A previous refactor of the `Service::Base::Step` class introduced a
non thread-safe behavior. `#call` mutates instance variables at runtime,
and since a step instance is the same for any given service class, this
can sometimes lead to `context` being the wrong one for the running
service.
This patch makes use of `Concurrent::ThreadLocalVar` to fix the issue.
There’s currently a bug when using a dedicated class as a policy in
services: if that class delegates its `#call` method (to an underlying
strategy object for example), then an error will be raised saying steps
aren’t allowed to provide default parameters.
This should not happen, and this patch fixes that issue.
Currently, there are two ways (kind of) for accessing `params` inside a
service:
- when there is no contract or it hasn’t been reached yet, `params` is
just the hash that was provided to the service. To access a key, you
have to use the bracket notation `params[:my_key]`.
- when there is a contract and it has been executed successfully,
`params` now references the contract and the attributes are accessible
using methods (`params.my_key`).
This patch unifies how `params` exposes its attributes. Now, even if
there is no contract at all in a service, `params` will expose its
attributes through methods, that way things are more consistent.
This patch also makes sure there is always a `params` object available
even when no `params` key is provided to the service (this allows a
contract to fail because its attributes are blank instead of having the
service raising an error because it doesn’t find `params` in its context).
With the current implementation, a service step can be written as:
```ruby
def my_step(a_default_value: 2)
…
end
```
That’s a pattern we want to avoid as default values (if needed) should
be probably defined in a contract.
This patch makes a service raise an exception if a default value is
encountered.
