1 files changed, 35 insertions, 5 deletions
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 5bec68ce4f6..c2bae379a94 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -375,15 +375,27 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def diff_start_sha
-    diff_start_commit.try(:sha)
+    if persisted?
+      merge_request_diff.start_commit_sha
+    else
+      target_branch_head.try(:sha)
+    end
   end
 
   def diff_base_sha
-    diff_base_commit.try(:sha)
+    if persisted?
+      merge_request_diff.base_commit_sha
+    else
+      branch_merge_base_commit.try(:sha)
+    end
   end
 
   def diff_head_sha
-    diff_head_commit.try(:sha)
+    if persisted?
+      merge_request_diff.head_commit_sha
+    else
+      source_branch_head.try(:sha)
+    end
   end
 
   # When importing a pull request from GitHub, the old and new branches may no
@@ -646,7 +658,7 @@ class MergeRequest < ActiveRecord::Base
     !ProtectedBranch.protected?(source_project, source_branch) &&
       !source_project.root_ref?(source_branch) &&
       Ability.allowed?(current_user, :push_code, source_project) &&
-      diff_head_commit == source_branch_head
+      diff_head_sha == source_branch_head.try(:sha)
   end
 
   def should_remove_source_branch?
@@ -853,7 +865,7 @@ class MergeRequest < ActiveRecord::Base
 
   def can_be_merged_by?(user)
     access = ::Gitlab::UserAccess.new(user, project: project)
-    access.can_push_to_branch?(target_branch) || access.can_merge_to_branch?(target_branch)
+    access.can_update_branch?(target_branch)
   end
 
   def can_be_merged_via_command_line_by?(user)
@@ -1075,4 +1087,22 @@ class MergeRequest < ActiveRecord::Base
 
     project.merge_requests.merged.where(author_id: author_id).empty?
   end
+
+  def allow_maintainer_to_push
+    maintainer_push_possible? && super
+  end
+
+  alias_method :allow_maintainer_to_push?, :allow_maintainer_to_push
+
+  def maintainer_push_possible?
+    source_project.present? && for_fork? &&
+      target_project.visibility_level > Gitlab::VisibilityLevel::PRIVATE &&
+      source_project.visibility_level > Gitlab::VisibilityLevel::PRIVATE &&
+      !ProtectedBranch.protected?(source_project, source_branch)
+  end
+
+  def can_allow_maintainer_to_push?(user)
+    maintainer_push_possible? &&
+      Ability.allowed?(user, :push_code, source_project)
+  end
 end