FirebaseAppDistributionApi().media.upload() failing to upload

[AbhijithKonnayil]

follow up of issue : #674

When trying to upload apk to firebase app distribution with FirebaseAppDistributionApi it fails with 400 without proper error message on what is the issue in the request.

DetailedApiRequestError(status: 400, message: No error details. HTTP status was: 400.)

I checked with the google-api-python-client and the firebase app distribution api is working fine in python and APK has successfully uploaded to firebase.

I compared the curl request sent by the python client and googleapis.dart.

The critical differences are as follows

Header	Python	dart
content-type	application/vnd.android.package-archive	multipart/related; boundary="314159265358979323846"
content-length	18,935,247 (actual apk size)	25,247,203

I experimented with the googleapis.dart and here are my findings

1. The protocol mentioned in disovery apis is simple with multipart set to true

            "protocols": {
                     "simple": {
                                "multipart": true,
                                "path": "/upload/v1/{+app}/releases:upload"
                            }
                        }

2. APK will get uploaded when using the simpleUpload() in discoveryapis_commons > api_requester.dart > ApiRequester - class > _request().
   1. But it will use simpleUpload() only if body parameter in _request() is null.
   2. The body parameter gets its value by JSON encoding the request object passed to upload() in generated > googleapis > firebaseappdistribution > v1.dart > MediaSource - class > upload()

      async.Future<GoogleLongrunningOperation> upload(
          GoogleFirebaseAppdistroV1UploadReleaseRequest request,
          core.String app, {
          core.String? $fields,
          commons.Media? uploadMedia,
        }) async {
          final body_ = convert.json.encode(request);

   3. Even if the requeset passed to upload has no paramentes passed , it will enocode as "{}"
   4. The toJson() method in GoogleFirebaseAppdistroV1UploadReleaseRequest is non nullable

in short , to upload using simpleUpload() the body_ should be null

3. By default , the package tries to upload using MultipartMediaUploader-class > upload()
   1. Here the content type is chaneged to multipart/related; boundary="314159265358979323846" and the content length is changed accordingly to combine Json data and encoded binary file.

   . Are we supposed to use this for upload. The rest api in firebase mention only about binary as body. https://firebase.google.com/docs/reference/app-distribution/rest/v1/upload.v1.projects.apps.releases/upload#http-request