fix: fall back to full recalculation when updating a task position

updateTaskPosition returned ErrNeedsFullRecalculation raw when the
neighbours around the target position were too close for a localized
repair, surfacing as an HTTP 500 on POST /api/v1/tasks/{id}/position.
It now falls back to a full recalculation of the view, like the task
creation path and the CLI repair command already do.

Ref: Sentry API-CLOUD-1R

pkg/models/task_position.go

@@ -277,6 +277,9 @@ func updateTaskPosition(s *xorm.Session, a web.Auth, tp *TaskPosition) (err erro
 
 	if len(conflicts) > 1 {
 		err = resolveTaskPositionConflicts(s, tp.ProjectViewID, conflicts)
+		if IsErrNeedsFullRecalculation(err) {
+			err = recalculateTaskPositionsForRepair(s, &ProjectView{ID: tp.ProjectViewID})
+		}
 		if err != nil {
 			return err
 		}

pkg/models/task_position_test.go

@@ -315,6 +315,52 @@ func TestUpdateTaskPositionWithConflictResolution(t *testing.T) {
 
 		assert.NotEqual(t, pos1.Position, pos2.Position)
 	})
+
+	t.Run("falls back to full recalculation when neighbours are too close", func(t *testing.T) {
+		db.LoadAndAssertFixtures(t)
+		s := db.NewSession()
+		defer s.Close()
+
+		_, err := s.Where("project_view_id = ?", 93).Delete(&TaskPosition{})
+		require.NoError(t, err)
+
+		// The gap between the neighbours of the target position only leaves
+		// 0.01/3 for two conflicting tasks, less than MinPositionSpacing.
+		conflictPosition := 100.005
+		_, err = s.Insert(&TaskPosition{TaskID: 900, ProjectViewID: 93, Position: 100})
+		require.NoError(t, err)
+		_, err = s.Insert(&TaskPosition{TaskID: 901, ProjectViewID: 93, Position: conflictPosition})
+		require.NoError(t, err)
+		_, err = s.Insert(&TaskPosition{TaskID: 902, ProjectViewID: 93, Position: 100.01})
+		require.NoError(t, err)
+
+		tp := &TaskPosition{
+			TaskID:        903,
+			ProjectViewID: 93,
+			Position:      conflictPosition,
+		}
+
+		err = updateTaskPosition(s, nil, tp)
+		require.NoError(t, err)
+
+		var positions []*TaskPosition
+		err = s.Where("project_view_id = ?", 93).OrderBy("position ASC").Find(&positions)
+		require.NoError(t, err)
+		require.Len(t, positions, 4)
+
+		for i := 1; i < len(positions); i++ {
+			assert.Greater(t, positions[i].Position, positions[i-1].Position,
+				"position of task %d should be greater than the one of task %d",
+				positions[i].TaskID, positions[i-1].TaskID)
+			assert.GreaterOrEqual(t, positions[i].Position-positions[i-1].Position, MinPositionSpacing)
+		}
+
+		updated := &TaskPosition{}
+		_, err = s.Where("task_id = ? AND project_view_id = ?", 903, 93).Get(updated)
+		require.NoError(t, err)
+		assert.InDelta(t, updated.Position, tp.Position, 0, "the returned position should be refreshed")
+		assert.Greater(t, tp.Position, 1000.0, "positions should have been spread over the full range")
+	})
 }
 
 func TestRepairTaskPositions(t *testing.T) {