pythongh-90102: Shortcut isatty in _io open()

cmaloney · Sep 10, 2024 · be84f90 · be84f90
1 parent 1a16077
commit be84f90
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Showing 6 changed files with 26 additions and 4 deletions.
diff --git a/Include/internal/pycore_global_objects_fini_generated.h b/Include/internal/pycore_global_objects_fini_generated.h
diff --git a/Include/internal/pycore_global_strings.h b/Include/internal/pycore_global_strings.h
@@ -248,6 +248,7 @@ struct _Py_global_strings {
  STRUCT_FOR_ID(_initializing)
  STRUCT_FOR_ID(_io)
  STRUCT_FOR_ID(_is_text_encoding)
+ STRUCT_FOR_ID(_isatty_openonly)
  STRUCT_FOR_ID(_layout_)
  STRUCT_FOR_ID(_length_)
  STRUCT_FOR_ID(_limbo)

diff --git a/Include/internal/pycore_runtime_init_generated.h b/Include/internal/pycore_runtime_init_generated.h
diff --git a/Include/internal/pycore_unicodeobject_generated.h b/Include/internal/pycore_unicodeobject_generated.h
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
@@ -346,7 +346,8 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
 
  /* buffering */
  if (buffering < 0) {
- PyObject *res = PyObject_CallMethodNoArgs(raw, &_Py_ID(isatty));
+ PyObject *res = PyObject_CallMethodNoArgs(raw,
+ &_Py_ID(_isatty_openonly));
  if (res == NULL)
  goto error;
  isatty = PyObject_IsTrue(res);

diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
@@ -12,9 +12,6 @@
 #ifdef HAVE_SYS_TYPES_H
 # include <sys/types.h>
 #endif
-#ifdef HAVE_SYS_STAT_H
-# include <sys/stat.h>
-#endif
 #ifdef HAVE_IO_H
 # include <io.h>
 #endif
@@ -1207,6 +1204,22 @@ _io_FileIO_isatty_impl(fileio *self)
  return PyBool_FromLong(res);
 }
 
+/* Checks whether the file is a TTY using an open-only optimization.
+
+ Normally isatty always makes a system call. In the case of open() there
+ is a _inside the same python call_ stat result which we can use to
+ skip that system call for non-character files. Outsid of that context
+ this is subject to TOCTOU issues (the FD has been returned to user code
+ and arbitrary syscalls could have happened). */
+static PyObject *
+_io_FileIO_isatty_openonly(fileio *self, void *Py_UNUSED(ignored))
+{
+ if (self->stat_atopen != NULL && !S_ISCHR(self->stat_atopen->st_mode)) {
+ Py_RETURN_FALSE;
+ }
+ return _io_FileIO_isatty_impl(self);
+}
+
 #include "clinic/fileio.c.h"
 
 static PyMethodDef fileio_methods[] = {
@@ -1223,6 +1236,7 @@ static PyMethodDef fileio_methods[] = {
  _IO_FILEIO_WRITABLE_METHODDEF
  _IO_FILEIO_FILENO_METHODDEF
  _IO_FILEIO_ISATTY_METHODDEF
+ {"_isatty_openonly", (PyCFunction)_io_FileIO_isatty_openonly, METH_NOARGS},
  {"_dealloc_warn", (PyCFunction)fileio_dealloc_warn, METH_O, NULL},
  {"__reduce__", _PyIOBase_cannot_pickle, METH_NOARGS},
  {"__reduce_ex__", _PyIOBase_cannot_pickle, METH_O},