Detect rebase state based on rebase directory

app/models/merge_request.rb

@@ -506,4 +506,12 @@ class MergeRequest < ActiveRecord::Base
   def ff_merge_possible?
     target_sha == source_sha_parent
   end
+
+  def rebase_dir_path
+    Rails.root.join('tmp', 'rebase', source_project.id.to_s, id.to_s).to_s
+  end
+
+  def rebase_in_progress?
+    File.exist?(rebase_dir_path)
+  end
 end

app/services/merge_requests/rebase_service.rb

@@ -52,6 +52,7 @@ module MergeRequests
 
       true
     ensure
+      clean_dir
       Gitlab::ShellEnv.reset_env
     end
 
@@ -64,11 +65,15 @@ module MergeRequests
     end
 
     def tree_path
-      @tree_path ||= Rails.root.join('tmp', 'rebase', source_project.id.to_s, SecureRandom.hex).to_s
+      @tree_path ||= merge_request.rebase_dir_path
     end
 
     def log(message)
       Gitlab::GitLogger.error(message)
     end
+
+    def clean_dir
+      FileUtils.rm_rf(tree_path) if File.exist?(tree_path)
+    end
   end
 end

app/views/projects/merge_requests/widget/open/_ff_accept.html.haml

@@ -14,9 +14,12 @@
     = hidden_field_tag :authenticity_token, form_authenticity_token
     .accept-merge-holder.clearfix.js-toggle-container
       - if @merge_request.target_project.merge_requests_rebase_enabled && can_rebase?(@merge_request.target_project, @merge_request.target_branch)
-        .accept-action
-          = f.button class: "btn btn-reopen rebase-mr" do
-            Rebase from #{@merge_request.target_branch}
+        - if @merge_request.rebase_in_progress?
+          %h4 Rebase in progress... It can take a while. Reload at will.
+        - else
+          .accept-action
+            = f.button class: "btn btn-reopen rebase-mr" do
+              Rebase from #{@merge_request.target_branch}
       .accept-control
         Fast-forward merge is not possible. Branch must be rebased first