Add Gem hook plugin support

Support loading Overcommit hooks supplied by Gems in the same
fashion as hooks that can be provided in Repo-Specific hooks.

README.md

@@ -41,6 +41,7 @@ any Ruby code.
   * [PreRebase](#prerebase)
 * [Repo-Specific Hooks](#repo-specific-hooks)
   * [Adding Existing Git Hooks](#adding-existing-git-hooks)
+* [Gem-provided Hooks](#gem-provided-hooks)
 * [Security](#security)
 * [Contributing](#contributing)
 * [Community](#community)
@@ -671,6 +672,41 @@ of hook, see the [git-hooks documentation][GHD].
 
 [GHD]: https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks
 
+## Gem-provided Hooks
+
+For hooks which should be available from multiple repositories, but which do not
+make sense to contribute back to Overcommit, you can package them as a Ruby gem
+and `overcommit` can use them as if they were repo-specific hooks. This would be
+useful for an organization that has custom hooks they wish to apply to many
+repositories, but manage the hooks in a central way.
+
+To enable gem-provided hooks you need to set the configuration option
+`gem_plugins_enabled` to `true`.
+
+If you are using the [`gemfile`](#gemfile) configuration option or otherwise
+invoking Overcommit via Bundler then you must ensure the gems are listed
+in your Bundler configuration and Bundler will ensure that the gems are in
+Ruby's `$LOAD_PATH` so that Overcommit can find and load them.
+
+If you are not using Bundler (the gems are installed directly in your Ruby
+environment) then you must provide a path that Overcommit can `require` so
+the gem will be added to the `$LOAD_PATH`.
+
+```yaml
+gem_plugins_require: ['overcommit_site_hooks']
+```
+
+The above would result in a `require 'overcommit_site_hooks'`. The file does
+not have to implement any functional code, but merely be a placeholder sufficient
+to allow the `require` to work.
+
+The hooks themselves are implemented similarly to Repo-Specific Hooks, but need
+to be placed in a gem in the normal `overcommit` hook path within the gem. For
+example, to add `MyCustomHook` as a pre_commit hook you would put it here:
+`./lib/overcommit/hooks/pre_commit/my_custom_hook.rb` within the gem file structure.
+
+See [Repo-Specific Hooks](#repo-specific-hooks) for details.
+
 ## Security
 
 While Overcommit can make managing Git hooks easier and more convenient,

config/default.yml

@@ -34,6 +34,19 @@
 # (Generate lock file by running `bundle install --gemfile=.overcommit_gems.rb`)
 gemfile: false
 
+# If enabled, Overcommit will look for plugins provided anywhere in the
+# load path and load them as it would with built-in or repo-specific
+# hooks, allowing plugins to be provided by gems.
+# Note: If you are providing them as system gems (and not using `gemfile`
+# or a Bundler context where they are specified) then you must also set the
+# `gem_plugins_require` option.
+gem_plugins_enabled: true
+
+# If `gem_plugins_enabled` is true and you are using system gems (not via a
+# Bundler context), in order to allow Ruby to find the gems, you must list a
+# valid `require` path for each gem providing custom hooks here:
+# gem_plugins_require: [ 'overcommit_custom_hook' ]
+
 # Where to store hook plugins specific to a repository. These are loaded in
 # addition to the default hooks Overcommit comes with. The location is relative
 # to the root of the repository.

lib/overcommit.rb

@@ -17,6 +17,7 @@
 require 'overcommit/hook_signer'
 require 'overcommit/hook_loader/base'
 require 'overcommit/hook_loader/built_in_hook_loader'
+require 'overcommit/hook_loader/gem_hook_loader'
 require 'overcommit/hook_loader/plugin_hook_loader'
 require 'overcommit/interrupt_handler'
 require 'overcommit/printer'

lib/overcommit/configuration.rb

@@ -119,6 +119,15 @@ def enabled_builtin_hooks(hook_context)
         select { |hook_name| hook_enabled?(hook_context, hook_name) }
     end
 
+    # Returns the gem-provided hooks that have been enabled for a hook type.
+    def enabled_gem_hooks(hook_context)
+      @hash[hook_context.hook_class_name].keys.
+        reject { |hook_name| hook_name == 'ALL' }.
+        reject { |hook_name| built_in_hook?(hook_context, hook_name) }.
+        select { |hook_name| gem_hook?(hook_context, hook_name) }.
+        select { |hook_name| hook_enabled?(hook_context, hook_name) }
+    end
+
     # Returns the ad hoc hooks that have been enabled for a hook type.
     def enabled_ad_hoc_hooks(hook_context)
       @hash[hook_context.hook_class_name].keys.
@@ -259,6 +268,7 @@ def ad_hoc_hook?(hook_context, hook_name)
       # Ad hoc hooks are neither built-in nor have a plugin file written but
       # still have a `command` specified to be run
       !built_in_hook?(hook_context, hook_name) &&
+        !gem_hook?(hook_context, hook_name) &&
         !plugin_hook?(hook_context, hook_name) &&
         (ad_hoc_conf['command'] || ad_hoc_conf['required_executable'])
     end
@@ -270,8 +280,18 @@ def built_in_hook?(hook_context, hook_name)
                             hook_context.hook_type_name, "#{hook_name}.rb"))
     end
 
