Indicate how much an MR branch diverges from the target branch

CHANGELOG

@@ -6,6 +6,7 @@ v 8.6.0 (unreleased)
   - Fix issue when pushing to projects ending in .wiki
   - Fix avatar stretching by providing a cropping feature (Johann Pardanaud)
   - Don't load all of GitLab in mail_room
+  - Indicate how much an MR diverged from the target branch (Pierre de La Morinerie)
   - Strip leading and trailing spaces in URL validator (evuez)
   - Return empty array instead of 404 when commit has no statuses in commit status API
   - Update documentation to reflect Guest role not being enforced on internal projects

app/models/merge_request.rb

@@ -524,6 +524,29 @@ class MergeRequest < ActiveRecord::Base
     end
   end
+  def diverged_commits_count
+    cache = Rails.cache.read(:"merge_request_#{id}_diverged_commits")
+
+    if cache.blank? || cache[:source_sha] != source_sha || cache[:target_sha] != target_sha
+      cache = {
+        source_sha: source_sha,
+        target_sha: target_sha,
+        diverged_commits_count: compute_diverged_commits_count
+      }
+      Rails.cache.write(:"merge_request_#{id}_diverged_commits", cache)
+    end
+
+    cache[:diverged_commits_count]
+  end
+
+  def compute_diverged_commits_count
+    Gitlab::Git::Commit.between(target_project.repository.raw_repository, source_sha, target_sha).size
+  end
+
+  def diverged_from_target_branch?
+    diverged_commits_count > 0
+  end
+
   def ci_commit
     @ci_commit ||= source_project.ci_commit(last_commit.id) if last_commit && source_project
   end

app/views/projects/merge_requests/_show.html.haml

@@ -34,6 +34,8 @@
         %span into
         = link_to namespace_project_commits_path(@project.namespace, @project, @merge_request.target_branch), class: "label-branch" do
           = @merge_request.target_branch
+        - if @merge_request.open? && @merge_request.diverged_from_target_branch?
+          %span (#{pluralize(@merge_request.diverged_commits_count, 'commit')} behind)
     = render "projects/merge_requests/show/how_to_merge"
     = render "projects/merge_requests/widget/show.html.haml"

features/project/merge_requests.feature

@@ -26,6 +26,16 @@ Feature: Project Merge Requests
     When I visit project "Shop" merge requests page
     Then I should see "other_branch" branch
+  Scenario: I should not see the numbers of diverged commits if the branch is rebased on the target
+    Given project "Shop" have "Bug NS-07" open merge request with rebased branch
+    When I visit merge request page "Bug NS-07"
+    Then I should not see the diverged commits count
+
+  Scenario: I should see the numbers of diverged commits if the branch diverged from the target
+    Given project "Shop" have "Bug NS-08" open merge request with diverged branch
+    When I visit merge request page "Bug NS-08"
+    Then I should see the diverged commits count
+
   Scenario: I should see rejected merge requests
     Given I click link "Closed"
     Then I should see "Feature NS-03" in merge requests

features/steps/project/merge_requests.rb

@@ -60,7 +60,6 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
     expect(page).not_to have_content "Feature NS-03"
   end
-
   step 'I should not see "Bug NS-04" in merge requests' do
     expect(page).not_to have_content "Bug NS-04"
   end
@@ -121,6 +120,22 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
            author: project.users.first)
   end
+  step 'project "Shop" have "Bug NS-07" open merge request with rebased branch' do
+    create(:merge_request, :rebased,
+      title: "Bug NS-07",
+      source_project: project,
+      target_project: project,
+      author: project.users.first)
+  end
+
+  step 'project "Shop" have "Bug NS-08" open merge request with diverged branch' do
+    create(:merge_request, :diverged,
+      title: "Bug NS-08",
+      source_project: project,
+      target_project: project,
+      author: project.users.first)
+  end
+
   step 'project "Shop" have "Feature NS-03" closed merge request' do
     create(:closed_merge_request,
            title: "Feature NS-03",
@@ -490,6 +505,18 @@ class Spinach::Features::ProjectMergeRequests < Spinach::FeatureSteps
     end
   end
+  step 'I should see the diverged commits count' do
+    page.within ".mr-source-target" do
+      expect(page).to have_content /([0-9]+ commits behind)/
+    end
+  end
+
+  step 'I should not see the diverged commits count' do
+    page.within ".mr-source-target" do
+      expect(page).not_to have_content /([0-9]+ commit[s]? behind)/
+    end
+  end
+
   step 'I should see "Bug NS-05" at the top' do
     expect(page.find('ul.content-list.mr-list li.merge-request:first-child')).to have_content("Bug NS-05")
   end

