fix: return Google Error when there is a missing required parameter

src/fallbackRest.ts

@@ -45,11 +45,13 @@ export function encodeRequest(
   if (typeof json !== 'object' || Array.isArray(json)) {
     throw new Error(`Request to RPC ${rpc.name} must be an object.`);
   }
+
   const transcoded = transcode(
     json,
     rpc.parsedOptions,
     rpc.resolvedRequestType!.fields
   );
+
   if (!transcoded) {
     throw new Error(
       `Cannot build HTTP request for ${JSON.stringify(json)}, method: ${

src/fallbackServiceStub.ts

@@ -86,19 +86,29 @@ export function generateServiceStub(
       // We cannot use async-await in this function because we need to return the canceller object as soon as possible.
       // Using plain old promises instead.
 
+      let fetchParameters: FetchParameters;
+      try {
+        fetchParameters = requestEncoder(
+          rpc,
+          protocol,
+          servicePath,
+          servicePort,
+          request
+        );
+      } catch (err) {
+        // we could not encode parameters; pass error to the callback
+        // and return a no-op canceler object.
+        callback(err);
+        return {
+          cancel() {},
+        };
+      }
+
       const cancelController = hasAbortController()
         ? new AbortController()
         : new NodeAbortController();
       const cancelSignal = cancelController.signal as AbortSignal;
       let cancelRequested = false;
-
-      const fetchParameters = requestEncoder(
-        rpc,
-        protocol,
-        servicePath,
-        servicePort,
-        request
-      );
       const url = fetchParameters.url;
       const headers = fetchParameters.headers;
       for (const key of Object.keys(options)) {

src/transcoding.ts

@@ -20,6 +20,7 @@
 import {JSONObject, JSONValue} from 'proto3-json-serializer';
 import {Field, Type} from 'protobufjs';
 import {google} from '../protos/http';
+import {GoogleError} from './googleError';
 import {camelToSnakeCase, toCamelCase as snakeToCamelCase} from './util';
 
 export interface TranscodedRequest {
@@ -340,7 +341,7 @@ export function transcode(
   // all fields annotated as REQUIRED MUST be emitted in the body.
   for (const requiredField of requiredFields) {
     if (!(requiredField in request) || request[requiredField] === undefined) {
-      throw new Error(
+      throw new GoogleError(
         `Required field ${requiredField} is not present in the request.`
       );
     }

test/unit/transcoding.ts

@@ -32,6 +32,7 @@ import {
   buildQueryStringComponents,
   requestChangeCaseAndCleanup,
   overrideHttpRules,
+  TranscodedRequest,
 } from '../../src/transcoding';
 import * as assert from 'assert';
 import {
@@ -615,7 +616,7 @@ describe('validate proto3 field with default value', () => {
     ];
     const transcoded = transcode(request, parsedOptions, badTestMessageFields);
     assert.deepStrictEqual(
-      transcoded?.url,
+      (transcoded as TranscodedRequest)?.url,
       'projects/test-project/contents/test-content'
     );
   });
@@ -633,7 +634,7 @@ describe('validate proto3 field with default value', () => {
     ];
     assert.throws(
       () => transcode(request, parsedOptions, testMessageFields),
-      Error
+      /Error: Required field content is not present in the request/
     );
   });
   it('when body="*", all required field should emitted in body', () => {
@@ -650,8 +651,13 @@ describe('validate proto3 field with default value', () => {
       },
     ];
     const transcoded = transcode(request, parsedOptions, testMessageFields);
-    assert.deepStrictEqual(transcoded?.url, 'projects/test-project');
-    assert.deepStrictEqual(transcoded?.data, {content: 'test-content'});
+    assert.deepStrictEqual(
+      (transcoded as TranscodedRequest)?.url,
+      'projects/test-project'
+    );
+    assert.deepStrictEqual((transcoded as TranscodedRequest)?.data, {
+      content: 'test-content',
+    });
   });
   it('when body="*", unset optional field should remove from body', () => {
     const request: RequestType = {
@@ -668,10 +674,10 @@ describe('validate proto3 field with default value', () => {
     ];
     const transcoded = transcode(request, parsedOptions, testMessageFields);
     assert.deepStrictEqual(
-      transcoded?.url,
+      (transcoded as TranscodedRequest)?.url,
       'projects/test-project/contents/test-content'
     );
-    assert.deepStrictEqual(transcoded?.data, {});
+    assert.deepStrictEqual((transcoded as TranscodedRequest)?.data, {});
   });
   it('unset optional fields should not appear in query params', () => {
     const request: RequestType = {
@@ -690,9 +696,15 @@ describe('validate proto3 field with default value', () => {
       },
     ];
     const transcoded = transcode(request, parsedOptions, testMessageFields);
-    assert.deepStrictEqual(transcoded?.url, 'projects/test-project');
-    assert.deepStrictEqual(transcoded?.data, 'test-content');
-    assert.strictEqual(transcoded.queryString, '');
+    assert.deepStrictEqual(
+      (transcoded as TranscodedRequest)?.url,
+      'projects/test-project'
+    );
+    assert.deepStrictEqual(
+      (transcoded as TranscodedRequest)?.data,
+      'test-content'
+    );
+    assert.deepStrictEqual((transcoded as TranscodedRequest).queryString, '');
   });
 });