Add xfchmod to libsupport

support/Makefile

@@ -96,6 +96,7 @@ libsupport-routines = \
   xdlfcn \
   xdlmopen \
   xdup2 \
+  xfchmod \
   xfclose \
   xfopen \
   xfork \

support/xfchmod.c

@@ -0,0 +1,28 @@
+/* fchmod with error checking.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/xunistd.h>
+#include <sys/stat.h>
+
+void
+xfchmod (int fd, mode_t mode)
+{
+  if (fchmod (fd, mode) != 0)
+    FAIL_EXIT1 ("fchmod (%d, 0%o): %m", fd, mode);
+}

support/xunistd.h

@@ -46,6 +46,7 @@ long long xlseek (int fd, long long offset, int whence);
 void xftruncate (int fd, long long length);
 void xsymlink (const char *target, const char *linkpath);
 void xchdir (const char *path);
+void xfchmod (int fd, mode_t mode);
 
 /* Equivalent of "mkdir -p".  */
 void xmkdirp (const char *, mode_t);