features/steps/shared/paths.rb

@@ -395,6 +395,14 @@ module SharedPaths
     visit merge_request_path("Bug NS-05")
   end
+  step 'I visit merge request page "Bug NS-07"' do
+    visit merge_request_path("Bug NS-07")
+  end
+
+  step 'I visit merge request page "Bug NS-08"' do
+    visit merge_request_path("Bug NS-08")
+  end
+
   step 'I visit merge request page "Bug CO-01"' do
     mr = MergeRequest.find_by(title: "Bug CO-01")
     visit namespace_project_merge_request_path(mr.target_project.namespace, mr.target_project, mr)

spec/factories/merge_requests.rb

@@ -69,6 +69,16 @@ FactoryGirl.define do
       target_branch "master"
     end
+    trait :rebased do
+      source_branch "markdown"
+      target_branch "improve/awesome"
+    end
+
+    trait :diverged do
+      source_branch "feature"
+      target_branch "master"
+    end
+
     trait :merge_when_build_succeeds do
       merge_when_build_succeeds true
       merge_user author

spec/models/merge_request_spec.rb

@@ -274,6 +274,70 @@ describe MergeRequest, models: true do
     end
   end
+  describe '#diverged_commits_count' do
+    let(:project)      { create(:project) }
+    let(:fork_project) { create(:project, forked_from_project: project) }
+
+    context 'diverged on same repository' do
+      subject(:merge_request_with_divergence) { create(:merge_request, :diverged, source_project: project, target_project: project) }
+
+      it 'counts commits that are on target branch but not on source branch' do
+        expect(subject.diverged_commits_count).to eq(5)
+      end
+    end
+
+    context 'diverged on fork' do
+      subject(:merge_request_fork_with_divergence) { create(:merge_request, :diverged, source_project: fork_project, target_project: project) }
+
+      it 'counts commits that are on target branch but not on source branch' do
+        expect(subject.diverged_commits_count).to eq(5)
+      end
+    end
+
+    context 'rebased on fork' do
+      subject(:merge_request_rebased) { create(:merge_request, :rebased, source_project: fork_project, target_project: project) }
+
+      it 'counts commits that are on target branch but not on source branch' do
+        expect(subject.diverged_commits_count).to eq(0)
+      end
+    end
+
+    describe 'caching' do
+      before(:example) do
+        allow(Rails).to receive(:cache).and_return(ActiveSupport::Cache::MemoryStore.new)
+      end
+
+      it 'caches the output' do
+        expect(subject).to receive(:compute_diverged_commits_count).
+          once.
+          and_return(2)
+
+        subject.diverged_commits_count
+        subject.diverged_commits_count
+      end
+
+      it 'invalidates the cache when the source sha changes' do
+        expect(subject).to receive(:compute_diverged_commits_count).
+          twice.
+          and_return(2)
+
+        subject.diverged_commits_count
+        allow(subject).to receive(:source_sha).and_return('123abc')
+        subject.diverged_commits_count
+      end
+
+      it 'invalidates the cache when the target sha changes' do
+        expect(subject).to receive(:compute_diverged_commits_count).
+          twice.
+          and_return(2)
+
+        subject.diverged_commits_count
+        allow(subject).to receive(:target_sha).and_return('123abc')
+        subject.diverged_commits_count
+      end
+    end
+  end
+
   it_behaves_like 'an editable mentionable' do
     subject { create(:merge_request) }