fileutils: handle Windows errors in non-blocking mode

Handle erroring operations on non-blocking pipes by reading the _doserrno code.

Author: RayyanAnsari

Python/fileutils.c

@@ -1750,7 +1750,16 @@ _Py_read(int fd, void *buf, size_t count)
         Py_BEGIN_ALLOW_THREADS
         errno = 0;
 #ifdef MS_WINDOWS
+        _doserrno = 0;
         n = read(fd, buf, (int)count);
+        int winerr = _doserrno;
+        if (n < 0 && errno == EINVAL) {
+                if (winerr == ERROR_NO_DATA) {
+                        /* read() on a non-blocking empty pipe
+                           returns EINVAL on Windows */
+                        errno = EAGAIN;
+                }
+        }
 #else
         n = read(fd, buf, count);
 #endif
@@ -1814,7 +1823,14 @@ _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)
             Py_BEGIN_ALLOW_THREADS
             errno = 0;
 #ifdef MS_WINDOWS
+            _doserrno = 0;
             n = write(fd, buf, (int)count);
+            int winerr = _doserrno;
+            if (n < 0 && errno == ENOSPC) {
+                /* write() on a non-blocking full pipe
+                   returns ENOSPC on Windows */
+                errno = EAGAIN;
+            }
 #else
             n = write(fd, buf, count);
 #endif
@@ -1829,7 +1845,14 @@ _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)
         do {
             errno = 0;
 #ifdef MS_WINDOWS
+            _doserrno = 0;
             n = write(fd, buf, (int)count);
+            int winerr = _doserrno;
+            if (n < 0 && errno == ENOSPC) {
+                /* write() on a non-blocking full pipe
+                   returns ENOSPC on Windows */
+                errno = EAGAIN;
+            }
 #else
             n = write(fd, buf, count);
 #endif