Tidy up role and host commander setup

Extract Kamal::Commander::Specifics to deal with host and role setup and
ensure that primary hosts and roles always come first. This means that
in a rolling deploy we deploy to the primary ones first.

This will be important when we remove the healthcheck step as we want
to confirm the primary host can be deployed to before completing a
deployment for other roles.

By setting the hosts and roles all together in one place we can sort
the primary ones to the front without creating infinite loops.

lib/kamal/commander.rb

@@ -3,11 +3,13 @@ require "active_support/core_ext/module/delegation"
 
 class Kamal::Commander
   attr_accessor :verbosity, :holding_lock, :hold_lock_on_error
+  delegate :hosts, :roles, :primary_host, :primary_role, :roles_on, :traefik_hosts, :accessory_hosts, to: :specifics
 
   def initialize
     self.verbosity = :info
     self.holding_lock = false
     self.hold_lock_on_error = false
+    @specifics = nil
   end
 
   def config
@@ -24,10 +26,12 @@ class Kamal::Commander
   attr_reader :specific_roles, :specific_hosts
 
   def specific_primary!
+    @specifics = nil
     self.specific_hosts = [ config.primary_host ]
   end
 
   def specific_roles=(role_names)
+    @specifics = nil
     if role_names.present?
       @specific_roles = Kamal::Utils.filter_specific_items(role_names, config.roles)
 
@@ -40,6 +44,7 @@ class Kamal::Commander
   end
 
   def specific_hosts=(hosts)
+    @specifics = nil
     if hosts.present?
       @specific_hosts = Kamal::Utils.filter_specific_items(hosts, config.all_hosts)
 
@@ -51,39 +56,6 @@ class Kamal::Commander
     end
   end
 
-  def primary_host
-    # Given a list of specific roles, make an effort to match up with the primary_role
-    specific_hosts&.first || specific_roles&.detect { |role| role == config.primary_role }&.primary_host || specific_roles&.first&.primary_host || config.primary_host
-  end
-
-  def primary_role
-    roles_on(primary_host).first
-  end
-
-  def roles
-    (specific_roles || config.roles).select do |role|
-      ((specific_hosts || config.all_hosts) & role.hosts).any?
-    end
-  end
-
-  def hosts
-    (specific_hosts || config.all_hosts).select do |host|
-      (specific_roles || config.roles).flat_map(&:hosts).include?(host)
-    end
-  end
-
-  def roles_on(host)
-    roles.select { |role| role.hosts.include?(host.to_s) }
-  end
-
-  def traefik_hosts
-    specific_hosts || config.traefik_hosts
-  end
-
-  def accessory_hosts
-    specific_hosts || config.accessories.flat_map(&:hosts)
-  end
-
   def accessory_names
     config.accessories&.collect(&:name) || []
   end
@@ -181,4 +153,8 @@ class Kamal::Commander
       SSHKit.config.command_map[:docker] = "docker" # No need to use /usr/bin/env, just clogs up the logs
       SSHKit.config.output_verbosity = verbosity
     end
+
+    def specifics
+      @specifics ||= Kamal::Commander::Specifics.new(config, specific_hosts, specific_roles)
+    end
 end

lib/kamal/commander/specifics.rb

@@ -0,0 +1,49 @@
+class Kamal::Commander::Specifics
+  attr_reader :primary_host, :primary_role, :hosts, :roles
+  delegate :stable_sort!, to: Kamal::Utils
+
+  def initialize(config, specific_hosts, specific_roles)
+    @config, @specific_hosts, @specific_roles = config, specific_hosts, specific_roles
+
+    @roles, @hosts = specified_roles, specified_hosts
+
+    @primary_host = specific_hosts&.first || primary_specific_role&.primary_host || config.primary_host
+    @primary_role = primary_or_first_role(roles_on(primary_host))
+
+    stable_sort!(roles) { |role| role == primary_role ? 0 : 1 }
+    stable_sort!(hosts) { |host| roles_on(host).any? { |role| role == primary_role } ? 0 : 1 }
+  end
+
+  def roles_on(host)
+    roles.select { |role| role.hosts.include?(host.to_s) }
+  end
+
+  def traefik_hosts
+    specific_hosts || config.traefik_hosts
+  end
+
+  def accessory_hosts
+    specific_hosts || config.accessories.flat_map(&:hosts)
+  end
+
+  private
+    attr_reader :config, :specific_hosts, :specific_roles
+
+    def primary_specific_role
+      primary_or_first_role(specific_roles) if specific_roles.present?
+    end
+
+    def primary_or_first_role(roles)
+      roles.detect { |role| role == config.primary_role } || roles.first
+    end
+
+    def specified_roles
+      (specific_roles || config.roles) \
+        .select { |role| ((specific_hosts || config.all_hosts) & role.hosts).any? }
+    end
+
+    def specified_hosts
+      (specific_hosts || config.all_hosts) \
+        .select { |host| (specific_roles || config.roles).flat_map(&:hosts).include?(host) }
+    end
+end

lib/kamal/utils.rb

@@ -74,4 +74,8 @@ module Kamal::Utils
 
     matches
   end
+
+  def stable_sort!(elements, &block)
+    elements.sort_by!.with_index { |element, index| [ block.call(element), index ] }
+  end
 end