Implement File#fsync on Windows

src/crystal/system/win32/file.cr

@@ -241,6 +241,8 @@ module Crystal::System::File
   end
 
   private def system_fsync(flush_metadata = true) : Nil
-    raise NotImplementedError.new("File#fsync")
+    if LibC._commit(fd) != 0
+      raise IO::Error.from_errno("Error syncing file")
+    end
   end
 end

src/file.cr

@@ -771,6 +771,9 @@ class File < IO::FileDescriptor
   # then the syscall *fdatasync* will be used and only data required for
   # subsequent data retrieval is flushed. Metadata such as modified time and
   # access time is not written.
+  #
+  # NOTE: Metadata is flushed even when *flush_metadata* is false on Windows
+  # and DragonFly BSD.
   def fsync(flush_metadata = true) : Nil
     flush
     system_fsync(flush_metadata)

src/lib_c/x86_64-windows-msvc/c/io.cr

@@ -15,4 +15,5 @@ lib LibC
   fun _get_osfhandle(fd : Int) : IntPtrT
   fun _pipe(pfds : Int*, psize : UInt, textmode : Int) : Int
   fun _dup2(fd1 : Int, fd2 : Int) : Int
+  fun _commit(fd : Int) : Int
 end