+    def gem_hook?(hook_context, hook_name)
+      hook_name = Overcommit::Utils.snake_case(hook_name)
+
+      $LOAD_PATH.any? do |path|
+        File.exist?(File.join(path, 'overcommit', 'hook',
+                              hook_context.hook_type_name, "#{hook_name}.rb"))
+      end
+    end
+
     def hook_exists?(hook_context, hook_name)
       built_in_hook?(hook_context, hook_name) ||
+        gem_hook?(hook_context, hook_name) ||
         plugin_hook?(hook_context, hook_name) ||
         ad_hoc_hook?(hook_context, hook_name)
     end

lib/overcommit/configuration_validator.rb

@@ -23,6 +23,7 @@ def validate(config, hash, options)
       check_for_missing_enabled_option(hash) unless @options[:default]
       check_for_too_many_processors(config, hash)
       check_for_verify_plugin_signatures_option(hash)
+      check_for_gem_plugins(hash)
 
       hash
     end
@@ -181,6 +182,43 @@ def check_for_verify_plugin_signatures_option(hash)
         @log.newline
       end
     end
+
+    # check for presence of Gems specified in gem_plugins by trying to load them
+    def check_for_gem_plugins(hash)
+      return unless @log
+      return unless hash['gem_plugins_enabled'] == true
+      return unless hash.key?('gem_plugins_require')
+
+      required = hash['gem_plugins_require']
+
+      unless required.is_a?(Array)
+        @log.error 'gem_plugins_require expects a list value to be set'
+        raise Overcommit::Exceptions::ConfigurationError,
+              'gem_plugins_require expects a list value'
+      end
+
+      errors = []
+
+      required.each do |path|
+        begin
+          require path
+        rescue LoadError
+          errors << "Unable to require path '#{path}' listed in gem_plugins_require."
+        end
+        @log.debug "Successfully loaded gem_plugins_require: #{path}"
+      end
+
+      return unless errors.any?
+
+      errors << 'Ensure that the gems providing requested gem_plugins_require are ' \
+                'installed on the system or specify them via the gemfile ' \
+                'configuration option'
+
+      @log.error errors.join("\n")
+      @log.newline
+      raise Overcommit::Exceptions::ConfigurationError,
+            'One or more gems specified in gem_plugins_require could not be loaded'
+    end
   end
 end
 # rubocop:enable Metrics/ClassLength, Metrics/CyclomaticComplexity, Metrics/MethodLength

lib/overcommit/hook_loader/gem_hook_loader.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Overcommit::HookLoader
+  # Responsible for loading custom hooks that users install via Gems
+  class GemHookLoader < Base
+    def load_hooks
+      @config.enabled_gem_hooks(@context).map do |hook_name|
+        underscored_hook_name = Overcommit::Utils.snake_case(hook_name)
+        require "overcommit/hook/#{@context.hook_type_name}/#{underscored_hook_name}"
+        create_hook(hook_name)
+      end
+    end
+  end
+end

lib/overcommit/hook_runner.rb

@@ -200,6 +200,11 @@ def load_hooks
 
       @hooks += HookLoader::BuiltInHookLoader.new(@config, @context, @log).load_hooks
 
+      # Load Gem-based hooks next, if gemfile in use or gem_plugins is explicitly enabled:
+      if @config['gem_plugins_enabled'] == true
+        @hooks += HookLoader::GemHookLoader.new(@config, @context, @log).load_hooks
+      end
+
       # Load plugin hooks after so they can subclass existing hooks
       @hooks += HookLoader::PluginHookLoader.new(@config, @context, @log).load_hooks
     rescue LoadError => ex

