feat(derive): Stage multiplexer

[clabby]

Overview

Implements a stage multiplexer macro, which creates a shim above several stage implementations and has the ability to re-link the pipeline at runtime. Upon configured fork activation, a new stage is created at runtime and the ownership of the previous stage is transferred to the new stage.

This method of stage multiplexing:

1. Preserves the use of generic types over dynamic dispatch.
2. Requires no duplication of the pipeline; It is re-linked at runtime.
3. Is generic enough to work with other stages, due to the multiplexer being implemented as a macro.
   • Doesn't make the nested generics worse, since the sub-stages are contained within the multiplexer, and only generic over the previous stage.

Downsides of this method:

1. With the current macro implementation, all stages must have the same new signature (rollup_cfg, previous stage). e: changed, can now also be used for other stages that need more parameters / generics and can benefit from multiplexing (BatchQueue)
2. This macro is currently partially unhygenic, but that should be fine for the sake of kona-derive.

ChannelAssembler

To test this abstraction, I implemented the ChannelAssembler - a simplified ChannelBank that only has to contend with a single channel, activating upon Holocene activation. This implementation is similar to the method upstream went with to simplify the pipeline along with Holocene.

Visual

[codecov]

Codecov Report

Attention: Patch coverage is 97.51381% with 9 lines in your changes missing coverage. Please review.

Project coverage is 94.1%. Comparing base (4e7f4d1) to head (e953b9a).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...tes/derive/src/stages/channel/channel_assembler.rs	97.5%	3 Missing ⚠️
...ates/derive/src/stages/channel/channel_provider.rs	97.8%	3 Missing ⚠️
crates/derive/src/stages/channel/channel_bank.rs	94.1%	2 Missing ⚠️
crates/derive/src/stages/multiplexed.rs	98.5%	1 Missing ⚠️

Additional details and impacted files

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[refcell]

I'll approve this, but I could see this macro biting us down the road. Hopefully it will be easy to modify if that happens