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 +1750,15 @@ _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);
+        // read() on a non-blocking empty pipe fails with EINVAL, which is
+        // mapped from the Windows error code ERROR_NO_DATA.
+        if (n < 0 && errno == EINVAL) {
+            if (_doserrno == ERROR_NO_DATA) {
+                errno = EAGAIN;
+            }
+        }
 #else
         n = read(fd, buf, count);
 #endif
@@ -1814,7 +1822,13 @@ _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);
+            // write() on a non-blocking pipe fails with ENOSPC on Windows if
+            // the pipe lacks available space for the entire buffer.
+            if (n < 0 && errno == ENOSPC && _doserrno == 0) {
+                errno = EAGAIN;
+            }
 #else
             n = write(fd, buf, count);
 #endif
@@ -1829,7 +1843,13 @@ _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);
+            // write() on a non-blocking pipe fails with ENOSPC on Windows if
+            // the pipe lacks available space for the entire buffer.
+            if (n < 0 && errno == ENOSPC && _doserrno == 0) {
+                errno = EAGAIN;
+            }
 #else
             n = write(fd, buf, count);
 #endif