Add ensure-file parser package.

cipd/client/cipd/ensure/file_test.go

Index: cipd/client/cipd/ensure/file_test.go
diff --git a/cipd/client/cipd/ensure/file_test.go b/cipd/client/cipd/ensure/file_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..5f2f8dcd6feb7c392092a4f18e9aa3cc70bfa1f6
--- /dev/null
+++ b/cipd/client/cipd/ensure/file_test.go
@@ -0,0 +1,83 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package ensure
+
+import (
+	"bytes"
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+var fileSerializationTests = []struct {
+	name   string
+	f      *File
+	expect string
+}{
+	{
+		"empty",
+		&File{},
+		f(""),
+	},
+
+	{
+		"ServiceURL",
+		&File{"https://something.example.com", nil},
+		f(
+			"$ServiceURL https://something.example.com",
+		),
+	},
+
+	{
+		"simple packages",
+		&File{"", map[string]PackageSlice{
+			"": {
+				PackageDef{"some/thing", "version", 0},
+				PackageDef{"some/other_thing", "latest", 0},
+			},
+		}},
+		f(
+			"some/other_thing latest",
+			"some/thing version",
+		),
+	},
+
+	{
+		"full file",
+		&File{"https://some.example.com", map[string]PackageSlice{
+			"": {
+				PackageDef{"some/thing", "version", 0},
+				PackageDef{"some/other_thing", "latest", 0},
+			},
+			"path/to root/with/spaces": {
+				PackageDef{"different/package", "some_tag:thingy", 0},
+			},
+		}},
+		f(
+			"$ServiceURL https://some.example.com",
+			"",
+			"some/other_thing latest",
+			"some/thing version",
+			"",
+			"@Root path/to root/with/spaces",
+			"different/package some_tag:thingy",
+		),
+	},
+}
+
+func TestFileSerialization(t *testing.T) {
+	t.Parallel()
+
+	Convey("File.Serialize", t, func() {
+		for _, tc := range fileSerializationTests {
+			Convey(tc.name, func() {
+				buf := &bytes.Buffer{}
+				_, err := tc.f.Serialize(buf)
+				So(err, ShouldBeNil)
+				So(buf.String(), ShouldEqual, tc.expect)
+			})
+		}
+	})
+}