From 52545d2857cfab49056f2e0334ed570a63be3e6d Mon Sep 17 00:00:00 2001
From: Collin Anderson <cmawebsite@gmail.com>
Date: Wed, 1 Dec 2021 00:04:24 -0500
Subject: [PATCH] bpo-45944: Avoid calling isatty() for most open() calls

---
 Lib/_pyio.py            |  3 ++-
 Modules/_io/_iomodule.c | 12 ++++++++++++
 Modules/_io/fileio.c    |  4 ++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index d7119742b9d22b..757c1d5718b5b2 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -237,7 +237,7 @@ 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 getattr(raw, '_size', 0) == 0 and raw.isatty():
             buffering = -1
             line_buffering = True
         if buffering < 0:
@@ -1594,6 +1594,7 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
             self._blksize = getattr(fdfstat, 'st_blksize', 0)
             if self._blksize <= 1:
                 self._blksize = DEFAULT_BUFFER_SIZE
+            self._size = fdfstat.st_size
 
             if _setmode:
                 # don't translate newlines (\r\n <=> \n)
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index b4743fbd5e04f0..6d37a4ea5d3067 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -237,11 +237,13 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
 
     char rawmode[6], *m;
     int line_buffering, is_number;
+    __int64_t size = 0;
     long isatty = 0;
 
     PyObject *raw, *modeobj = NULL, *buffer, *wrapper, *result = NULL, *path_or_fd = NULL;
 
     _Py_IDENTIFIER(_blksize);
+    _Py_IDENTIFIER(_size);
     _Py_IDENTIFIER(isatty);
     _Py_IDENTIFIER(mode);
     _Py_IDENTIFIER(close);
@@ -381,6 +383,16 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
 
     /* buffering */
     if (buffering < 0) {
+        PyObject *size_obj;
+        size_obj = _PyObject_GetAttrId(raw, &PyId__size);
+        if (size_obj == NULL)
+            goto error;
+        size = PyLong_AsLongLong(size_obj);
+        Py_DECREF(size_obj);
+        if (size == -1 && PyErr_Occurred())
+            goto error;
+    }
+    if (buffering < 0 && size == 0) {
         PyObject *res = _PyObject_CallMethodIdNoArgs(raw, &PyId_isatty);
         if (res == NULL)
             goto error;
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index dd215e89399442..a42983e8bda9f2 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -66,6 +66,7 @@ typedef struct {
     unsigned int closefd : 1;
     char finalizing;
     unsigned int blksize;
+    __int64_t size;
     PyObject *weakreflist;
     PyObject *dict;
 } fileio;
@@ -186,6 +187,7 @@ fileio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         self->appending = 0;
         self->seekable = -1;
         self->blksize = 0;
+        self->size = 0;
         self->closefd = 1;
         self->weakreflist = NULL;
     }
@@ -469,6 +471,7 @@ _Py_COMP_DIAG_POP
         if (fdfstat.st_blksize > 1)
             self->blksize = fdfstat.st_blksize;
 #endif /* HAVE_STRUCT_STAT_ST_BLKSIZE */
+	self->size = fdfstat.st_size;
     }
 
 #if defined(MS_WINDOWS) || defined(__CYGWIN__)
@@ -1184,6 +1187,7 @@ static PyGetSetDef fileio_getsetlist[] = {
 
 static PyMemberDef fileio_members[] = {
     {"_blksize", T_UINT, offsetof(fileio, blksize), 0},
+    {"_size", T_UINT, offsetof(fileio, size), 0},
     {"_finalizing", T_BOOL, offsetof(fileio, finalizing), 0},
     {NULL}
 };