Notify only when unmergeable due to conflict

There is still the edge case when 'no commits' changes to 'conflict'
would not trigger notification, which we ignore for now.

Calling can_be_merged? can cause exception (e.g. non-UTF8)
Ignore those by rescueing.

Remove unmergeable_reason as now only conflict is notified

Update spec

5 files changed, 13 insertions, 24 deletions

diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 5ba3a4a322c..70509e9066d 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -59,8 +59,6 @@ module Emails
     def merge_request_unmergeable_email(recipient_id, merge_request_id, reason = nil)
       setup_merge_request_mail(merge_request_id, recipient_id)
 
-      @reasons = MergeRequestPresenter.new(@merge_request, current_user: current_user).unmergeable_reasons
-
       mail_answer_thread(@merge_request, merge_request_thread_options(@merge_request.author_id, recipient_id, reason))
     end
 
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 324065c1162..4b78ba1029f 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -128,8 +128,17 @@ class MergeRequest < ActiveRecord::Base
     end
 
     after_transition unchecked: :cannot_be_merged do |merge_request, transition|
-      NotificationService.new.merge_request_unmergeable(merge_request)
-      TodoService.new.merge_request_became_unmergeable(merge_request)
+      begin
+        # Merge request can become unmergeable due to many reasons.
+        # We only notify if it is due to conflict.
+        unless merge_request.project.repository.can_be_merged?(merge_request.diff_head_sha, merge_request.target_branch)
+          NotificationService.new.merge_request_unmergeable(merge_request)
+          TodoService.new.merge_request_became_unmergeable(merge_request)
+        end
+      rescue Gitlab::Git::CommandError
+        # Checking mergeability can trigger exception, e.g. non-utf8
+        # We ignore this type of errors.
+      end
     end
 
     def check_state?(merge_status)
diff --git a/app/presenters/merge_request_presenter.rb b/app/presenters/merge_request_presenter.rb
index 8d466c33510..eb54ab2cda6 100644
--- a/app/presenters/merge_request_presenter.rb
+++ b/app/presenters/merge_request_presenter.rb
@@ -20,17 +20,6 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
     end
   end
 
-  def unmergeable_reasons
-    strong_memoize(:unmergeable_reasons) do
-      reasons = []
-      reasons << "no commits" if merge_request.has_no_commits?
-      reasons << "source branch is missing" unless merge_request.source_branch_exists?
-      reasons << "target branch is missing" unless merge_request.target_branch_exists?
-      reasons << "has merge conflicts" unless merge_request.project.repository.can_be_merged?(merge_request.diff_head_sha, merge_request.target_branch)
-      reasons
-    end
-  end
-
   def cancel_merge_when_pipeline_succeeds_path
     if can_cancel_merge_when_pipeline_succeeds?(current_user)
       cancel_merge_when_pipeline_succeeds_project_merge_request_path(project, merge_request)
diff --git a/app/views/notify/merge_request_unmergeable_email.html.haml b/app/views/notify/merge_request_unmergeable_email.html.haml
index 578fa1fbce7..7ec0c1ef390 100644
--- a/app/views/notify/merge_request_unmergeable_email.html.haml
+++ b/app/views/notify/merge_request_unmergeable_email.html.haml
@@ -1,6 +1,2 @@
 %p
-  Merge Request #{link_to @merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request)} can no longer be merged due to the following #{'reason'.pluralize(@reasons.count)}:
-
-  %ul
-    - @reasons.each do |reason|
-      %li= reason
+  Merge Request #{link_to @merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request)} can no longer be merged due to conflict.
diff --git a/app/views/notify/merge_request_unmergeable_email.text.haml b/app/views/notify/merge_request_unmergeable_email.text.haml
index e4f9f1bf5e7..dcdd6db69d6 100644
--- a/app/views/notify/merge_request_unmergeable_email.text.haml
+++ b/app/views/notify/merge_request_unmergeable_email.text.haml
@@ -1,7 +1,4 @@
-Merge Request #{@merge_request.to_reference} can no longer be merged due to the following #{'reason'.pluralize(@reasons.count)}:
-
-- @reasons.each do |reason|
-  * #{reason}
+Merge Request #{@merge_request.to_reference} can no longer be merged due to conflict.
 
 Merge Request url: #{project_merge_request_url(@merge_request.target_project, @merge_request)}