Do not trigger rebalance when we have DB timeouts

Statement timeouts can be transient and a rebalance is disruptive
because it blocks positioning for the duration of the rebalance.

This changes the code so that we only trigger a rebalance when there is
no space left.

app/models/concerns/relative_positioning.rb

@@ -135,21 +135,21 @@ module RelativePositioning
     before, after = [before, after].sort_by(&:relative_position) if before && after
 
     RelativePositioning.mover.move(self, before, after)
-  rescue ActiveRecord::QueryCanceled, NoSpaceLeft => e
+  rescue NoSpaceLeft => e
     could_not_move(e)
     raise e
   end
 
   def move_after(before = self)
     RelativePositioning.mover.move(self, before, nil)
-  rescue ActiveRecord::QueryCanceled, NoSpaceLeft => e
+  rescue NoSpaceLeft => e
     could_not_move(e)
     raise e
   end
 
   def move_before(after = self)
     RelativePositioning.mover.move(self, nil, after)
-  rescue ActiveRecord::QueryCanceled, NoSpaceLeft => e
+  rescue NoSpaceLeft => e
     could_not_move(e)
     raise e
   end
@@ -159,9 +159,6 @@ module RelativePositioning
   rescue NoSpaceLeft => e
     could_not_move(e)
     self.relative_position = MAX_POSITION
-  rescue ActiveRecord::QueryCanceled => e
-    could_not_move(e)
-    raise e
   end
 
   def move_to_start
@@ -169,9 +166,6 @@ module RelativePositioning
   rescue NoSpaceLeft => e
     could_not_move(e)
     self.relative_position = MIN_POSITION
-  rescue ActiveRecord::QueryCanceled => e
-    could_not_move(e)
-    raise e
   end
 
   # This method is used during rebalancing - override it to customise the update

spec/models/issue_spec.rb

@@ -1435,19 +1435,19 @@ RSpec.describe Issue do
   describe 'scheduling rebalancing' do
     before do
       allow_next_instance_of(RelativePositioning::Mover) do |mover|
-        allow(mover).to receive(:move) { raise ActiveRecord::QueryCanceled }
+        allow(mover).to receive(:move) { raise RelativePositioning::NoSpaceLeft }
       end
     end
 
     shared_examples 'schedules issues rebalancing' do
       let(:issue) { build_stubbed(:issue, relative_position: 100, project: project) }
 
-      it 'schedules rebalancing if we time-out when moving' do
+      it 'schedules rebalancing if there is no space left' do
         lhs = build_stubbed(:issue, relative_position: 99, project: project)
         to_move = build(:issue, project: project)
         expect(IssueRebalancingWorker).to receive(:perform_async).with(nil, project_id, namespace_id)
 
-        expect { to_move.move_between(lhs, issue) }.to raise_error(ActiveRecord::QueryCanceled)
+        expect { to_move.move_between(lhs, issue) }.to raise_error(RelativePositioning::NoSpaceLeft)
       end
     end