bpo-44219: Release the GIL during isatty syscalls (GH-28250)

Release the GIL while performing isatty() system calls on arbitrary
file descriptors. In particular, this affects os.isatty(),
os.device_encoding() and io.TextIOWrapper. By extension,
io.open() in text mode is also affected.

1 files changed, 14 insertions, 2 deletions

diff --git a/Python/fileutils.c b/Python/fileutils.c
index e8a7eda..9e732dd 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -67,9 +67,11 @@ PyObject *
 _Py_device_encoding(int fd)
 {
     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;
 
@@ -1776,12 +1778,22 @@ _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) {