spec/overcommit/configuration_spec.rb

@@ -294,4 +294,62 @@
       end
     end
   end
+
+  describe '#enabled_gem_hooks' do
+    let(:hash) do
+      {
+        'PreCommit' => {
+          'MyCustomHook' => {
+            'enabled' => true
+          },
+          'MyOtherHook' => {
+            'enabled' => false
+          },
+        }
+      }
+    end
+
+    let(:context) { double('context') }
+    subject { config.enabled_gem_hooks(context) }
+
+    before do
+      context.stub(hook_class_name: 'PreCommit',
+                   hook_type_name: 'pre_commit')
+    end
+
+    it 'excludes hooks that are not found' do
+      subject.should_not include 'MyCustomHook'
+      subject.should_not include 'MyOtherHook'
+    end
+
+    context 'when custom hooks are found' do
+      before do
+        $LOAD_PATH.unshift('/my/custom/path/lib')
+        allow(File).to receive(:exist?).
+          with('/my/custom/path/lib/overcommit/hook/pre_commit/my_custom_hook.rb').
+          and_return(true)
+        allow(File).to receive(:exist?).
+          with(File.join(Overcommit::HOME, 'lib/overcommit/hook/pre_commit/my_custom_hook.rb')).
+          and_return(false)
+        allow(File).to receive(:exist?).
+          with('/my/custom/path/lib/overcommit/hook/pre_commit/my_other_hook.rb').
+          and_return(true)
+        allow(File).to receive(:exist?).
+          with(File.join(Overcommit::HOME, 'lib/overcommit/hook/pre_commit/my_other_hook.rb')).
+          and_return(false)
+      end
+
+      after do
+        $LOAD_PATH.shift
+      end
+
+      it 'includes hooks that are enabled and found' do
+        subject.should include 'MyCustomHook'
+      end
+
+      it 'excludes hooks that are not enable but found' do
+        subject.should_not include 'MyOtherHook'
+      end
+    end
+  end
 end

spec/overcommit/configuration_validator_spec.rb

@@ -7,8 +7,9 @@
   let(:logger) { Overcommit::Logger.new(output) }
   let(:options) { { logger: logger } }
   let(:config) { Overcommit::Configuration.new(config_hash, validate: false) }
+  let(:instance) { described_class.new }
 
-  subject { described_class.new.validate(config, config_hash, options) }
+  subject { instance.validate(config, config_hash, options) }
 
   context 'when hook has an invalid name' do
     let(:config_hash) do
@@ -115,4 +116,63 @@
       end
     end
   end
+
+  context 'when gem_plugins_require is set' do
+    let(:plugins_enabled) { true }
+    let(:plugins_require) { nil }
+
+    let(:config_hash) do
+      {
+        'gem_plugins_enabled' => plugins_enabled,
+        'gem_plugins_require' => plugins_require,
+      }
+    end
+
+    context 'when plugins_enabled is true' do
+      let(:plugins_enabled) { true }
+
+      context 'and it is not an array' do
+        let(:plugins_require) { true }
+
+        it 'raises an error' do
+          expect { subject }.to raise_error Overcommit::Exceptions::ConfigurationError
+        end
+      end
+
+      context 'and one does not load' do
+        let(:plugins_require) { %w[mygem missinggem] }
+
+        before do
+          allow(instance).to receive(:require).with('mygem').and_return(true)
+          allow(instance).to receive(:require).with('missinggem').and_raise(LoadError)
+        end
+
+        it 'raises an error' do
+          expect(logger).to receive(:error).with(/installed on the system/)
+
+          expect { subject }.to raise_error Overcommit::Exceptions::ConfigurationError
+        end
+      end
+
+      context 'and the gems load' do
+        let(:plugins_require) { ['mygem'] }
+
+        it 'is valid' do
+          expect(instance).to receive(:require).with('mygem').and_return(true)
+
+          expect { subject }.not_to raise_error
+        end
+      end
+    end
+
+    context 'when plugins_enabled is false' do
+      let(:plugins_enabled) { false }
+      let(:plugins_require) { ['one'] }
+
+      it 'loads nothing' do
+        expect(instance).not_to receive(:require)
+        subject
+      end
+    end
+  end
 end

spec/overcommit/hook_loader/gem_hook_loader_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Overcommit::HookLoader::GemHookLoader do
+  let(:hash) { {} }
+  let(:config) { Overcommit::Configuration.new(hash) }
+  let(:logger) { double('logger') }
+  let(:context) { double('context') }
+  let(:loader) { described_class.new(config, context, logger) }
+
+  describe '#load_hooks' do
+    subject(:load_hooks) { loader.send(:load_hooks) }
+
+    before do
+      context.stub(hook_class_name: 'PreCommit',
+                   hook_type_name: 'pre_commit')
+    end
+
+    it 'loads enabled gem hooks' do
+      allow(config).to receive(:enabled_gem_hooks).with(context).and_return(['MyCustomHook'])
+
+      allow(loader).to receive(:require).
+        with('overcommit/hook/pre_commit/my_custom_hook').
+        and_return(true)
+      allow(loader).to receive(:create_hook).with('MyCustomHook')
+      expect(loader).to receive(:require).with('overcommit/hook/pre_commit/my_custom_hook')
+      load_hooks
+    end
+  end
+end