Add cannot_be_merged_recheck merge_status

First, transitions between can_be_merged & cannot_be_merged are removed,
as they are currently blocked in `check_if_can_be_merged`.
`can_be_merge` always returns to `unchecked` first,
before it can transition to `cannot_be_merged` (and vice versa).

We want to avoid repeated notification triggered by repeated transition
between `cannot_be_merged` & `unchecked`.

So we added `cannot_be_merged_recheck` state, similar to `unchecked`,
but as a mean to remember it’s from cannot_be_merged.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18042/#note_65945407

Since `unchecked` and `cannot_be_merged_recheck` both mean
“we are in the middle of checking if it is mergeable”,
quite often we need to see if merge_status is in either one of them,
so `check_state?` is added to achieve this.

2 files changed, 11 insertions, 5 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 628c61d5d69..ea5057c2a78 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -104,24 +104,30 @@ class MergeRequest < ActiveRecord::Base
 
   state_machine :merge_status, initial: :unchecked do
     event :mark_as_unchecked do
-      transition [:can_be_merged, :cannot_be_merged] => :unchecked
+      transition [:can_be_merged] => :unchecked
+      transition [:cannot_be_merged] => :cannot_be_merged_recheck
     end
 
     event :mark_as_mergeable do
-      transition [:unchecked, :cannot_be_merged] => :can_be_merged
+      transition [:unchecked, :cannot_be_merged_recheck] => :can_be_merged
     end
 
     event :mark_as_unmergeable do
-      transition [:unchecked, :can_be_merged] => :cannot_be_merged
+      transition [:unchecked, :cannot_be_merged_recheck] => :cannot_be_merged
     end
 
     state :unchecked
+    state :cannot_be_merged_recheck
     state :can_be_merged
     state :cannot_be_merged
 
     around_transition do |merge_request, transition, block|
       Gitlab::Timeless.timeless(merge_request, &block)
     end
+
+    def check_state?(merge_status)
+      [:unchecked, :cannot_be_merged_recheck].include?(merge_status.to_sym)
+    end
   end
 
   validates :source_project, presence: true, unless: [:allow_broken, :importing?, :closed_without_fork?]
@@ -602,7 +608,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def check_if_can_be_merged
-    return unless unchecked? && Gitlab::Database.read_write?
+    return unless self.class.state_machines[:merge_status].check_state?(merge_status) && Gitlab::Database.read_write?
 
     can_be_merged =
       !broken? && project.repository.can_be_merged?(diff_head_sha, target_branch)
diff --git a/app/models/project_services/drone_ci_service.rb b/app/models/project_services/drone_ci_service.rb
index 71b10fc6bc1..a4bf427ac0b 100644
--- a/app/models/project_services/drone_ci_service.rb
+++ b/app/models/project_services/drone_ci_service.rb
@@ -115,6 +115,6 @@ class DroneCiService < CiService
 
   def merge_request_valid?(data)
     data[:object_attributes][:state] == 'opened' &&
-      data[:object_attributes][:merge_status] == 'unchecked'
+      MergeRequest.state_machines[:merge_status].check_state?(data[:object_attributes][:merge_status])
   end
 end