io: Check at runtime if timestamp supports nanoseconds

Now that non-LFS stat function is implemented on to on LFS, it will
use statx when available.  It allows to check for nanosecond timestamp
if the kernel supports __NR_statx.

Checked on s390-linux-gnu with 4.12.14 kernel.

io/tst-stat.c

@@ -25,6 +25,7 @@
 #include <support/xunistd.h>
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
+#include <stdio.h>
 #include <unistd.h>
 
 static void
@@ -73,6 +74,10 @@ do_test (void)
 
   test_t tests[] = { stat_check, lstat_check, fstat_check, fstatat_check };
 
+  bool check_ns = support_stat_nanoseconds (path);
+  if (!check_ns)
+    printf ("warning: timestamp with nanoseconds not supportted\n");
+
   for (int i = 0; i < array_length (tests); i++)
     {
       struct stat st;
@@ -92,7 +97,7 @@ do_test (void)
 
       TEST_COMPARE (stx.stx_ctime.tv_sec, st.st_ctim.tv_sec);
       TEST_COMPARE (stx.stx_mtime.tv_sec, st.st_mtim.tv_sec);
-      if (support_stat_nanoseconds ())
+      if (check_ns)
 	{
 	  TEST_COMPARE (stx.stx_ctime.tv_nsec, st.st_ctim.tv_nsec);
 	  TEST_COMPARE (stx.stx_mtime.tv_nsec, st.st_mtim.tv_nsec);

support/support.h

@@ -142,7 +142,7 @@ static __inline bool support_path_support_time64 (const char *path)
 }
 
 /* Return true if stat supports nanoseconds resolution.  */
-extern bool support_stat_nanoseconds (void);
+extern bool support_stat_nanoseconds (const char *path);
 
 __END_DECLS
 

support/support_stat_nanoseconds.c

@@ -16,16 +16,42 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <errno.h>
+#include <fcntl.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/timespec.h>
 #include <stdbool.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
 
 bool
-support_stat_nanoseconds (void)
+support_stat_nanoseconds (const char *path)
 {
-  /* s390 stat64 compat symbol does not support nanoseconds resolution
-     and it used on non-LFS [f,l]stat[at] implementations.  */
-#if defined __linux__ && !defined __s390x__ && defined __s390__
-  return false;
-#else
-  return true;
+  bool support;
+#ifdef __linux__
+  /* Obtain the original timestamp to restore at the end.  */
+  struct stat ost;
+  TEST_VERIFY_EXIT (stat (path, &ost) == 0);
+
+  const struct timespec tsp[] = { { 0, TIMESPEC_HZ - 1 },
+				  { 0, TIMESPEC_HZ / 2 } };
+  TEST_VERIFY_EXIT (utimensat (AT_FDCWD, path, tsp, 0) == 0);
+
+  struct stat st;
+  TEST_VERIFY_EXIT (stat (path, &st) == 0);
+
+  support = st.st_atim.tv_nsec == tsp[0].tv_nsec
+	    && st.st_mtim.tv_nsec == tsp[1].tv_nsec;
+
+  /* Reset to original timestamps.  */
+  const struct timespec otsp[] =
+  {
+    { ost.st_atim.tv_sec, ost.st_atim.tv_nsec },
+    { ost.st_mtim.tv_sec, ost.st_mtim.tv_nsec },
+  };
+  TEST_VERIFY_EXIT (utimensat (AT_FDCWD, path, otsp, 0) == 0);
 #endif
+  return support;
 }