Nexus: Better handling of nil and empty payloads (#6616)

## What changed?

Improve conversion of payloads with empty metadata and nil workflow
results to Nexus Content.

NOTE: This also upgrades the Go SDK from 1.27.0 to 1.29.1 to test Nexus
via the SDK.

## Why?

Handle a couple of edge cases that weren't properly handled before.
The fix in `mutable_state_impl.go` is the more critical one and may
cause surprising behavior in the SDKs.

## How did you test it?

Added tests.

common/nexus/payload_serializer.go

@@ -129,11 +129,13 @@ func (payloadSerializer) Serialize(v any) (*nexus.Content, error) {
 		return nil, fmt.Errorf("%w: cannot serialize %v", errSerializer, v)
 	}
 
+	// Use the "nil" Nexus Content representation for nil Payloads.
 	if payload == nil {
-		return &nexus.Content{}, nil
+		// Use same structure as the nil serializer from the Nexus Go SDK.
+		return &nexus.Content{Header: nexus.Header{}}, nil
 	}
 
-	if payload.GetMetadata() == nil {
+	if len(payload.GetMetadata()) == 0 {
 		return xTemporalPayload(payload)
 	}
 

common/nexus/payload_serializer_test.go

@@ -64,6 +64,13 @@ func TestNexusPayloadSerializer(t *testing.T) {
 			inputPayload: mustToPayload(t, nil),
 			header:       nexus.Header{},
 		},
+		{
+			// Empty payload is preserved.
+			name:            "empty",
+			inputPayload:    &commonpb.Payload{},
+			expectedPayload: &commonpb.Payload{},
+			header:          nexus.Header{"type": "application/x-temporal-payload"},
+		},
 		{
 			name:         "json proto",
 			inputPayload: mustToPayload(t, commonpb.RetryPolicy{}),
@@ -104,15 +111,16 @@ func TestNexusPayloadSerializer(t *testing.T) {
 					"encoding": []byte("binary/null"),
 				},
 			},
-			// Yes this is the default value, but this test should have an explicit expectation.
-			header: nil,
+			header: nexus.Header{},
 		},
 		{
-			name:         "nil metadata",
-			inputPayload: &commonpb.Payload{},
+			name: "nil metadata non-empty data",
+			inputPayload: &commonpb.Payload{
+				Data: []byte("not empty"),
+			},
 			expectedPayload: &commonpb.Payload{
 				Metadata: map[string][]byte{},
-				Data:     []byte{},
+				Data:     []byte("not empty"),
 			},
 			header: nexus.Header{"type": "application/x-temporal-payload"},
 		},

go.mod

@@ -56,7 +56,7 @@ require (
 	go.opentelemetry.io/otel/sdk/metric v1.27.0
 	go.opentelemetry.io/otel/trace v1.27.0
 	go.temporal.io/api v1.39.1-0.20240910163028-b13574e18f3c
-	go.temporal.io/sdk v1.27.0
+	go.temporal.io/sdk v1.29.1
 	go.temporal.io/version v0.3.0
 	go.uber.org/atomic v1.11.0
 	go.uber.org/automaxprocs v1.5.3

go.sum

@@ -323,8 +323,8 @@ go.opentelemetry.io/proto/otlp v1.2.0 h1:pVeZGk7nXDC9O2hncA6nHldxEjm6LByfA2aN8IO
 go.opentelemetry.io/proto/otlp v1.2.0/go.mod h1:gGpR8txAl5M03pDhMC79G6SdqNV26naRm/KDsgaHD8A=
 go.temporal.io/api v1.39.1-0.20240910163028-b13574e18f3c h1:1mORGBzFqhOn4I+Yg/KT07UQtHV59wptGxsPRB0rGIk=
 go.temporal.io/api v1.39.1-0.20240910163028-b13574e18f3c/go.mod h1:1WwYUMo6lao8yl0371xWUm13paHExN5ATYT/B7QtFis=
-go.temporal.io/sdk v1.27.0 h1:C5oOE/IRyLcZaFoB13kEHsjvSHEnGcwT6bNys0HFFHk=
-go.temporal.io/sdk v1.27.0/go.mod h1:PnOq5f3dWuU2NAbY+yczXkIeycsIIdBtoCO62ZE0aak=
+go.temporal.io/sdk v1.29.1 h1:y+sUMbUhTU9rj50mwIZAPmcXCtgUdOWS9xHDYRYSgZ0=
+go.temporal.io/sdk v1.29.1/go.mod h1:kp//DRvn3CqQVBCtjL51Oicp9wrZYB2s6row1UgzcKQ=
 go.temporal.io/version v0.3.0 h1:dMrei9l9NyHt8nG6EB8vAwDLLTwx2SvRyucCSumAiig=
 go.temporal.io/version v0.3.0/go.mod h1:UA9S8/1LaKYae6TyD9NaPMJTZb911JcbqghI2CBSP78=
 go.uber.org/atomic v1.7.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=

service/history/workflow/mutable_state_impl.go

@@ -575,14 +575,12 @@ func (ms *MutableStateImpl) GetNexusCompletion(ctx context.Context) (nexus.Opera
 	switch ce.GetEventType() {
 	case enumspb.EVENT_TYPE_WORKFLOW_EXECUTION_COMPLETED:
 		payloads := ce.GetWorkflowExecutionCompletedEventAttributes().GetResult().GetPayloads()
-		var p *commonpb.Payload
+		var p *commonpb.Payload // default to nil, the payload serializer converts nil to Nexus nil Content.
 		if len(payloads) > 0 {
 			// All of our SDKs support returning a single value from workflows, we can safely ignore the
 			// rest of the payloads. Additionally, even if a workflow could return more than a single value,
 			// Nexus does not support it.
 			p = payloads[0]
-		} else {
-			p = &commonpb.Payload{}
 		}
 		completion, err := nexus.NewOperationCompletionSuccessful(p, nexus.OperationCompletionSuccesfulOptions{
 			Serializer: commonnexus.PayloadSerializer,