operations: fix setting the timestamp on Windows for multithread copy

Before this fix we attempted to set the modification time on the file
when it was open. This works fine on Linux but not on Windows. The
test was also incorrect testing the source file rather than the
destination file.

This closes the file before setting the modification time and fixes
the tests.

Fixes #3994

fs/operations/multithread.go

@@ -165,7 +165,6 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 	if err != nil {
 		return nil, errors.Wrap(err, "multpart copy: failed to open destination")
 	}
-	defer fs.CheckClose(mc.wc, &err)
 
 	fs.Debugf(src, "Starting multi-thread copy with %d parts of size %v", mc.streams, fs.SizeSuffix(mc.partSize))
 	for stream := 0; stream < mc.streams; stream++ {
@@ -175,9 +174,13 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 		})
 	}
 	err = g.Wait()
+	closeErr := mc.wc.Close()
 	if err != nil {
 		return nil, err
 	}
+	if closeErr != nil {
+		return nil, errors.Wrap(closeErr, "multi-thread copy: failed to close object after copy")
+	}
 
 	obj, err := f.NewObject(ctx, remote)
 	if err != nil {

fs/operations/multithread_test.go

@@ -138,7 +138,7 @@ func TestMultithreadCopy(t *testing.T) {
 			assert.Equal(t, src.Size(), dst.Size())
 			assert.Equal(t, "file1", dst.Remote())
 
-			fstest.CheckListingWithPrecision(t, r.Fremote, []fstest.Item{file1}, nil, fs.ModTimeNotSupported)
+			fstest.CheckItems(t, r.Flocal, file1)
 			require.NoError(t, dst.Remove(context.Background()))
 		})
 	}