Module sc_executor_common::runtime_blob

This module allows for inspection and instrumentation, i.e. modifying the module to alter it’s structure or behavior, of a wasm module.

Instrumentation

In ideal world, there would be no instrumentation. However, in the real world the execution engines we use are somewhat limited in their APIs or abilities.

To give you some examples:

We need to reset the globals because when we execute the Substrate Runtime, we do not drop and create the instance anew, instead we restore some selected parts of the state.

• stack depth metering can be performed via instrumentation or deferred to the engine and say be added directly in machine code. Implementing this in machine code is rather cumbersome so instrumentation looks like a good solution.

  Stack depth metering is needed to make a wasm blob execution deterministic, which in turn is needed by the Parachain Validation Function in Polkadot.

Inspection

Inspection of a wasm module may be needed to extract some useful information, such as to extract data segment snapshot, which is helpful for quickly restoring the initial state of instances. Inspection can be also useful to prove that a wasm module possesses some properties, such as, is free of any floating point operations, which is a useful step towards making instances produced from such a module deterministic.

Structs

• RuntimeBlob
  A program blob containing a Substrate runtime.