Add EVENT with subscription ID message type.

envelope/find.go

@@ -33,7 +33,7 @@ func ParseElement(element json.RawMessage, value interface{}, position string) e
 	return nil
 }
 
-// FindEvent extracts an event from an EVENT envelope with no subscription ID.
+// FindEvent extracts an event from an EVENT envelope
 // Expected Format: ["EVENT", event]
 func FindEvent(env Envelope) ([]byte, error) {
 	var arr []json.RawMessage
@@ -57,6 +57,35 @@ func FindEvent(env Envelope) ([]byte, error) {
 	return arr[1], nil
 }
 
+// FindEventWithReq extracts an event from an EVENT envelope with a subscription ID.
+// Expected Format: ["EVENT", "SUBID", event]
+func FindEventWithReq(env Envelope) (string, []byte, error) {
+	var arr []json.RawMessage
+	if err := json.Unmarshal(env, &arr); err != nil {
+		return "", nil, fmt.Errorf("%w: %v", errors.InvalidJSON, err)
+	}
+
+	if err := CheckArrayLength(arr, 3); err != nil {
+		return "", nil, err
+	}
+
+	var label string
+	if err := ParseElement(arr[0], &label, "envelope label"); err != nil {
+		return "", nil, err
+	}
+
+	if err := CheckLabel(label, "EVENT"); err != nil {
+		return "", nil, err
+	}
+
+	var req string
+	if err := ParseElement(arr[1], &req, "request id"); err != nil {
+		return "", nil, err
+	}
+
+	return req, arr[2], nil
+}
+
 // FindSubscriptionEvent extracts an event and subscription ID from an EVENT envelope.
 // Expected Format: ["EVENT", subID, event]
 func FindSubscriptionEvent(env Envelope) (subID string, event []byte, err error) {

envelope/find_test.go

@@ -65,6 +65,68 @@ func TestFindEvent(t *testing.T) {
 	}
 }
 
+func TestFindEventWithReq(t *testing.T) {
+	cases := []struct {
+		name        string
+		env         Envelope
+		wantReq     string
+		wantEvent   []byte
+		wantErr     error
+		wantErrText string
+	}{
+		{
+			name:      "valid event",
+			env:       []byte(`["EVENT","SUBID",{"id":"abc123","kind":1}]`),
+			wantReq:   "SUBID",
+			wantEvent: []byte(`{"id":"abc123","kind":1}`),
+		},
+		{
+			name:        "wrong label",
+			env:         []byte(`["REQ","SUBID",{"id":"abc123","kind":1}]`),
+			wantErr:     errors.WrongEnvelopeLabel,
+			wantErrText: "expected EVENT, got REQ",
+		},
+		{
+			name:    "invalid json",
+			env:     []byte(`invalid`),
+			wantErr: errors.InvalidJSON,
+		},
+		{
+			name:        "missing elements",
+			env:         []byte(`["EVENT","SUBID"]`),
+			wantErr:     errors.InvalidEnvelope,
+			wantErrText: "expected 3 elements, got 2",
+		},
+		{
+			name:      "extraneous elements",
+			env:       []byte(`["EVENT","SUBID",{"id":"abc123"},"extra"]`),
+			wantReq:   "SUBID",
+			wantEvent: []byte(`{"id":"abc123"}`),
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			gotReq, gotEvent, err := FindEventWithReq(tc.env)
+
+			if tc.wantErr != nil || tc.wantErrText != "" {
+				if tc.wantErr != nil {
+					assert.ErrorIs(t, err, tc.wantErr)
+				}
+
+				if tc.wantErrText != "" {
+					assert.ErrorContains(t, err, tc.wantErrText)
+				}
+				return
+			}
+
+			assert.NoError(t, err)
+			assert.Equal(t, tc.wantReq, gotReq)
+			assert.Equal(t, tc.wantEvent, gotEvent)
+		})
+	}
+}
+
 func TestFindSubscriptionEvent(t *testing.T) {
 	cases := []struct {
 		name        string