gh-118422: Fix run_fileexflags() test (#118429)

Don't test the undefined behavior of fileno()
on a closed file, but use fstat() as a reliable
test if the file was closed or not.

Python/fileutils.c

@@ -3050,3 +3050,52 @@ _Py_GetTicksPerSecond(long *ticks_per_second)
     return 0;
 }
 #endif
+
+
+/* Check if a file descriptor is valid or not.
+   Return 0 if the file descriptor is invalid, return non-zero otherwise. */
+int
+_Py_IsValidFD(int fd)
+{
+/* dup() is faster than fstat(): fstat() can require input/output operations,
+   whereas dup() doesn't. There is a low risk of EMFILE/ENFILE at Python
+   startup. Problem: dup() doesn't check if the file descriptor is valid on
+   some platforms.
+
+   fcntl(fd, F_GETFD) is even faster, because it only checks the process table.
+   It is preferred over dup() when available, since it cannot fail with the
+   "too many open files" error (EMFILE).
+
+   bpo-30225: On macOS Tiger, when stdout is redirected to a pipe and the other
+   side of the pipe is closed, dup(1) succeed, whereas fstat(1, &st) fails with
+   EBADF. FreeBSD has similar issue (bpo-32849).
+
+   Only use dup() on Linux where dup() is enough to detect invalid FD
+   (bpo-32849).
+*/
+    if (fd < 0) {
+        return 0;
+    }
+#if defined(F_GETFD) && ( \
+        defined(__linux__) || \
+        defined(__APPLE__) || \
+        (defined(__wasm__) && !defined(__wasi__)))
+    return fcntl(fd, F_GETFD) >= 0;
+#elif defined(__linux__)
+    int fd2 = dup(fd);
+    if (fd2 >= 0) {
+        close(fd2);
+    }
+    return (fd2 >= 0);
+#elif defined(MS_WINDOWS)
+    HANDLE hfile;
+    _Py_BEGIN_SUPPRESS_IPH
+    hfile = (HANDLE)_get_osfhandle(fd);
+    _Py_END_SUPPRESS_IPH
+    return (hfile != INVALID_HANDLE_VALUE
+            && GetFileType(hfile) != FILE_TYPE_UNKNOWN);
+#else
+    struct stat st;
+    return (fstat(fd, &st) == 0);
+#endif
+}