fix(lambda.js): catch unhandled promise rejections (#403)

src/wrappers/lambda.js

@@ -111,6 +111,17 @@ function baseLambdaWrapper(
                 process._events.beforeExit = originalBeforeExit;
             });
         };
+        // catch an unhandled promise rejection within the user's code
+        process.once('unhandledRejection', (reason) => {
+            utils.debugLog('Unhandled Promise Rejection caught. Please handle the rejected promise');
+            const rejectedError = {
+                name: 'UnhandledPromiseRejectionError',
+                message: reason || '',
+                stack: '',
+            };
+            eventInterface.setException(runner, rejectedError, false);
+        });
+
 
         const handleUserExecutionDone = (error, result, sendSync) => {
             clearTimeout(timeoutHandler);

test/unit_tests/wrappers/test_lambda.js

@@ -100,6 +100,7 @@ describe('lambdaWrapper tests', () => {
             done() {},
         };
         consts.COLD_START = true;
+        process.setMaxListeners(0);
     });
 
     afterEach(() => {
@@ -277,6 +278,17 @@ describe('lambdaWrapper tests', () => {
         }, 1);
     });
 
+    it('lambdaWrapper: catch unhandled rejected promise', (done) => {
+        this.stubFunction = sinon.spy(() => {
+            // eslint-disable-next-line prefer-promise-reject-errors,no-new
+            new Promise((_, reject) => reject('Unauthorized'));
+            return 'success';
+        });
+        this.wrappedStub = lambdaWrapper.lambdaWrapper(this.stubFunction);
+        expect(this.wrappedStub({}, null, this.callbackStub)).to.equal('success');
+        done();
+    });
+
     it('lambdaWrapper: update COLD_START value', () => {
         consts.COLD_START = true;
         this.wrappedStub({}, this.context, this.callbackStub);