DEV: Provide user input to services using params key

Currently in services, we don’t make a distinction between input
parameters, options and dependencies.

This can lead to user input modifying the service behavior, whereas it
was not the developer intention.

This patch addresses the issue by changing how data is provided to
services:
- `params` is now used to hold all data coming from outside (typically
  user input from a controller) and a contract will take its values from
  `params`.
- `options` is a new key to provide options to a service. This typically
  allows changing a service behavior at runtime. It is, of course,
  totally optional.
- `dependencies` is actually anything else provided to the service (like
  `guardian`) and available directly from the context object.

The `service_params` helper in controllers has been updated to reflect
those changes, so most of the existing services didn’t need specific
changes.

The options block has the same DSL as contracts, as it’s also based on
`ActiveModel`. There aren’t any validations, though. Here’s an example:
```ruby
options do
  attribute :allow_changing_hidden, :boolean, default: false
end
```
And here’s an example of how to call a service with the new keys:
```ruby
MyService.call(params: { key1: value1, … }, options: { my_option: true }, guardian:, …)
```

lib/service.rb

@@ -40,9 +40,9 @@ module Service
   #
   # @example An example from the {TrashChannel} service
   #   class TrashChannel
-  #     include Base
+  #     include Service::Base
   #
-  #     model :channel, :fetch_channel
+  #     model :channel
   #     policy :invalid_access
   #     transaction do
   #       step :prevents_slug_collision
@@ -79,17 +79,15 @@ module Service
   #   end
   # @example An example from the {UpdateChannelStatus} service which uses a contract
   #   class UpdateChannelStatus
-  #     include Base
+  #     include Service::Base
   #
-  #     model :channel, :fetch_channel
-  #     contract
-  #     policy :check_channel_permission
-  #     step :change_status
-  #
-  #     class Contract
+  #     model :channel
+  #     contract do
   #       attribute :status
   #       validates :status, inclusion: { in: Chat::Channel.editable_statuses.keys }
   #     end
+  #     policy :check_channel_permission
+  #     step :change_status
   #
   #     …
   #   end

lib/service/base.rb

@@ -18,7 +18,7 @@ module Service
 
     # Simple structure to hold the context of the service during its whole lifecycle.
     class Context
-      delegate :slice, to: :store
+      delegate :slice, :dig, to: :store
 
       def initialize(context = {})
         @store = context.symbolize_keys
@@ -115,6 +115,12 @@ module Service
       def transaction(&block)
         steps << TransactionStep.new(&block)
       end
+
+      def options(&block)
+        klass = Class.new(Service::OptionsBase).tap { _1.class_eval(&block) }
+        const_set("Options", klass)
+        steps << OptionsStep.new(:default, class_name: klass)
+      end
     end
 
     # @!visibility private
@@ -196,7 +202,7 @@ module Service
         attributes = class_name.attribute_names.map(&:to_sym)
         default_values = {}
         default_values = context[default_values_from].slice(*attributes) if default_values_from
-        contract = class_name.new(default_values.merge(context.slice(*attributes)))
+        contract = class_name.new(default_values.merge(context[:params].slice(*attributes)))
         context[contract_name] = contract
         context[result_key] = Context.build
         if contract.invalid?
@@ -208,9 +214,13 @@ module Service
       private
 
       def contract_name
-        return :contract if name.to_sym == :default
+        return :contract if default?
         :"#{name}_contract"
       end
+
+      def default?
+        name.to_sym == :default
+      end
     end
 
     # @!visibility private
@@ -229,6 +239,14 @@ module Service
       end
     end
 
+    # @!visibility private
+    class OptionsStep < Step
+      def call(instance, context)
+        context[result_key] = Context.build
+        context[:options] = class_name.new(context[:options])
+      end
+    end
+
     included do
       # The global context which is available from any step.
       attr_reader :context
@@ -263,7 +281,7 @@ module Service
     # customized by providing the +name+ argument).
     #
     # @example
-    #   model :channel, :fetch_channel
+    #   model :channel
     #
     #   private
     #
@@ -361,6 +379,17 @@ module Service
     #     step :log_channel_deletion
     #   end
 
+    # @!scope class
+    # @!method options(&block)
+    # @param block [Proc] a block containing options definition
+    # This is used to define options allowing to parameterize the service
+    # behavior. The resulting options are available in `context[:options]`.
+    #
+    # @example
+    #   options do
+    #     attribute :my_option, :boolean, default: false
+    #   end
+
     # @!visibility private
     def initialize(initial_context = {})
       @context = Context.build(initial_context.merge(__steps__: self.class.steps))

lib/service/options_base.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class Service::OptionsBase
+  include ActiveModel::API
+  include ActiveModel::Attributes
+  include ActiveModel::AttributeMethods
+end

lib/service/steps_inspector.rb

@@ -98,6 +98,10 @@ class Service::StepsInspector
       nil
     end
   end
+  #
+  # @!visibility private
+  class Options < Step
+  end
 
   attr_reader :steps, :result