To ensure consistent fieldpath matching for events, we generate the
fieldpath matching using protobuf definitions. This is done through a
plugin called "fieldpath" that defines a `Field` method for each type
with the plugin enabled. Generated code handles top-level envelope
fields, as well as deferred serialization for matching any types.
In practice, this means that we can cheaply match events on `topic` and
`namespace`. If we want to match on attributes within the event, we can
use the `event` prefix to address these fields. For example, the
following will match all envelopes that have a field named
`container_id` that has the value `testing`:
```
ctr events "event.container_id==testing"
```
The above will decode the underlying event and check that particular
field. Accordingly, if only `topic` or `namespace` is used, the event
will not be decoded and only match on the envelope.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
