gh-90102: Remove isatty call during regular open

Include/internal/pycore_global_strings.h

@@ -246,6 +246,7 @@ struct _Py_global_strings {
  STRUCT_FOR_ID(_initializing)
  STRUCT_FOR_ID(_io)
  STRUCT_FOR_ID(_is_text_encoding)
+ STRUCT_FOR_ID(_isatty_open_only)
  STRUCT_FOR_ID(_length_)
  STRUCT_FOR_ID(_limbo)
  STRUCT_FOR_ID(_lock_unlock_module)

Lib/_pyio.py

@@ -238,7 +238,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
  result = raw
  try:
  line_buffering = False
- if buffering == 1 or buffering < 0 and raw.isatty():
+ if buffering == 1 or buffering < 0 and raw._isatty_open_only():
  buffering = -1
  line_buffering = True
  if buffering < 0:
@@ -1794,6 +1794,21 @@ def isatty(self):
  self._checkClosed()
  return os.isatty(self._fd)
 
+ def _isatty_open_only(self):
+ """Checks whether the file is a TTY using an open-only optimization.
+
+ TTYs are always character devices. If the interpreter knows a file is
+ not a character device when it would call ``isatty``, can skip that
+ call. Inside ``open()`` there is a fresh stat result that contains that
+ information. Use the stat result to skip a system call. Outside of that
+ context TOCTOU issues (the fd could be arbitrarily modified by
+ surrounding code).
+ """
+ if (self._stat_atopen is not None
+ and not stat.S_ISCHR(self._stat_atopen.st_mode)):
+ return True
+ return os.isatty(self._fd)
+
  @property
  def closefd(self):
  """True if the file descriptor will be closed by close()."""

Misc/NEWS.d/next/Library/2024-10-02-22-53-48.gh-issue-90102.4qX52R.rst

@@ -0,0 +1,3 @@
+Skip the ``isatty`` system call during open() when the file is known to not
+be a character device. This provides a slight performance improvement when
+reading whole files.

Modules/_io/_iomodule.c

@@ -346,7 +346,7 @@ _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_open_only));
  if (res == NULL)
  goto error;
  isatty = PyObject_IsTrue(res);

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
@@ -1218,6 +1215,24 @@ _io_FileIO_isatty_impl(fileio *self)
  return PyBool_FromLong(res);
 }
 
+/* Checks whether the file is a TTY using an open-only optimization.
+
+ TTYs are always character devices. If the interpreter knows a file is
+ not a character device when it would call ``isatty``, can skip that
+ call. Inside ``open()`` there is a fresh stat result that contains that
+ information. Use the stat result to skip a system call. Outside of that
+ context TOCTOU issues (the fd could be arbitrarily modified by
+ surrounding code). */
+static PyObject *
+_io_FileIO_isatty_open_only(PyObject *op, PyObject *Py_UNUSED(ignored))
+{
+ fileio *self = _PyFileIO_CAST(op);
+ 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[] = {
@@ -1234,6 +1249,7 @@ static PyMethodDef fileio_methods[] = {
  _IO_FILEIO_WRITABLE_METHODDEF
  _IO_FILEIO_FILENO_METHODDEF
  _IO_FILEIO_ISATTY_METHODDEF
+ {"_isatty_open_only", _io_FileIO_isatty_open_only, METH_NOARGS},
  {"_dealloc_warn", fileio_dealloc_warn, METH_O, NULL},
  {"__reduce__", _PyIOBase_cannot_pickle, METH_NOARGS},
  {"__reduce_ex__", _PyIOBase_cannot_pickle, METH_O},

Modules/_io/winconsoleio.c

@@ -1128,6 +1128,7 @@ static PyMethodDef winconsoleio_methods[] = {
  _IO__WINDOWSCONSOLEIO_WRITABLE_METHODDEF
  _IO__WINDOWSCONSOLEIO_FILENO_METHODDEF
  _IO__WINDOWSCONSOLEIO_ISATTY_METHODDEF
+ {"_isatty_open_only", (PyCFunction)_io__WindowsConsoleIO_isatty, METH_NOARGS},
  {NULL, NULL} /* sentinel */
 };