Modify aggregate to only encode a write concern if $out is specified.

GODRIVER-666

Change-Id: Icfbb86359ce3e3a2b5678a75c3727993053ccdcc

Author: Divjot Arora

x/network/command/aggregate.go

@@ -58,11 +58,12 @@ func (a *Aggregate) encode(desc description.SelectedServer) (*Read, error) {
 	}
 
 	cursor := bsonx.Doc{}
+	hasOutStage := a.HasDollarOut()
 
 	for _, opt := range a.Opts {
 		switch opt.Key {
 		case "batchSize":
-			if opt.Value.Int32() == 0 && a.HasDollarOut() {
+			if opt.Value.Int32() == 0 && hasOutStage {
 				continue
 			}
 			cursor = append(cursor, opt)
@@ -73,7 +74,7 @@ func (a *Aggregate) encode(desc description.SelectedServer) (*Read, error) {
 	command = append(command, bsonx.Elem{"cursor", bsonx.Document(cursor)})
 
 	// add write concern because it won't be added by the Read command's Encode()
-	if a.WriteConcern != nil {
+	if desc.WireVersion.Max >= 5 && hasOutStage && a.WriteConcern != nil {
 		element, err := a.WriteConcern.MarshalBSONElement()
 		if err != nil {
 			return nil, err

x/network/command/aggregate_test.go

@@ -0,0 +1,64 @@
+package command
+
+import (
+	"github.com/mongodb/mongo-go-driver/internal/testutil/helpers"
+	"github.com/mongodb/mongo-go-driver/mongo/writeconcern"
+	"github.com/mongodb/mongo-go-driver/x/bsonx"
+	"github.com/mongodb/mongo-go-driver/x/network/description"
+	"testing"
+)
+
+func TestAggregate(t *testing.T) {
+	wc := writeconcern.New(writeconcern.W(10))
+	legacyDesc := description.SelectedServer{
+		Server: description.Server{
+			WireVersion: &description.VersionRange{
+				Max: 4,
+			},
+		},
+	}
+	desc := description.SelectedServer{
+		Server: description.Server{
+			WireVersion: &description.VersionRange{
+				Max: 5,
+			},
+		},
+	}
+	outDoc := bsonx.Doc{{"$out", bsonx.Int32(1)}}
+	outPipeline := bsonx.Arr{bsonx.Document(outDoc)}
+
+	testCases := []struct {
+		name       string
+		desc       description.SelectedServer
+		pipeline   bsonx.Arr
+		wcExpected bool
+	}{
+		{"LegacyDescNoOut", legacyDesc, bsonx.Arr{}, false},
+		{"LegacyDescOut", legacyDesc, outPipeline, false},
+		{"NewDescNoOut", desc, bsonx.Arr{}, false},
+		{"NewDescOut", desc, outPipeline, true},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := Aggregate{
+				NS: Namespace{
+					DB:         "db",
+					Collection: "coll",
+				},
+				Pipeline:     tc.pipeline,
+				WriteConcern: wc,
+			}
+
+			readCmd, err := cmd.encode(tc.desc)
+			testhelpers.RequireNil(t, err, "error encoding: %s", err)
+
+			_, err = readCmd.Command.LookupErr("writeConcern")
+			nilErr := err == nil
+			// err should be nil if wc was expected
+			if nilErr != tc.wcExpected {
+				t.Fatalf("write concern mismatch: expected %v got %v", tc.wcExpected, nilErr)
+			}
+		})
+	}
+}