fix(services): decode GetEventInformation without object identifier

lib/services/get-event-information.js

@@ -7,14 +7,23 @@ module.exports.encode = (buffer, lastReceivedObjectId) => {
   baAsn1.encodeContextObjectId(buffer, 0, lastReceivedObjectId.type, lastReceivedObjectId.instance);
 };
 
-module.exports.decode = (buffer, offset) => {
+module.exports.decode = (buffer, offset, apduLen) => {
   let len = 0;
   const value = {};
-  const result = baAsn1.decodeTagNumberAndValue(buffer, offset + len);
-  len += result.len;
-  const decodedValue = baAsn1.decodeObjectId(buffer, offset + len);
-  len += decodedValue.len;
-  value.lastReceivedObjectId = {type: decodedValue.objectType, instance: decodedValue.instance};
+  value.lastReceivedObjectId = null;
+  // The 'lastReceivedObjectIdentifier' is optional (ASHRAE 135, 13.12.1.1). A
+  // client asking for the first batch of events omits it, which results in a
+  // service request without any payload.
+  const hasObjectId = (apduLen === undefined || apduLen > 0) &&
+    offset < buffer.length &&
+    baAsn1.decodeIsContextTag(buffer, offset, 0);
+  if (hasObjectId) {
+    const result = baAsn1.decodeTagNumberAndValue(buffer, offset + len);
+    len += result.len;
+    const decodedValue = baAsn1.decodeObjectId(buffer, offset + len);
+    len += decodedValue.len;
+    value.lastReceivedObjectId = {type: decodedValue.objectType, instance: decodedValue.instance};
+  }
   value.len = len;
   return value;
 };

test/unit/service-get-event-information.spec.js

@@ -14,6 +14,24 @@ describe('bacstack - Services layer GetEventInformation unit', () => {
       lastReceivedObjectId: {type: 8, instance: 15}
     });
   });
+
+  it('should successfully decode a request without the optional object identifier', () => {
+    const buffer = utils.getBuffer();
+    const result = baServices.getEventInformation.decode(buffer.buffer, 0, 0);
+    delete result.len;
+    expect(result).toEqual({
+      lastReceivedObjectId: null
+    });
+  });
+
+  it('should not read beyond the end of a request without payload', () => {
+    const buffer = Buffer.from([]);
+    const result = baServices.getEventInformation.decode(buffer, 0, 0);
+    delete result.len;
+    expect(result).toEqual({
+      lastReceivedObjectId: null
+    });
+  });
 });
 
 describe('GetEventInformationAcknowledge', () => {