Feature: Dynamic Workflows

[koonpeng]

New feature implementation

Implemented feature

Add support for dynamic workflows, dynamic workflows are workflows that is built at runtime.

Next steps

Add deserialization of dynamic workflows.

[mxgrey]

This review is primarily an audit of the use of .unwrap() inside the critical path.

It's imperative that we bring the risk of crashing/aborting/panicking as close to zero as possible, or else we undermine the crash-free exception-free selling points of Rust.

There will certainly be cases where we can guarantee that .unwrap() is perfectly safe, and in those places we should add clear documentation to explain how we know this.

But in general we should prefer to return Err results if we aren't positively 100% certain that the unwrap will not panic.

[koonpeng]

It is hard to explain how the unwraps in workflow_builder.rs are safe. We usually access edges with a key that comes from vertices and both are built using the same data, so in theory it would never panic. But we do modify edges while building the workflow so the preconditions became very complex.

I added a UnknownDiagramError, for these unwraps, it captures the backtrace to help debug if it ever happens.

[mxgrey]

Thanks for all the rigorous iterating @koonpeng

I'm super happy with how this capability turned out. The JSON syntax is very clean and intuitive without limiting what can be done with it. The fact that the schema can be generated from native Rust structs is even better.

We still have plenty more operations to add, but this initial set is already very powerful, and most importantly it sets a strong foundation for us to continue building off of.