Add support for pre-rebase hooks

config/default.yml

@@ -489,3 +489,11 @@ PrePush:
     enabled: false
     description: 'Running rspec test suite'
     required_executable: 'rspec'
+
+# Hooks that run during `git rebase`, before any commits are rebased.
+# If a hook fails, the rebase is aborted.
+PreRebase:
+  ALL:
+    requires_files: false
+    required: false
+    quiet: false

lib/overcommit/hook/pre_rebase/base.rb

@@ -0,0 +1,11 @@
+require 'forwardable'
+
+module Overcommit::Hook::PreRebase
+  # Functionality common to all pre-rebase hooks.
+  class Base < Overcommit::Hook::Base
+    extend Forwardable
+
+    def_delegators :@context,
+                   :upstream_branch, :rebased_branch, :fast_forward?, :rebased_commits
+  end
+end

lib/overcommit/hook_context/pre_rebase.rb

@@ -0,0 +1,36 @@
+module Overcommit::HookContext
+  # Contains helpers related to contextual information used by pre-rebase
+  # hooks.
+  class PreRebase < Base
+    # Returns the name of the branch we are rebasing onto.
+    def upstream_branch
+      @args[0]
+    end
+
+    # Returns the name of the branch being rebased. Empty if rebasing a
+    # detached HEAD.
+    def rebased_branch
+      @rebased_branch ||=
+        @args[1] || `git symbolic-ref --short --quiet HEAD`.chomp
+    end
+
+    # Returns whether we are rebasing a detached HEAD rather than a branch
+    def detached_head?
+      rebased_branch.empty?
+    end
+
+    # Returns whether this rebase is a fast-forward
+    def fast_forward?
+      rebased_commits.empty?
+    end
+
+    # Returns the SHA1-sums of the series of commits to be rebased
+    # in reverse topological order.
+    def rebased_commits
+      rebased_ref = detached_head? ? 'HEAD' : rebased_branch
+      @rebased_commits ||=
+        `git rev-list --topo-order --reverse #{upstream_branch}..#{rebased_ref}`.
+          split("\n")
+    end
+  end
+end

spec/overcommit/hook_context/pre_rebase_spec.rb

@@ -0,0 +1,107 @@
+require 'spec_helper'
+require 'overcommit/hook_context/pre_rebase'
+
+describe Overcommit::HookContext::PreRebase do
+  let(:config) { double('config') }
+  let(:args) { [upstream_branch, rebased_branch] }
+  let(:upstream_branch) { 'master' }
+  let(:rebased_branch) { 'topic' }
+  let(:input) { double('input') }
+  let(:context) { described_class.new(config, args, input) }
+
+  describe '#upstream_branch' do
+    subject { context.upstream_branch }
+
+    it { should == upstream_branch }
+  end
+
+  describe '#rebased_branch' do
+    subject { context.rebased_branch }
+
+    it { should == rebased_branch }
+
+    context 'when rebasing current branch' do
+      let(:rebased_branch) { nil }
+      let(:current_branch) { 'master' }
+
+      around do |example|
+        repo do
+          `git checkout -b #{current_branch} &> /dev/null`
+          example.run
+        end
+      end
+
+      it { should == current_branch }
+    end
+  end
+
+  describe '#fast_forward?' do
+    subject { context.fast_forward? }
+
+    context 'when upstream branch is descendent from rebased branch' do
+      before do
+        context.stub(:rebased_commits).and_return([])
+      end
+
+      it { should == true }
+    end
+
+    context 'when upstream branch is not descendent from rebased branch' do
+      before do
+        context.stub(:rebased_commits).and_return([random_hash])
+      end
+
+      it { should == false }
+    end
+  end
+
+  describe '#detached_head?' do
+    subject { context.detached_head? }
+
+    context 'when rebasing a detached HEAD' do
+      let(:rebased_branch) { '' }
+
+      it { should == true }
+    end
+
+    context 'when rebasing a branch' do
+      let(:rebased_branch) { 'topic' }
+
+      it { should == false }
+    end
+  end
+
+  describe '#rebased_commits' do
+    subject { context.rebased_commits }
+
+    let(:base_branch) { 'master' }
+    let(:topic_branch_1) { 'topic-1' }
+    let(:topic_branch_2) { 'topic-2' }
+
+    around do |example|
+      repo do
+        `git checkout -b #{base_branch} &> /dev/null`
+        `git commit --allow-empty -m "Initial Commit"`
+        `git checkout -b #{topic_branch_1} &> /dev/null`
+        `git commit --allow-empty -m "Hello World"`
+        `git checkout -b #{topic_branch_2} #{base_branch} &> /dev/null`
+        `git commit --allow-empty -m "Hello Again"`
+        example.run
+      end
+    end
+
+    context 'when upstream branch is descendent from rebased branch' do
+      let(:upstream_branch) { topic_branch_1 }
+      let(:rebased_branch) { base_branch }
+
+      it { should be_empty }
+    end
+
+    context 'when upstream branch is not descendent from rebased branch' do
+      let(:upstream_branch) { topic_branch_1 }
+      let(:rebased_branch) { topic_branch_2 }
+
+      it { should_not be_empty }
+    end
+  end
+end

spec/overcommit/utils_spec.rb

@@ -109,15 +109,15 @@
     subject { described_class.supported_hook_types }
 
     # rubocop:disable Metrics/LineLength
-    it { should =~ %w[commit-msg pre-commit post-checkout post-commit post-merge post-rewrite pre-push] }
+    it { should =~ %w[commit-msg pre-commit post-checkout post-commit post-merge post-rewrite pre-push pre-rebase] }
     # rubocop:enable Metrics/LineLength
   end
 
   describe '.supported_hook_type_classes' do
     subject { described_class.supported_hook_type_classes }
 
     # rubocop:disable Metrics/LineLength
-    it { should =~ %w[CommitMsg PreCommit PostCheckout PostCommit PostMerge PostRewrite PrePush] }
+    it { should =~ %w[CommitMsg PreCommit PostCheckout PostCommit PostMerge PostRewrite PrePush PreRebase] }
     # rubocop:enable Metrics/LineLength
   end
 

template-dir/hooks/pre-rebase

@@ -0,0 +1 @@
+overcommit-hook