python · miss-islington · Sep 9, 2021 · Sep 9, 2021
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-09-09-10-32-33.bpo-44219.WiYyjz.rst b/Misc/NEWS.d/next/Core and Builtins/2021-09-09-10-32-33.bpo-44219.WiYyjz.rst
@@ -0,0 +1,4 @@
+Release the GIL while performing ``isatty`` system calls on arbitrary file
+descriptors. In particular, this affects :func:`os.isatty`,
+:func:`os.device_encoding` and :class:`io.TextIOWrapper`. By extension,
+:func:`io.open` in text mode is also affected.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -10058,9 +10058,11 @@ os_isatty_impl(PyObject *module, int fd)
 /*[clinic end generated code: output=6a48c8b4e644ca00 input=08ce94aa1eaf7b5e]*/
 {
    int return_value;
+    Py_BEGIN_ALLOW_THREADS
    _Py_BEGIN_SUPPRESS_IPH
    return_value = isatty(fd);
    _Py_END_SUPPRESS_IPH
+    Py_END_ALLOW_THREADS
    return return_value;
 }


diff --git a/Python/fileutils.c b/Python/fileutils.c
@@ -69,9 +69,11 @@ _Py_device_encoding(int fd)
    UINT cp;
 #endif
    int valid;
+    Py_BEGIN_ALLOW_THREADS
    _Py_BEGIN_SUPPRESS_IPH
    valid = isatty(fd);
    _Py_END_SUPPRESS_IPH
+    Py_END_ALLOW_THREADS
    if (!valid)
        Py_RETURN_NONE;

@@ -1737,12 +1739,22 @@ _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)

    _Py_BEGIN_SUPPRESS_IPH
 #ifdef MS_WINDOWS
-    if (count > 32767 && isatty(fd)) {
+    if (count > 32767) {
        /* Issue #11395: the Windows console returns an error (12: not
           enough space error) on writing into stdout if stdout mode is
           binary and the length is greater than 66,000 bytes (or less,
           depending on heap usage). */
-        count = 32767;
+        if (gil_held) {
+            Py_BEGIN_ALLOW_THREADS
+            if (isatty(fd)) {
+                count = 32767;
+            }
+            Py_END_ALLOW_THREADS
+        } else {
+            if (isatty(fd)) {
+                count = 32767;
+            }
+        }
    }
 #endif
    if (count > _PY_WRITE_MAX) {