Add BrokenSymlinks pre-commit hook

The recent bug in the pre-commit `HookContext`'s handling of broken
symlinks prompted the need for a hook that detects these problems
explicitly (otherwise the user might get a cryptic error in one of the
other hooks).

Being explicitly told that a symlink is broken is useful, so add it as a
pre-commit hook.

Change-Id: I1a1330bdf812550c9bbb0631c785b0016a727b38
Reviewed-on: http://gerrit.causes.com/42396
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

CHANGELOG.md

@@ -12,6 +12,7 @@
 * All built-in hooks will now fail if the required executable is not present
 * Fix bug where pre-commit hook would crash if user attempted to commit a
   broken symlink
+* Add `BrokenSymlinks` pre-commit hook which checks for broken symlinks
 
 ## 0.17.0
 

config/default.yml

@@ -64,6 +64,10 @@ PreCommit:
     include:
       - '**/*.rb'
 
+  BrokenSymlinks:
+    description: 'Checking for broken symlinks'
+    quiet: true
+
   BundleCheck:
     description: 'Checking Gemfile dependencies'
     required_executable: 'bundle'

lib/overcommit/hook/pre_commit/broken_symlinks.rb

@@ -0,0 +1,15 @@
+module Overcommit::Hook::PreCommit
+  # Checks for broken symlinks.
+  class BrokenSymlinks < Base
+    def run
+      broken_symlinks = applicable_files.
+        select { |file| Overcommit::Utils.broken_symlink?(file) }
+
+      if broken_symlinks.any?
+        return :fail, "Broken symlinks detected:\n#{broken_symlinks.join("\n")}"
+      end
+
+      :pass
+    end
+  end
+end

spec/overcommit/hook/pre_commit/broken_symlinks_spec.rb

@@ -0,0 +1,28 @@
+require 'spec_helper'
+
+describe Overcommit::Hook::PreCommit::BrokenSymlinks do
+  let(:config)  { Overcommit::ConfigurationLoader.default_configuration }
+  let(:context) { double('context') }
+  let(:staged_file) { 'staged-file.txt' }
+  subject { described_class.new(config, context) }
+
+  before do
+    subject.stub(:applicable_files).and_return([staged_file])
+  end
+
+  before do
+    Overcommit::Utils.stub(:broken_symlink?).with(staged_file).and_return(broken)
+  end
+
+  context 'when the symlink is broken' do
+    let(:broken) { true }
+
+    it { should fail_hook }
+  end
+
+  context 'when the symlink is not broken' do
+    let(:broken) { false }
+
+    it { should pass }
+  end
+end