern PyObject *_PyIO_empty_str;
 extern PyObject *_PyIO_empty_bytes;
 
 extern PyTypeObject _PyBytesIOBuffer_Type;
-
-extern void _PyIO_atexit_flush(void);

Modules/_io/bufferedio.c

@@ -197,7 +197,7 @@ bufferediobase_write(PyObject *self, PyObject *args)
 }
 
 
-typedef struct _buffered {
+typedef struct {
     PyObject_HEAD
 
     PyObject *raw;
@@ -239,18 +239,8 @@ typedef struct _buffered {
 
     PyObject *dict;
     PyObject *weakreflist;
-
-    /* a doubly-linked chained list of "buffered" objects that need to
-       be flushed when the process exits */
-    struct _buffered *next, *prev;
 } buffered;
 
-/* the actual list of buffered objects */
-static buffered buffer_list_end = {
-    .next = &buffer_list_end,
-    .prev = &buffer_list_end
-};
-
 /*
     Implementation notes:
 
@@ -389,21 +379,10 @@ _enter_buffered_busy(buffered *self)
         (self->buffer_size * (size / self->buffer_size)))
 
 
-static void
-remove_from_linked_list(buffered *self)
-{
-    self->next->prev = self->prev;
-    self->prev->next = self->next;
-    self->prev = NULL;
-    self->next = NULL;
-}
-
 static void
 buffered_dealloc(buffered *self)
 {
     self->finalizing = 1;
-    if (self->next != NULL)
-        remove_from_linked_list(self);
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
@@ -1827,38 +1806,10 @@ _io_BufferedWriter___init___impl(buffered *self, PyObject *raw,
     self->fast_closed_checks = (Py_TYPE(self) == &PyBufferedWriter_Type &&
                                 Py_TYPE(raw) == &PyFileIO_Type);
 
-    if (self->next == NULL) {
-        self->prev = &buffer_list_end;
-        self->next = buffer_list_end.next;
-        buffer_list_end.next->prev = self;
-        buffer_list_end.next = self;
-    }
-
     self->ok = 1;
     return 0;
 }
 
-/*
-* Ensure all buffered writers are flushed before proceeding with
-* normal shutdown.  Otherwise, if the underlying file objects get
-* finalized before the buffered writer wrapping it then any buffered
-* data will be lost.
-*/
-void _PyIO_atexit_flush(void)
-{
-    while (buffer_list_end.next != &buffer_list_end) {
-        buffered *buf = buffer_list_end.next;
-        remove_from_linked_list(buf);
-        if (buf->ok && !buf->finalizing) {
-            /* good state and not finalizing */
-            Py_INCREF(buf);
-            buffered_flush(buf, NULL);
-            Py_DECREF(buf);
-            PyErr_Clear();
-        }
-    }
-}
-
 static Py_ssize_t
 _bufferedwriter_raw_write(buffered *self, char *start, Py_ssize_t len)
 {