Shared I2C in RTIC - exercise

Basically, this solution consists of 2 major elements.

• I2cHandlerProxy which pretends to be the embedded_hal's i2c implementation. It is supposed to be consumed by a driver object during init phase.

  • It spawns i2c-related task via RTIC's Spawn object.

• ScopedTaskSpawnProvider which creates something similar to scope.

  • On creation, it populates I2cHandlerProxy with Spawn instance.
  • On drop, it sets I2cHandlerProxy Spawn reference to None.
    • After drop, accessing i2c via I2cHandlerProxy causes panic.

I reworked it and now it is runnable in qemu-system-arm with cargo run.

Major soundness issues

• STSP accepts any Spawn object (actually any object that implements I2cHandlerCallable)
  • Therefore, it will be transmuted into any other I2cHandlerCallable implementing type causing UB.
  • It is a result of struggles with lifetimes. More information in the code and comments.
• I2cCommand holds references with static lifetimes. Passing it to any spawned task with lower priority than i2c handler is a potential use-after-free.