pkg/util/walkers: refactor CopyWalker and add some tests

So at least some changes to this function can be easily tested.

Signed-off-by: Mateusz Gozdek <mateusz@kinvolk.io>

pkg/util/walkers/copying.go

@@ -15,6 +15,7 @@
 package walkers
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -36,16 +37,24 @@ func CopyingWalker(path string, newDirPerms os.FileMode) assets.WalkFunc {
 			return errors.Wrap(err, "failed to create dir")
 		}
 
-		// TODO: If we start packing binaries, make sure they have executable bit set.
-		targetFile, err := os.OpenFile(fileName, os.O_RDWR|os.O_CREATE, 0644)
-		if err != nil {
-			return errors.Wrap(err, "failed to open target file")
-		}
-		defer targetFile.Close()
+		return writeFile(fileName, r)
+	}
+}
 
-		if _, err := io.Copy(targetFile, r); err != nil {
-			return errors.Wrap(err, "failed to write file")
-		}
-		return nil
+// writeFile writes data from given io.Reader to the file and makes sure, that
+// this is the only content stored in the file.
+func writeFile(p string, r io.Reader) error {
+	// TODO: If we start packing binaries, make sure they have executable bit set.
+	f, err := os.OpenFile(p, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		return fmt.Errorf("failed to open target file %s: %v", p, err)
 	}
+
+	defer f.Close()
+
+	if _, err := io.Copy(f, r); err != nil {
+		return fmt.Errorf("failed writing to file %s: %v", p, err)
+	}
+
+	return nil
 }

pkg/util/walkers/copying_test.go

@@ -0,0 +1,36 @@
+package walkers
+
+import (
+	"io/ioutil"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+const (
+	tmpPattern = "lokomotive-test"
+	testData   = "222"
+)
+
+func TestWriteFile(t *testing.T) {
+	f, err := ioutil.TempFile("", tmpPattern)
+	if err != nil {
+		t.Fatalf("Creating temp file should succeed, got: %v", err)
+	}
+
+	defer os.Remove(f.Name())
+
+	if err := writeFile(f.Name(), strings.NewReader(testData)); err != nil {
+		t.Fatalf("Writing to file should succeed, got: %v", err)
+	}
+
+	d, err := ioutil.ReadFile(f.Name())
+	if err != nil {
+		t.Fatalf("Reading tmp file should succeed, got: %v", err)
+	}
+
+	if !reflect.DeepEqual(testData, string(d)) {
+		t.Fatalf("expected: '%s', got '%s'", testData, string(d))
+	}
+}