add skip_update as a replacement for returning nothing in ComputePipeline callbacks

[ffreyer]

Description

This is mostly in response to #5717 (comment)

Currently edge callbacks can return nothing to mark all outputs or a specific output as "not updated". This can be problematic when an output is fixed to Union{Nothing, ...} as it makes it impossible to switch to nothing. This pr adds struct SkipUpdate end and an exported const skip_update = SkipUpdate() to fix this.

Note that this can't be deprecated cleanly because the old and new use of nothing can't be differentiate. So instead I added a Preferneces.jl based global to enable/disable warnings. These also give a bunch of information about the edge to hopefully make it easier to find the actual issue.

I also deprecated some special casing for edge initialization which allows return nothing to initialize all outputs rather than the usual on value per output. I don't see why nothing should be treated differently here.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[ffreyer]

/benchmark 20

[MakieBot]

Benchmark Results

SHA: eb33c7e6de6ec169614ebe0c157b384993640007 · 20 samples

Warning

These results are subject to substantial noise because GitHub's CI runs on shared machines that are not ideally suited for benchmarking.