DigitalWaveform.signals introduces a reference cycle

[bkeryan]

Tech Debt

There is a reference cycle between DigitalWaveform and DigitalWaveformSignal/DigitalWaveformSignalCollection. Python normally uses reference counting to free objects, but it also has a generational GC that can clean up reference cycles, and apparently this can affect application performance.

We should evaluate whether to use weakref to break this cycle, and if so, how.

• Should the parent->child link be a weak ref? The parent caches the children and can reconstruct them if needed.
• Should the child->parent link be a weak ref? Once this is invalidated, the child object is unusable, but many use cases will reference the parent.

AB#3178116