git2go: Serialize results directly into os.Stdout

When spawning Git2Go commands, communication happens via serialized
messages between parent and its child process. This is currently done by
serializing into a string, only to print that string to standard output
immediately afterwards. This pattern is quite inefficient, as we first
need to serialize the whole string into memory.

This commit implements a new function to serialize directly into a
reader in order to avoid this overhead and adjusts the merge command to
use it.

5 files changed, 46 insertions, 9 deletions

diff --git a/cmd/gitaly-git2go/merge.go b/cmd/gitaly-git2go/merge.go
index dab99c4bc..97ea6f49a 100644
--- a/cmd/gitaly-git2go/merge.go
+++ b/cmd/gitaly-git2go/merge.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"os"
 	"strings"
 	"time"
 
@@ -88,12 +89,9 @@ func (cmd *mergeSubcommand) Run() error {
 		CommitID: commit.String(),
 	}
 
-	serialized, err := response.Serialize()
-	if err != nil {
+	if err := response.SerializeTo(os.Stdout); err != nil {
 		return err
 	}
 
-	fmt.Println(serialized)
-
 	return nil
 }
diff --git a/internal/git2go/command.go b/internal/git2go/command.go
index df94a9c0a..2409b847c 100644
--- a/internal/git2go/command.go
+++ b/internal/git2go/command.go
@@ -6,6 +6,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os/exec"
 	"path"
 	"strings"
@@ -46,3 +47,10 @@ func deserialize(serialized string, v interface{}) error {
 	jsonDecoder := json.NewDecoder(base64Decoder)
 	return jsonDecoder.Decode(v)
 }
+
+func serializeTo(writer io.Writer, v interface{}) error {
+	base64Encoder := base64.NewEncoder(base64.StdEncoding, writer)
+	defer base64Encoder.Close()
+	jsonEncoder := json.NewEncoder(base64Encoder)
+	return jsonEncoder.Encode(v)
+}
diff --git a/internal/git2go/command_test.go b/internal/git2go/command_test.go
new file mode 100644
index 000000000..14f1c0a3a
--- /dev/null
+++ b/internal/git2go/command_test.go
@@ -0,0 +1,28 @@
+package git2go
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSerialization_SerializeTo(t *testing.T) {
+	type testStruct struct {
+		Contents string `json:"contents"`
+	}
+
+	var buf bytes.Buffer
+
+	input := testStruct{
+		Contents: "foobar",
+	}
+	err := serializeTo(&buf, &input)
+	require.NoError(t, err)
+	require.NotZero(t, buf.Len())
+
+	var output testStruct
+	err = deserialize(buf.String(), &output)
+	require.NoError(t, err)
+	require.Equal(t, input, output)
+}
diff --git a/internal/git2go/merge.go b/internal/git2go/merge.go
index 39cf88b4a..a60ff51c8 100644
--- a/internal/git2go/merge.go
+++ b/internal/git2go/merge.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"time"
 
 	"gitlab.com/gitlab-org/gitaly/internal/gitaly/config"
@@ -52,9 +53,9 @@ func MergeCommandFromSerialized(serialized string) (MergeCommand, error) {
 	return request, nil
 }
 
-// Serialize serializes the merge response into its JSON representation and encodes it with base64.
-func (m MergeResult) Serialize() (string, error) {
-	return serialize(m)
+// SerializeTo serializes the merge result and writes it into the writer.
+func (m MergeResult) SerializeTo(w io.Writer) error {
+	return serializeTo(w, m)
 }
 
 // Merge performs a merge via gitaly-git2go.
diff --git a/internal/git2go/merge_test.go b/internal/git2go/merge_test.go
index 1de4f3cab..5f0bc60c4 100644
--- a/internal/git2go/merge_test.go
+++ b/internal/git2go/merge_test.go
@@ -1,6 +1,7 @@
 package git2go
 
 import (
+	"bytes"
 	"testing"
 	"time"
 
@@ -108,9 +109,10 @@ func TestGit2Go_MergeCommandSerialization(t *testing.T) {
 
 func TestGit2Go_MergeResultSerialization(t *testing.T) {
 	serializeResult := func(t *testing.T, result MergeResult) string {
-		serialized, err := result.Serialize()
+		var buf bytes.Buffer
+		err := result.SerializeTo(&buf)
 		require.NoError(t, err)
-		return serialized
+		return buf.String()
 	}
 
 	testcases := []struct {