Describe workaround when restore fails because of `Errno::EBUSY`

When `Errno::EBUSY` is raised during restore, this could indicate that
the directory being restored into is a mountpoint. In this case we
explain the user how to retry the restore.

lib/backup/files.rb

@@ -53,6 +53,8 @@ module Backup
           FileUtils.mv(files, timestamped_files_path)
         rescue Errno::EACCES
           access_denied_error(app_files_dir)
+        rescue Errno::EBUSY
+          resource_busy_error(app_files_dir)
         end
       end
     end

lib/backup/helper.rb

@@ -13,5 +13,19 @@ module Backup
       EOS
       raise message
     end
+
+    def resource_busy_error(path)
+      message = <<~EOS
+
+      ### NOTICE ###
+      As part of restore, the task tried to rename `#{path}` before restoring.
+      This could not be completed, perhaps `#{path}` is a mountpoint?
+
+      To complete the restore, please move the contents of `#{path}` to a
+      different location and run the restore task again.
+
+      EOS
+      raise message
+    end
   end
 end

lib/backup/repository.rb

@@ -81,6 +81,8 @@ module Backup
           FileUtils.mv(files, bk_repos_path)
         rescue Errno::EACCES
           access_denied_error(path)
+        rescue Errno::EBUSY
+          resource_busy_error(path)
         end
       end
     end

spec/lib/backup/files_spec.rb

@@ -62,5 +62,19 @@ describe Backup::Files do
         subject.restore
       end
     end
+
+    describe 'folders that are a mountpoint' do
+      before do
+        allow(FileUtils).to receive(:mv).and_raise(Errno::EBUSY)
+        allow(subject).to receive(:run_pipeline!).and_return(true)
+      end
+
+      it 'shows error message' do
+        expect(subject).to receive(:resource_busy_error).with("/var/gitlab-registry")
+                             .and_call_original
+
+        expect { subject.restore }.to raise_error(/is a mountpoint/)
+      end
+    end
   end
 end

spec/lib/backup/repository_spec.rb

@@ -81,6 +81,18 @@ describe Backup::Repository do
         subject.restore
       end
     end
+
+    describe 'folder that is a mountpoint' do
+      before do
+        allow(FileUtils).to receive(:mv).and_raise(Errno::EBUSY)
+      end
+
+      it 'shows error message' do
+        expect(subject).to receive(:resource_busy_error).and_call_original
+
+        expect { subject.restore }.to raise_error(/is a mountpoint/)
+      end
+    end
   end
 
   describe '#empty_repo?' do