Add gemfile option for setting Bundler Gemfile

There is a real need for specifying which versions of a gem Overcommit
hooks should use during the hook run, especially when those versions are
not yet released or older versions that what is currently on the machine
(since Ruby will always load the latest version by default).

Add support for a `gemfile` option which allows the developer to specify
the explicit list of gems and their respective versions to use during
hook runs.

Author: sds

CHANGELOG.md

@@ -14,6 +14,8 @@
   lists and the operating system command length limit
 * Add `modified_files` helper to `PostCheckout` hooks
 * Add `rewritten_commits` helper to `PostRewrite` hooks
+* Add `gemfile` option to configuration file which allows a `Gemfile` to be
+  loaded by Bundler to enforce particular gem versions during hook runs
 
 ## 0.26.0
 

README.md

@@ -25,6 +25,7 @@ repository (but unlike regular Git hooks, are stored in source control).
 * [Configuration](#configuration)
   * [Hooks](#hooks)
   * [Hook Categories](#hook-categories)
+  * [Gemfile](#gemfile)
   * [Plugin Directory](#plugin-directory)
   * [Signature Verification](#signature-verification)
 * [Built-In Hooks](#built-in-hooks)
@@ -61,6 +62,10 @@ Depending on the hooks you enable/disable for your repository, you'll need to
 ensure your development environment already has those dependencies installed.
 Most hooks will display a warning if a required executable isn't available.
 
+If you are using Bundler to manage your Ruby gem dependencies, you'll likely
+want to use the [`gemfile`](#gemfile) option to control which gem versions are
+available during your hook runs.
+
 ## Installation
 
 `overcommit` is installed via [RubyGems](https://rubygems.org/):
@@ -239,6 +244,44 @@ hooks.
 Again, you can consult the [default configuration](config/default.yml) for
 detailed examples of how the `ALL` hook can be used.
 
+### Gemfile
+
+You may want to enforce the version of Overcommit or other gems that you use in
+your git hooks. This can be done by specifying the `gemfile` option in your
+`.overcommit.yml`.
+
+The `gemfile` option tells Overcommit to load the specified file with
+[Bundler](http://bundler.io/), the standard gem dependency manager for Ruby.
+This is useful if you would like to:
+
+  - Enforce a specific version of Overcommit to use for all hook runs
+    (or to use a version from the master branch that has not been released yet)
+  - Enforce a specific version or unreleased branch is used for a gem you want
+    to use in your git hooks
+
+Loading a Bundler context necessarily adds a startup delay to your hook runs
+as Bundler parses the specified `Gemfile` and checks that the dependencies are
+satisfied. Thus for projects with many gems this can introduce a noticeable
+delay.
+
+The recommended workaround is to create a separate `Gemfile` in the root of
+your repository (call it `.overcommit_gems.rb`), and include only the gems that
+your Overcommit hooks need in order to run. Generate the associated lock file
+by running:
+
+```bash
+bundle install --gemfile=.overcommit_gems.rb
+```
+
+...and commit `.overcommit_gems.rb` and the resulting
+`.overcommit_gems.rb.lock` file to your repository. Set your `gemfile` option
+to `.overcommit_gems.rb`, and you're all set.
+
+Using a smaller Gemfile containing only the gems used by your Overcommit hooks
+significantly reduces the startup delay in your hook runs. It is thus the
+recommended approach unless your project has a relatively small number of gems
+in your `Gemfile`.
+
 ### Plugin Directory
 
 You can change the directory that project-specific hooks are loaded from via

config/default.yml

@@ -2,6 +2,37 @@
 #
 # This is an opinionated list of which hooks are valuable to run and what their
 # out-of-the-box settings should be.
+#-------------------------------------------------------------------------------
+
+# Loads Bundler context from a Gemfile. If false, does nothing (default).
+#
+# Specifying a Gemfile for Bundler to load allows you to control which gems are
+# available in the load path (i.e. loadable via `require`) within your hook
+# runs. Note that having a Gemfile requires you to include `overcommit` itself
+# in your Gemfile (otherwise Overcommit can't load itself!).
+#
+# This is useful if you want to:
+#
+#   - Enforce a specific version of Overcommit to use for all hook runs
+#     (or to use a version from the master branch that has not been released yet)
+#   - Enforce a specific version or unreleased branch is used for a gem you want
+#     to use in your git hooks
+#
+# WARNING: This makes your hook runs slower, but you can work around this!
+#
+# Loading a Bundler context necessarily adds a startup delay to your hook runs
+# as Bundler parses the Gemfile and checks that the dependencies are satisfied.
+# Thus for projects with many gems this can introduce a noticeable delay.
+#
+# The recommended workaround is to create a separate Gemfile in the root of your
+# repository (call it `.overcommit_gems.rb`), and include only the gems that
+# your Overcommit hooks need in order to run. This significantly reduces the
+# startup delay in your hook runs. Make sure to commit both
+# `.overcommit_gems.rb` and the resulting `.overcommit_gems.rb.lock` file to
+# your repository, and then set the `gemfile` option below to the name you gave
+# the file.
+# (Generate lock file by running `bundle install --gemfile=.overcommit_gems.rb`)
+gemfile: false
 
 # 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

template-dir/hooks/overcommit-hook

@@ -25,13 +25,27 @@ if hook_type == 'overcommit-hook'
   exit 64 # EX_USAGE
 end
 
+# Check if Overcommit should invoke a Bundler context for loading gems
+require 'yaml'
+if gemfile = YAML.load_file('.overcommit.yml')['gemfile'] rescue nil
+  ENV['BUNDLE_GEMFILE'] = gemfile
+  require 'bundler/setup'
+end
+
 begin
   require 'overcommit'
 rescue LoadError
-  puts 'This repository contains hooks installed by Overcommit, but the ' \
-       "overcommit gem is not installed.\n" \
-       'Install it with `gem install overcommit`.'
-  exit
+  if gemfile
+    puts 'You have specified the `gemfile` option in your Overcommit ' \
+         'configuration but have not added the `overcommit` gem to ' \
+         "#{gemfile}."
+  else
+    puts 'This repository contains hooks installed by Overcommit, but the ' \
+         "`overcommit` gem is not installed.\n" \
+         'Install it with `gem install overcommit`.'
+  end
+
+  exit 64 # EX_USAGE
 end
 
 begin