Currently it is still possible to modify Pipeline::_sources after exec::agg::Pipeline has been created. This can cause unexpected problems.

The following non-const public methods modify Pipeline::_sources directly:
 - appendPipeline()

• popFrontWithNameAndCriteria()
• popFrontWithName()
• pushBack()
• popBack()
• popFront()
• addInitialSource()
• addFinalSource()
• optimizePipeline()

Additionally, there are some ways to modify Pipeline::_sources which are harder to control:

 - friend class PipelineD
 - getSources() (non-const)