Add sync to file_helper (#2343)

include/spdlog/details/os-inl.h

@@ -23,9 +23,10 @@
 
 #ifdef _WIN32
 
-#    include <io.h>      // _get_osfhandle and _isatty support
-#    include <process.h> //  _get_pid support
+#    include <io.h>      // for _get_osfhandle, _isatty, _fileno
+#    include <process.h> // for _get_pid
 #    include <spdlog/details/windows_include.h>
+#    include <fileapi.h> // for FlushFileBuffers
 
 #    ifdef __MINGW32__
 #        include <share.h>
@@ -601,6 +602,17 @@ std::string SPDLOG_INLINE getenv(const char *field)
 #endif
 }
 
+// Do fsync by FILE descriptor
+// Return true on success
+SPDLOG_INLINE bool fsync(FILE *fd)
+{
+#ifdef _WIN32
+    return FlushFileBuffers(reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fd)))) != 0;
+#else
+    return ::fsync(fileno(fd)) == 0;
+#endif
+}
+
 } // namespace os
 } // namespace details
 } // namespace spdlog