feat(apmgrpc): wrap the server-stream with transaction-ctx (#1151)

* feat(apmgrpc): wrap the server-stream with transaction-ctx

This allows the handler to retrieve the transaction from it's context, to e.g. create spans.

* define wrappedServerStream instead of importing from go-grpc-middleware

module/apmgrpc/go.sum

@@ -48,8 +48,6 @@ github.com/santhosh-tekuri/jsonschema v1.2.4/go.mod h1:TEAUOeZSmIxTTuHatJzrvARHi
 github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
-github.com/stretchr/testify v1.4.0 h1:2E4SXV/wtOkTonXsotYi4li6zVWxYlZuYNCXe9XRJyk=
-github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
 github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
 github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/yuin/goldmark v1.1.27 h1:nqDD4MMMQA0lmWq03Z2/myGPYLQoXtmi0rGVs95ntbo=

module/apmgrpc/server.go

@@ -134,6 +134,9 @@ func NewStreamServerInterceptor(o ...ServerOption) grpc.StreamServerInterceptor
 		tx, ctx := startTransaction(ctx, opts.tracer, info.FullMethod)
 		defer tx.End()
 
+		wrapped := wrapServerStream(stream)
+		wrapped.wrappedContext = ctx
+
 		// TODO(axw) define span context schema for RPC,
 		// including at least the peer address.
 
@@ -153,7 +156,7 @@ func NewStreamServerInterceptor(o ...ServerOption) grpc.StreamServerInterceptor
 			}
 			setTransactionResult(tx, err)
 		}()
-		return handler(srv, stream)
+		return handler(srv, wrapped)
 	}
 }
 
@@ -311,3 +314,20 @@ func WithServerStreamIgnorer(s StreamIgnorerFunc) ServerOption {
 		o.streamIgnorer = s
 	}
 }
+
+// wrappedServerStream is a thin wrapper around grpc.ServerStream that allows modifying context.
+type wrappedServerStream struct {
+	grpc.ServerStream
+	// wrappedContext is the wrapper's own Context. You can assign it.
+	wrappedContext context.Context
+}
+
+// Context returns the wrapper's WrappedContext, overwriting the nested grpc.ServerStream.Context()
+func (w *wrappedServerStream) Context() context.Context {
+	return w.wrappedContext
+}
+
+// wrapServerStream returns a ServerStream that has the ability to overwrite context.
+func wrapServerStream(stream grpc.ServerStream) *wrappedServerStream {
+	return &wrappedServerStream{ServerStream: stream, wrappedContext: stream.Context()}
+}

module/apmgrpc/server_test.go

@@ -261,7 +261,7 @@ func TestServerStream(t *testing.T) {
 	tracer, transport := transporttest.NewRecorderTracer()
 	defer tracer.Close()
 
-	s, _, addr := newAccumulatorServer(t, tracer, apmgrpc.WithRecovery())
+	s, accumulatorServer, addr := newAccumulatorServer(t, tracer, apmgrpc.WithRecovery())
 	defer s.GracefulStop()
 
 	conn, client := newAccumulatorClient(t, addr)
@@ -290,6 +290,13 @@ func TestServerStream(t *testing.T) {
 	tracer.Flush(nil)
 	transactions := transport.Payloads().Transactions
 	require.Len(t, transactions, 1)
+
+	// The transaction should have propagated into the accumulatorServer
+	require.NotNil(t, accumulatorServer.transactionFromContext)
+	expectedTraceID := fmt.Sprintf("%x", transactions[0].TraceID)
+	actualTraceID := accumulatorServer.transactionFromContext.TraceContext().Trace.String()
+	require.NotEmpty(t, expectedTraceID)
+	require.Equal(t, expectedTraceID, actualTraceID)
 }
 
 func TestServerTLS(t *testing.T) {
@@ -457,9 +464,12 @@ func (s *helloworldServer) SayHello(ctx context.Context, req *pb.HelloRequest) (
 type accumulator struct {
 	panic bool
 	err   error
+
+	transactionFromContext *apm.Transaction
 }
 
 func (a *accumulator) Accumulate(srv testservice.Accumulator_AccumulateServer) error {
+	a.transactionFromContext = apm.TransactionFromContext(srv.Context())
 	if a.panic {
 		panic(a.err)
 	}