python · vstinner · Oct 8, 2024 · Oct 3, 2024 · Oct 3, 2024 · Oct 3, 2024
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
@@ -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_openonly)
        STRUCT_FOR_ID(_length_)
        STRUCT_FOR_ID(_limbo)
        STRUCT_FOR_ID(_lock_unlock_module)

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/Lib/_pyio.py b/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_openonly():
            buffering = -1
            line_buffering = True
        if buffering < 0:
@@ -1794,6 +1794,19 @@ def isatty(self):
        self._checkClosed()
        return os.isatty(self._fd)

+    def _isatty_openonly(self):
+        """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. Outside of that context
+        this is subject to TOCTOU issues (the FD has been returned to user code
+        and arbitrary syscalls could have happened).
+        """
+        if (self._stat_atopen is not None and
+            not stat.S_ISCHR(self._stat_atopen.st_mode)):
+            return True
-            return True
+            return False
-            return True
+            return False
+        return os.isatty(self._fd)
+
    @property
    def closefd(self):
        """True if the file descriptor will be closed by close()."""

diff --git a/Misc/NEWS.d/next/Library/2024-10-02-22-53-48.gh-issue-90102.4qX52R.rst b/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.
diff --git a/Modules/_io/_iomodule.c b/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_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
@@ -1208,6 +1205,21 @@ _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. Outside 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[] = {
@@ -1224,6 +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},