Make all SCMs inherit from SCM::Plugin

This makes it much easier (no hacks needed) to determine whether the
user has installed an SCM plugin: simply check that the plugin responds
to `scm?` and that it returns true. A new SCM::Plugin base class makes
it easy to write such a plugin.

This commit also updates the website docs that explain how to write SCM
plugins so that they refer to this new technique.

docs/documentation/advanced-features/custom-scm/index.markdown

@@ -20,20 +20,20 @@ Capistrano checks out your application's source code. SCM plugins can be
 packaged as Ruby gems and distributed to other users.
 
 This document is a short guide to writing your own plugin. *It applies to
-Capistrano 3.5.0 and newer.*
+Capistrano 3.7.0 and newer.*
 
-### 1. Write a Ruby class that extends Capistrano::Plugin
+### 1. Write a Ruby class that extends Capistrano::SCM::Plugin
 
 Let's say you want to create a "Foo" SCM. You'll need to write a plugin class,
 like this:
 
 ```ruby
-require "capistrano/plugin"
+require "capistrano/scm/plugin"
 
 # By convention, Capistrano plugins are placed in the
 # Capistrano namespace. This is completely optional.
 module Capistrano
-  class FooPlugin < ::Capistrano::Plugin
+  class FooPlugin < ::Capistrano::SCM::Plugin
     def set_defaults
       # Define any variables needed to configure the plugin.
       # set_if_empty :myvar, "my-default-value"
@@ -42,11 +42,13 @@ module Capistrano
 end
 ```
 
-### 2. Implement the create_release task
+### 2. Implement a create_release task
 
 When the user runs `cap deploy`, your SCM is responsible for creating the
 release directory and copying the application source code into it. You need to
-do this using a `create_release` task that is namespaced to your plugin.
+do this using a task that is registered to run after `deploy:new_release_path`.
+
+By convention (not a requirement), this task is called `create_release`.
 
 Inside your plugin class, use the `define_tasks` and `register_hooks` methods
 like this:
@@ -56,7 +58,6 @@ def define_tasks
   # The namespace can be whatever you want, but its best
   # to choose a name that matches your plugin name.
   namespace :foo do
-    # The task *must* be named `create_release`
     task :create_release do
       # Your code to create the release directory and copy
       # the source code into it goes here.

lib/capistrano/configuration.rb

@@ -140,6 +140,10 @@ module Capistrano
                         load_immediately: load_immediately)
     end
 
+    def scm_plugin_installed?
+      installer.scm_installed?
+    end
+
     def servers
       @servers ||= Servers.new
     end

lib/capistrano/configuration/plugin_installer.rb

@@ -26,6 +26,7 @@ module Capistrano
 
         plugin.define_tasks
         plugin.register_hooks if load_hooks
+        @scm_installed ||= provides_scm?(plugin)
 
         if load_immediately
           plugin.set_defaults
@@ -35,6 +36,16 @@ module Capistrano
           end
         end
       end
+
+      def scm_installed?
+        @scm_installed
+      end
+
+      private
+
+      def provides_scm?(plugin)
+        plugin.respond_to?(:scm?) && plugin.scm?
+      end
     end
   end
 end

lib/capistrano/configuration/scm_resolver.rb

@@ -29,7 +29,8 @@ module Capistrano
         set(:scm, :git) if using_default_scm?
 
         print_deprecation_warnings_if_applicable
-        return if scm_plugin_loaded?
+        # Note that `scm_plugin_installed?` comes from Capistrano::DSL
+        return if scm_plugin_installed?
 
         if built_in_scm_name?
           load_built_in_scm
@@ -47,13 +48,6 @@ module Capistrano
         @using_default_scm = (fetch(:scm) == DEFAULT_GIT)
       end
 
-      # This is somewhat of a hack, because there is no guarantee that a third-
-      # party SCM will necessarily implement the typical SCM tasks. But it works
-      # well enough for the built-in SCMs.
-      def scm_plugin_loaded?
-        Rake::Task.tasks.any? { |t| t.name =~ /^[^:]+:create_release/ }
-      end
-
       def scm_name
         fetch(:scm)
       end
@@ -102,7 +96,7 @@ module Capistrano
 
       def print_deprecation_warnings_if_applicable
         if using_default_scm?
-          warn_add_git_to_capfile unless scm_plugin_loaded?
+          warn_add_git_to_capfile unless scm_plugin_installed?
         elsif built_in_scm_name?
           warn_set_scm_is_deprecated
         elsif third_party_scm_name?

lib/capistrano/dsl/env.rb

@@ -8,7 +8,7 @@ module Capistrano
                      :configure_backend, :fetch, :set, :set_if_empty, :delete,
                      :ask, :role, :server, :primary, :validate, :append,
                      :remove, :dry_run?, :install_plugin, :any?, :is_question?,
-                     :configure_scm
+                     :configure_scm, :scm_plugin_installed?
 
       def roles(*names)
         env.roles_for(names.flatten)

lib/capistrano/scm/git.rb

@@ -1,7 +1,6 @@
-require "capistrano/plugin"
-require "capistrano/scm"
+require "capistrano/scm/plugin"
 
-class Capistrano::SCM::Git < Capistrano::Plugin
+class Capistrano::SCM::Git < Capistrano::SCM::Plugin
   def set_defaults
     set_if_empty :git_shallow_clone, false
     set_if_empty :git_wrapper_path, lambda {

lib/capistrano/scm/hg.rb

@@ -1,7 +1,6 @@
-require "capistrano/plugin"
-require "capistrano/scm"
+require "capistrano/scm/plugin"
 
-class Capistrano::SCM::Hg < Capistrano::Plugin
+class Capistrano::SCM::Hg < Capistrano::SCM::Plugin
   def register_hooks
     after "deploy:new_release_path", "hg:create_release"
     before "deploy:check", "hg:check"

lib/capistrano/scm/plugin.rb

@@ -0,0 +1,13 @@
+require "capistrano/plugin"
+require "capistrano/scm"
+
+# Base class for all built-in and third-party SCM plugins. Notice that this
+# class doesn't really do anything other than provide an `scm?` predicate. This
+# tells Capistrano that the plugin provides SCM functionality. All other plugin
+# features are inherited from Capistrano::Plugin.
+#
+class Capistrano::SCM::Plugin < Capistrano::Plugin
+  def scm?
+    true
+  end
+end

lib/capistrano/scm/svn.rb

@@ -1,7 +1,6 @@
-require "capistrano/plugin"
-require "capistrano/scm"
+require "capistrano/scm/plugin"
 
-class Capistrano::SCM::Svn < Capistrano::Plugin
+class Capistrano::SCM::Svn < Capistrano::SCM::Plugin
   def register_hooks
     after "deploy:new_release_path", "svn:create_release"
     before "deploy:check", "svn:check"