support: Provide xclock_settime test helper function

The xclock_settime is a wrapper function on the clock_settime syscall
to be used in the test code.

It checks if the GLIBC_TEST_ALLOW_TIME_SETTING env variable is defined
in the environment in which test is executed. If it is not - the test
ends as unsupported. Otherwise, the clock-settime is executed and return
value is assessed.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

support/Makefile

@@ -90,6 +90,7 @@ libsupport-routines = \
   xchdir \
   xchroot \
   xclock_gettime \
+  xclock_settime \
   xclose \
   xchmod \
   xconnect \

support/xclock_settime.c

@@ -0,0 +1,34 @@
+/* clock_settime 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 <stdlib.h>
+#include <support/check.h>
+#include <support/xtime.h>
+
+void
+xclock_settime (clockid_t clockid,
+                const struct timespec *ts)
+{
+  if (getenv (SETTIME_ENV_NAME) == NULL)
+    FAIL_UNSUPPORTED ("clock_settime is executed only when "\
+                      SETTIME_ENV_NAME" is set\n");
+
+  int ret = clock_settime (clockid, ts);
+  if (ret < 0)
+    FAIL_EXIT1 ("clock_settime (%d): %m", clockid);
+}

support/xtime.h

@@ -23,10 +23,15 @@
 
 __BEGIN_DECLS
 
+/* Name of the env variable, which indicates if it is possible to
+   adjust time on target machine.  */
+#define SETTIME_ENV_NAME "GLIBC_TEST_ALLOW_TIME_SETTING"
+
 /* The following functions call the corresponding libc functions and
    terminate the process on error.  */
 
 void xclock_gettime (clockid_t clock, struct timespec *ts);
+void xclock_settime (clockid_t clock, const struct timespec *ts);
 
 /* This helper can often simplify tests by avoiding an explicit
    variable declaration or allowing that declaration to be const. */