Handle negative times in filesystem::last_write_time

	* src/filesystem/ops.cc
	(last_write_time(const path&, file_time_type, error_code&)): Handle
	negative times correctly.
	* testsuite/experimental/filesystem/operations/last_write_time.cc:
	Test writing file times.

From-SVN: r241522

libstdc++-v3/ChangeLog

 2016-10-25  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/filesystem/ops.cc
+	(last_write_time(const path&, file_time_type, error_code&)): Handle
+	negative times correctly.
+	* testsuite/experimental/filesystem/operations/last_write_time.cc:
+	Test writing file times.
+
 	* src/filesystem/ops.cc (do_copy_file): Report an error if source or
 	destination is not a regular file (LWG 2712).
 	(equivalent): Fix error handling and result when only one file exists.

libstdc++-v3/src/filesystem/ops.cc

@@ -1100,6 +1100,11 @@ fs::last_write_time(const path& p __attribute__((__unused__)),
   auto s = chrono::duration_cast<chrono::seconds>(d);
 #if _GLIBCXX_USE_UTIMENSAT
   auto ns = chrono::duration_cast<chrono::nanoseconds>(d - s);
+  if (ns < ns.zero()) // tv_nsec must be non-negative and less than 10e9.
+    {
+      --s;
+      ns += chrono::seconds(1);
+    }
   struct ::timespec ts[2];
   ts[0].tv_sec = 0;
   ts[0].tv_nsec = UTIME_OMIT;

libstdc++-v3/testsuite/experimental/filesystem/operations/last_write_time.cc

@@ -35,6 +35,8 @@
 void
 test01()
 {
+  // read times
+
   using time_type = std::experimental::filesystem::file_time_type;
 
   auto p = __gnu_test::nonexistent_path();
@@ -103,8 +105,46 @@ test01()
 #endif
 }
 
+void
+test02()
+{
+  // write times
+
+  using time_type = std::experimental::filesystem::file_time_type;
+
+  __gnu_test::scoped_file f;
+  std::error_code ec;
+  time_type time;
+
+  time = last_write_time(f.path);
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( last_write_time(f.path) == time );
+
+  time -= std::chrono::milliseconds(1000 * 60 * 10 + 15);
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( last_write_time(f.path) == time );
+
+  time += std::chrono::milliseconds(1000 * 60 * 20 + 15);
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( last_write_time(f.path) == time );
+
+  time = time_type();
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( last_write_time(f.path) == time );
+
+  time -= std::chrono::milliseconds(1000 * 60 * 10 + 15);
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( last_write_time(f.path) == time );
+}
+
 int
 main()
 {
   test01();
+  test02();
 }