Add tests for file/symlink/dir deleted while copy

Files, symlinks and directories may be deleted while or after directory
list is read. Add test to simulate this so we can fix the desired
behavior.

ref #72

all_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"testing"
@@ -86,6 +87,48 @@ func TestCopy(t *testing.T) {
 		err = os.Chmod(dest, 0o755)
 		Expect(t, err).ToBe(nil)
 	})
+	When(t, "file is deleted while copying", func(t *testing.T) {
+		src := t.TempDir()
+		dest := t.TempDir()
+
+		file := filepath.Join(src, "file")
+		f, err := os.Create(file)
+		Expect(t, err).ToBe(nil)
+		f.Close()
+
+		opt := Options{Skip: func(info os.FileInfo, src, dest string) (bool, error) {
+			os.Remove(src)
+			return false, nil
+		}}
+		err = Copy(src, dest, opt)
+		Expect(t, err).ToBe(nil)
+	})
+	When(t, "symlink is deleted while copying", func(t *testing.T) {
+		src := t.TempDir()
+		dest := t.TempDir()
+
+		Expect(t, os.Symlink(".", filepath.Join(src, "symlink"))).ToBe(nil)
+
+		opt := Options{Skip: func(info os.FileInfo, src, dest string) (bool, error) {
+			os.Remove(src)
+			return false, nil
+		}}
+		err = Copy(src, dest, opt)
+		Expect(t, err).ToBe(nil)
+	})
+	When(t, "directory is deleted while copying", func(t *testing.T) {
+		src := t.TempDir()
+		dest := t.TempDir()
+
+		Expect(t, os.Mkdir(filepath.Join(src, "dir"), 0755)).ToBe(nil)
+
+		opt := Options{Skip: func(info os.FileInfo, src, dest string) (bool, error) {
+			os.Remove(src)
+			return false, nil
+		}}
+		err = Copy(src, dest, opt)
+		Expect(t, err).ToBe(nil)
+	})
 }
 
 func TestCopy_NamedPipe(t *testing.T) {