Refactor tmp dir detection to improve testability

pkg/minikube/command/kic_runner.go

@@ -186,17 +186,13 @@ func (k *kicRunner) Copy(f assets.CopyableFile) error {
 		}
 	}
 	klog.Infof("%s (temp): %s --> %s (%d bytes)", k.ociBin, src, dst, f.GetLength())
-	tmpFolder := ""
 
-	// Snap only allows an application to see its own files in /tmp, making Docker unable to copy memory assets
-	// https://github.com/kubernetes/minikube/issues/10020
-	if isSnapBinary() {
-		home, err := os.UserHomeDir()
-		if err != nil {
-			return errors.Wrap(err, "detecting home dir")
-		}
-		tmpFolder = home
+	isSnap := isSnapBinary()
+	tmpFolder, err := tempDirectory(isSnap)
+	if err != nil {
+		return errors.Wrap(err, "determining temp directory")
 	}
+
 	tf, err := ioutil.TempFile(tmpFolder, "tmpf-memory-asset")
 	if err != nil {
 		return errors.Wrap(err, "creating temporary file")
@@ -209,6 +205,24 @@ func (k *kicRunner) Copy(f assets.CopyableFile) error {
 	return k.copy(tf.Name(), dst)
 }
 
+// tempDirectory returns the directory to use as the temp directory
+// or an empty string if it should use the os default temp directory.
+func tempDirectory(isSnap bool) (string, error) {
+	if !isSnap {
+		return "", nil
+	}
+
+	// Snap only allows an application to see its own files in /tmp, making Docker unable to copy memory assets
+	// https://github.com/kubernetes/minikube/issues/10020
+
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", errors.Wrap(err, "detecting home dir")
+	}
+	return home, nil
+}
+
+// isSnapBinary returns true if the binary path includes "snap".
 func isSnapBinary() bool {
 	ex, err := os.Executable()
 	if err != nil {

pkg/minikube/command/kic_runner_test.go

@@ -0,0 +1,54 @@
+/*
+Copyright 2019 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package command
+
+import (
+	"os"
+	"testing"
+)
+
+func TestKICRunner(t *testing.T) {
+	t.Parallel()
+
+	t.Run("TestTempDirectory", func(t *testing.T) {
+		t.Parallel()
+
+		home, err := os.UserHomeDir()
+		if err != nil {
+			t.Fatalf("failed to get user home directory: %v", err)
+		}
+
+		tests := []struct {
+			in   bool
+			want string
+		}{
+			{false, ""},
+			{true, home},
+		}
+
+		for _, tt := range tests {
+			got, err := tempDirectory(tt.in)
+			if err != nil {
+				t.Fatalf("failed to get temp directory: %v", err)
+			}
+
+			if got != tt.want {
+				t.Errorf("tempDirectory(%t) = %s; want %s", tt.in, got, tt.want)
+			}
+		}
+	})
+}