Throw an error when a used source is removed and not readded

[lucaswoj]

Currently, the below code does not throw an error. Instead it quietly fails to render mapbox-layer.

map.setStyle({
  'version': 8,
  'sources': {
    'mapbox-source': {...}
  },
  'layers': [{
    'id': 'mapbox-layer',
    'source': 'mapbox-source',
    'type': 'circle'
  }]
});

map.removeSource('mapbox-source');
// THROWS

This PR causes an error to be fired in the above case while still allowing the below case where a source is removed and then re-added before the next call to Style#update

map.removeSource('mapbox-source');
map.addSource('mapbox-source', {...});
// DOES NOT THROW

fixes #1711

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[jfirebaugh]

In the aim of simplicity and consistency with native, I don't think we should support the "temporarily remove an in use source" case. If you attempt to remove an in use source, it should immediately produce an error.

[lucaswoj]

@jfirebaugh I simplified per your feedback. I prefer this way but it may cause headaches for some users relying on the edge case. What do you think about using a warning & deprecation notice rather than an error?

[jfirebaugh]

Signal the error via this.fire('error', ...) rather than an exception -- that's the intent for all style errors, and will automatically make the error behavior softer. It'll be a console error rather than stack unwinding.

Secondarily, I favor making the change now to return immediately in the error case (i.e. not removing the layer), but if you want to delay making that change for a release or two that's ok too. Let's leave the issue open if you do though.

[mourner]

Congrats on the first PR after a while!

[lucaswoj]

✅ I made the suggested changes to the error handling flow. We'll should come back to this file and get rid of existing uses of throw with some kind of migration strategy for users who depend on catching those.

[jfirebaugh]

💯