What does "=!= Unknown error" indicate on Heroku build?

Question

I have a Django application running on Heroku with a web dyno on a container stack. On adding a worker dyno via my app.json and heroku.yml files, Heroku's build system for my Review app logs:

=== Fetching app code
=!= Unknown error

The application builds properly locally via Docker and via Docker Compose. It has been building properly on Heroku until I modified my heroku.yml file and app.json file to add a worker.

My questions are:

• What does "=?= Unknown error" signify, What part of Heroku's stack is throwing it? and what part of my configuration is likely causing it?
• How can I debug this problem?

Relevant files:

heroku.yml:

build:
  docker:
    web: Dockerfile
release:
  command:
    - ./release_commands.sh
  image: web
run:
  web:
    command: newrelic-admin run-program python manage.py runserver 0.0.0.0:$PORT
  worker:
    command: newrelic-admin run-program python manage.py rqworker app
    image: web

app.json:

{
  "name": "my-app",
  "stack": "container",
  "formation": {
    "web": {
      "quantity": 1
    },
    "worker": {
      "quantity": 1
    }
  },
  "environments": {
    "review": {
      "formation": {
        "web": {
          "quantity": 1,
          "size": "hobby"
        },
        "worker": {
          "quantity": 1,
          "size": "hobby"
        }
      },
      "addons": [
        "heroku-redis",
        {
          "plan": "heroku-postgresql",
          "options": {
            "version": 13
          }
        }
      ],
      "env": {
        "DEBUG": 1,
        "ENVIRONMENT": "dev",
        "ALLOWED_HOST": ".herokuapp.com",
        "CSRF_TRUSTED_DOMAIN": "*.herokuapp.com"
      }
    }
  }
}

Answer 1

For the next person to encounter this problem: this was corner case between two heroku.yml issues, caused by YAML ordering.

1. Heroku expects the run.<app>.command to be an array. If it isn't, Heroku will throw a parsing error (below).
2. Heroku expects run to be defined above release . If it isn't, and condition (1) is present, Heroku will fail to throw a parsing error and instead throw Unknown error .

Here is the error the user would see if the above heroku.yml were modified:

Properly ordered heroku.yml with command improperly set as a string instead of an array:

build:
  docker:
    web: Dockerfile
run:
  web:
    command: newrelic-admin run-program python manage.py runserver 0.0.0.0:$PORT
  worker:
    command: newrelic-admin run-program python manage.py rqworker app
    image: web
release:
  command:
    - ./release_commands.sh
  image: web

Yields the build error:

=== Fetching app code
=!= There were problems parsing your heroku.yml. We've detected the following issues:
run.worker.command needs to be in array format, such as:
run:
  worker:
    command:
      - newrelic-admin run-program python manage.py rqworker app heap braze
    image: web

Order shouldn't matter to YAML keys ( https://yaml.org/spec/1.2.2/#3221-mapping-key-order ). But when run is defined below release , and no other changes are made, Heroku's error output looks like this:

=== Fetching app code
=!= Unknown error

So, one cause of this Heroku error output is YAML ordering. Switching keys around fixes the problem.