kola/cluster: upload files to temporary path and rename

As good practice, upload to a separate file first before renaming it to
its final location. That way there's no chance of the file sticking
around in its official spot even if we fail. But also, other services
that are watching the path won't get a partially written file.

Prep for next patch which also adds labeling to that.

Author: jlebon

mantle/kola/cluster/cluster.go

@@ -24,6 +24,7 @@ import (
 	"github.com/coreos/mantle/harness"
 	"github.com/coreos/mantle/platform"
 	"github.com/coreos/pkg/capnslog"
+	"github.com/pkg/errors"
 )
 
 var (
@@ -103,9 +104,16 @@ func DropFile(machines []platform.Machine, localPath string) error {
 		if _, err := in.Seek(0, 0); err != nil {
 			return err
 		}
-		if err := platform.InstallFile(in, m, filepath.Base(localPath)); err != nil {
+		// write to a separate path first, then rename to its final location so
+		// that anything watching the path can only get a complete file
+		base := filepath.Base(localPath)
+		partial := base + ".partial"
+		if err := platform.InstallFile(in, m, partial); err != nil {
 			return err
 		}
+		if out, stderr, err := m.SSH(fmt.Sprintf("mv %[1]s.partial %[1]s", base)); err != nil {
+			return errors.Wrapf(err, "running mv %[1]s.partial %[1]s: %s: %s", base, out, stderr)
+		}
 	}
 	return nil
 }