CloudEvents to legacy events mapping for legacy pubsub and legacy storage events.

[hdp617]

When converting CloudEvents to legacy events, a legacy pubsub CloudEvent has type attribute "google.cloud.pubsub.topic.v1.messagePublished". This can be converted to a legacy event eventType as

• google.pubsub.topic.publish
• providers/cloud.pubsub/eventTypes/topic.publish (deprecated).

Similarly, the CloudEvent source attribute e.g. //pubsub.googleapis.com/projects/sample-project/topics/gcf-test can be converted into a legacy event resource as

• structured format:

{
        "service":"pubsub.googleapis.com",
        "name":"projects/sample-project/topics/gcf-test",
        "type":"type.googleapis.com/google.pubsub.v1.PubsubMessage"
}

• raw path: projects/sample-project/topics/gcf-test (deprecated).

(This is also the case for legacy storage events.)

1. Is this reasonable to always convert to the non-deprecated format? User's function might still depend on the deprecated format.
2. Is there a way to distinguish CloudEvents for legacy pubsub/storage and CloudEvents for normal pubsub/storage?

cc: @grant @jskeet

[jskeet]

The legacy Storage events won't be supported as CloudEvents, as far as I'm aware.

For the legacy PubSub messages - I don't know whether there's any way of detecting that we're "expecting" the legacy event.