test: cover test_scoped_lists_are_independent_per_scope_value

test/test_orm_list_position.rb

@@ -159,3 +159,150 @@ class OrmListPositionTest < Minitest::Test
     assert_equal [1, 2, 3, 4], [a.reload, c.reload, d.reload, restored.reload].map(&:list_position).sort
   end
 
+  def test_scoped_lists_are_independent_per_scope_value
+    a1 = Card.create!(title: 'a1', board_id: 1)
+    a2 = Card.create!(title: 'a2', board_id: 1)
+    b1 = Card.create!(title: 'b1', board_id: 2)
+    assert_equal 1, a1.list_position
+    assert_equal 2, a2.list_position
+    assert_equal 1, b1.list_position # own scope, own sequence
+
+    a1.move_lower
+    assert_equal 1, b1.reload.list_position # untouched by a1/a2's board
+  end
+
+  def test_explicit_position_on_create_shifts_existing_siblings_down
+    a = Task.create!(title: 'a')
+    b = Task.create!(title: 'b')
+    inserted = Task.create!(title: 'new-first', position: 1)
+    assert_equal 1, inserted.list_position
+    assert_equal 2, a.reload.list_position
+    assert_equal 3, b.reload.list_position
+  end
+
+  def test_changing_the_scope_column_moves_a_record_between_lists_on_update
+    a1 = Card.create!(title: 'a1', board_id: 1)
+    a2 = Card.create!(title: 'a2', board_id: 1)
+    a3 = Card.create!(title: 'a3', board_id: 1)
+    b1 = Card.create!(title: 'b1', board_id: 2)
+
+    a2.update!(board_id: 2)
+
+    # board 1's gap at a2's old slot (2) closed
+    assert_equal 1, a1.reload.list_position
+    assert_equal 2, a3.reload.list_position
+    # a2 joined board 2 at the bottom, not colliding with b1
+    assert_equal 1, b1.reload.list_position
+    assert_equal 2, a2.reload.list_position
+  end
+
+  def test_updating_without_touching_the_scope_column_leaves_position_alone
+    a = Card.create!(title: 'a', board_id: 1)
+    b = Card.create!(title: 'b', board_id: 1)
+    a.update!(title: 'a renamed')
+    assert_equal 1, a.list_position
+    assert_equal 2, b.reload.list_position
+  end
+
+  # Everything below forks real OS processes, same technique and same
+  # justification as test_orm_pessimistic.rb (Store#within_transaction takes
+  # a real File::LOCK_EX/LOCK_SH per transaction - this is not a simulation).
+  #
+  # These pin down a real bug that existed before move_higher/move_lower/
+  # move_to_top/move_to_bottom/insert_at were wrapped in with_lock:
+  # higher_item/lower_item/minimum/maximum used to run in their OWN
+  # read-only transaction, which releases Store's file lock the moment it
+  # returns, before the write transaction opened - a stale neighbor read by
+  # one process could still be used to compute a swap after another process
+  # already moved that same neighbor, corrupting the dense 1..N position
+  # invariant every method in this file depends on.
+  def test_swap_list_position_with_is_racy_without_a_held_lock
+    skip 'fork not supported on this platform' unless Process.respond_to?(:fork)
+
+    a = Task.create!(title: 'a') # position 1
+    b = Task.create!(title: 'b') # position 2
+    c = Task.create!(title: 'c') # position 3
+
+    a_read, a_wrote_read_signal = IO.pipe
+    b_read, b_wrote_read_signal = IO.pipe
+
+    # Forces both processes to read their (self, neighbor) positions from
+    # the ORIGINAL 1/2/3 layout before either one calls the private,
+    # lock-free swap_list_position_with directly (bypassing with_lock,
+    # exactly the shape move_higher/move_lower had before this fix) - only
+    # the reads are ordered, deliberately not the writes, since the race
+    # only needs both computed deltas to be based on the same stale
+    # snapshot, not any particular write order.
+    pid_x = Process.fork do
+      a_read.close
+      b_wrote_read_signal.close
+      x_self = Task.find(a.id) # position 1
+      x_neighbor = Task.find(b.id) # position 2, read before b moves at all
+      a_wrote_read_signal.write('r')
+      a_wrote_read_signal.close
+      b_read.read(1)
+      b_read.close
+      x_self.send(:swap_list_position_with, x_neighbor)
+      exit!(0)
+    end
+
+    pid_y = Process.fork do
+      a_wrote_read_signal.close
+      b_read.close
+      a_read.read(1)
+      a_read.close
+      y_self = Task.find(c.id) # position 3
+      y_neighbor = Task.find(b.id) # position 2, same stale snapshot as x_neighbor
+      b_wrote_read_signal.write('r')
+      b_wrote_read_signal.close
+      y_self.send(:swap_list_position_with, y_neighbor)
+      exit!(0)
+    end
+
+    [a_read, a_wrote_read_signal, b_read, b_wrote_read_signal].each(&:close)
+    Process.waitpid(pid_x)
+    Process.waitpid(pid_y)
+
+    # Both swaps computed their delta against B's stale on-disk value (2),
+    # so B absorbs both deltas (-1 then +1, or +1 then -1 depending on
+    # write order - either way net zero relative to its OWN stale
+    # baseline) while A and C both independently land on B's old slot: A
+    # gets +1 (1 -> 2), C gets -1 (3 -> 2), B ends up wherever the second
+    # writer's delta leaves the FIRST writer's already-moved value, which
+    # collapses to 2 either way. All three end up sharing position 2 -
+    # positions 1 and 3 are left completely empty, a corruption of the
+    # dense 1..N invariant that no valid outcome of this framework's API
+    # can otherwise produce.
+    assert_equal [2, 2, 2], [a, b, c].map { |t| t.reload.list_position }.sort
+  end
+
+  def test_move_higher_and_move_lower_stay_race_free_under_real_concurrency
+    skip 'fork not supported on this platform' unless Process.respond_to?(:fork)
+
+    a = Task.create!(title: 'a') # position 1
+    b = Task.create!(title: 'b') # position 2
+    c = Task.create!(title: 'c') # position 3
+
+    # No pipe-forced ordering needed here (unlike the racy test above): each
+    # move_lower/move_higher call now takes Store's real file lock for its
+    # ENTIRE reload-lookup-swap sequence via with_lock, so whichever process
+    # gets there first runs to completion before the other's lock! reload
+    # even sees the table - the second call always computes its neighbor
+    # from genuinely current data, not a stale snapshot, no matter how the
+    # OS schedules the two forks.
+    pid_x = Process.fork do
+      Task.find(a.id).move_lower
+      exit!(0)
+    end
+    pid_y = Process.fork do
+      Task.find(c.id).move_higher
+      exit!(0)
+    end
+    Process.waitpid(pid_x)
+    Process.waitpid(pid_y)
+
+    positions = [a, b, c].map { |t| t.reload.list_position }
+    assert_equal [1, 2, 3], positions.sort, 'no duplicate or skipped positions after concurrent moves'
+    assert_equal 3, positions.uniq.size
+  end
+end