Add tests for non-clobbering codec registration

This should fail, until the following commit which switches the test
to use the "nonclobbering" versions of these codecs.

internal/deptest/deptest_test.go

@@ -0,0 +1,43 @@
+// Copyright 2020 Mostyn Bramley-Moore.
+//
+// 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 deptest ensures that the go-grpc-compression sub-packages will
+// not clobber an existing registration.
+package deptest
+
+import (
+	"testing"
+
+	// This import happens first for the test, keep it ahead of
+	// the three unnamed imports below.
+	"github.com/mostynb/go-grpc-compression/internal/deptest/testlib"
+
+	// If these were moved above, the test would fail because
+	// testlib has the same no-clobber logic as the main packages.
+	_ "github.com/mostynb/go-grpc-compression/lz4"
+	_ "github.com/mostynb/go-grpc-compression/snappy"
+	_ "github.com/mostynb/go-grpc-compression/zstd"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc/encoding"
+)
+
+func TestCompressorNotClobbered(t *testing.T) {
+	// Because the deptest/lib imports first, it's init() function
+	// registers dummy compressors. The following libraries do not
+	// clobber, so we should find Dummy compressors.
+	for _, name := range testlib.AllNames {
+		_, ok := encoding.GetCompressor(name).(testlib.Dummy)
+		require.True(t, ok)
+	}
+}

internal/deptest/testlib/testlib.go

@@ -0,0 +1,52 @@
+// Copyright 2020 Mostyn Bramley-Moore.
+//
+// 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 testlib
+
+import (
+	"fmt"
+	"io"
+
+	"google.golang.org/grpc/encoding"
+)
+
+type Dummy string
+
+var _ encoding.Compressor = Dummy("")
+
+// AllNames lists every compressor in this repo, for testing.
+var AllNames = []string{"zstd", "snappy", "lz4"}
+
+func (d Dummy) Compress(io.Writer) (io.WriteCloser, error) {
+	return nil, fmt.Errorf("not implemented")
+}
+
+func (d Dummy) Decompress(r io.Reader) (io.Reader, error) {
+	return nil, fmt.Errorf("not implemented")
+}
+
+func (d Dummy) Name() string {
+	return string(d)
+}
+
+func init() {
+	for _, name := range AllNames {
+		// This test will not register the dummies in case
+		// of existing registrations, this ensures the import
+		// order of the deptest actually tests no-clobbering.
+		if encoding.GetCompressor(name) == nil {
+			encoding.RegisterCompressor(Dummy(name))
+		}
+	}
+}