MessageTemplate post-checkout hook

config/default.yml

@@ -785,6 +785,14 @@ PostCheckout:
     quiet: true
     recursive: false
 
+  MessageTemplate:
+    enabled: false
+    description: 'Write commit template message based on the new branch name'
+    quiet: true
+    recursive: false
+    branch_pattern: '\A.*\w+[-_](\d+).*\z'
+    replacement_text: '[#\1] [ci skip]'
+
 # Hooks that run after a commit is created.
 PostCommit:
   ALL:

lib/overcommit/hook/post_checkout/message_template.rb

@@ -0,0 +1,50 @@
+module Overcommit::Hook::PostCheckout
+  class MessageTemplate < Base
+    def run
+      Overcommit::Utils.log.debug("Checking if '#{Overcommit::GitRepo.current_branch}' matches #{branch_pattern}")
+      if branch_pattern.match?(Overcommit::GitRepo.current_branch)
+        set_commit_template
+        :pass
+      else
+        :warn
+      end
+    end
+
+    def set_commit_template
+      Overcommit::Utils.log.debug("Writing #{git_template_filename} with #{new_template}")
+      File.write(git_template_filename, new_template)
+      `git config commit.template #{git_template_filename}`
+    end
+
+    def new_template
+      new_template ||= Overcommit::GitRepo.current_branch.gsub(branch_pattern, replacement_text)
+    end
+
+    def branch_pattern
+      @branch_pattern ||=
+        begin
+          pattern = config['branch_pattern']
+          Regexp.new(pattern.empty? ? '\A.*\w+[-_](\d+).*\z' : pattern)
+        end
+    end
+
+    def replacement_text
+      @replacement_text ||=
+        begin
+          if File.exists?(replacement_text_config)
+            File.read(replacement_text_config)
+          else
+            replacement_text_config
+          end
+        end
+    end
+
+    def replacement_text_config
+      @replacement_text_config ||= config['replacement_text']
+    end
+
+    def git_template_filename
+      config['git_template_filename'] || 'overcommit_message_template.txt'
+    end
+  end
+end

spec/overcommit/hook/post_checkout/message_template_spec.rb

@@ -0,0 +1,58 @@
+require 'spec_helper'
+
+describe Overcommit::Hook::PostCheckout::MessageTemplate do
+  let(:config)  { Overcommit::ConfigurationLoader.default_configuration }
+  let(:context) { double('context') }
+
+  subject(:hook) { described_class.new(config, context) }
+
+  before do
+    # stubbing this so we don't litter this repo with useless template files /
+    # configurations
+    hook.stub(:set_commit_template)
+    allow(Overcommit::Utils).to receive_message_chain(:log, :debug)
+    allow(Overcommit::GitRepo).to receive(:current_branch).and_return(new_head)
+  end
+
+  let(:new_head) { 'userbeforeid-12345-branch-description' }
+
+  describe '#run' do
+    context 'when the checked out branch matches the pattern' do
+      it { is_expected.to pass }
+
+      context 'template contents' do
+        subject(:template) { hook.new_template }
+
+        before do
+          hook.stub(:replacement_text).and_return('Id is: \1')
+        end
+
+        it { is_expected.to eq('Id is: 12345') }
+      end
+    end
+
+    context 'when the checked out branch does not match the pattern' do
+      let(:new_head) { "this shouldn't match the default pattern" }
+
+      it { is_expected.to warn }
+    end
+  end
+
+  describe '#replacement_text' do
+    subject(:replacement_text) { hook.replacement_text }
+    let(:replacement_template_file) { 'valid_filename.txt' }
+    let(:replacement) { 'Id is: \1' }
+
+    context 'when the replacement text points to a valid filename' do
+      before do
+        hook.stub(:replacement_text_config).and_return(replacement_template_file)
+        File.stub(:exists?).and_return(true)
+        File.stub(:read).with(replacement_template_file).and_return(replacement)
+      end
+
+      describe 'it reads it as the replacement template' do
+        it { is_expected.to eq(replacement) }
+      end
+    end
+  end
+end