Fix build without cgo

The github.com/DataDog/zstd library wraps a c library, so it requires
cgo to compile. Add zstd wrapper funcs to sarama and put the zstd
import behind a build flag requiring cgo. Without cgo, the wrapper
funcs return an error saying zstd doesn't work without cgo.

message.go

@@ -7,7 +7,6 @@ import (
 	"io/ioutil"
 	"time"
 
-	"github.com/DataDog/zstd"
 	"github.com/eapache/go-xerial-snappy"
 	"github.com/pierrec/lz4"
 )
@@ -116,7 +115,7 @@ func (m *Message) encode(pe packetEncoder) error {
 			m.compressedCache = buf.Bytes()
 			payload = m.compressedCache
 		case CompressionZSTD:
-			c, err := zstd.CompressLevel(nil, m.Value, m.CompressionLevel)
+			c, err := zstdCompressLevel(nil, m.Value, m.CompressionLevel)
 			if err != nil {
 				return err
 			}
@@ -219,7 +218,7 @@ func (m *Message) decode(pd packetDecoder) (err error) {
 		if m.Value == nil {
 			break
 		}
-		if m.Value, err = zstd.Decompress(nil, m.Value); err != nil {
+		if m.Value, err = zstdDecompress(nil, m.Value); err != nil {
 			return err
 		}
 		if err := m.decodeSet(); err != nil {

record_batch.go

@@ -7,7 +7,6 @@ import (
 	"io/ioutil"
 	"time"
 
-	"github.com/DataDog/zstd"
 	"github.com/eapache/go-xerial-snappy"
 	"github.com/pierrec/lz4"
 )
@@ -195,7 +194,7 @@ func (b *RecordBatch) decode(pd packetDecoder) (err error) {
 			return err
 		}
 	case CompressionZSTD:
-		if recBuffer, err = zstd.Decompress(nil, recBuffer); err != nil {
+		if recBuffer, err = zstdDecompress(nil, recBuffer); err != nil {
 			return err
 		}
 	default:
@@ -254,7 +253,7 @@ func (b *RecordBatch) encodeRecords(pe packetEncoder) error {
 		}
 		b.compressedRecords = buf.Bytes()
 	case CompressionZSTD:
-		c, err := zstd.CompressLevel(nil, raw, b.CompressionLevel)
+		c, err := zstdCompressLevel(nil, raw, b.CompressionLevel)
 		if err != nil {
 			return err
 		}

zstd_cgo.go

@@ -0,0 +1,13 @@
+// +build cgo
+
+package sarama
+
+import "github.com/DataDog/zstd"
+
+func zstdDecompress(dst, src []byte) ([]byte, error) {
+	return zstd.Decompress(dst, src)
+}
+
+func zstdCompressLevel(dst, src []byte, level int) ([]byte, error) {
+	return zstd.CompressLevel(dst, src, level)
+}

zstd_fallback.go

@@ -0,0 +1,17 @@
+// +build !cgo
+
+package sarama
+
+import (
+	"errors"
+)
+
+var errZstdCgo = errors.New("zstd compression requires building with cgo enabled")
+
+func zstdDecompress(dst, src []byte) ([]byte, error) {
+	return nil, errZstdCgo
+}
+
+func zstdCompressLevel(dst, src []byte, level int) ([]byte, error) {
+	return nil, errZstdCgo
+}