Backward compatible changes in the runtime

Question

If I add a new field to a struct that is not stored in storage but used as a parameter to an extrinsic, will it break decoding if a node with the changed runtime is syncing (i.e. executing older blocks where the struct didn't have the extra field? Does it matter where/how I add the extra field? example:

struct exampleStruct {
  a: u32,
  b: u32
}

// Example 1 - new field added at the begining
struct exampleStruct {
  newField: u32
  a: u32,
  b: u32
}

// Example 2 - new field added at the end
struct exampleStruct {
  a: u32,
  b: u32,
  newField: u32
}

I saw this old discussion here: https://github.com/paritytech/substrate/issues/1221 but not sure how it works for structs.

Thanks.

Answer 1

The runtime itself is stored on-chain. This is what actually executes the extrinsics for block inclusion. This means that all clients run the exact same on-chain code at any single point in time to execute extrinsics.

Once an upgrade happens, structures can be changed and the new runtime will once again be able to execute those structures. since the spec_version is used inside the signing payload, transactions created before the upgrade become invalid, so cannot be included into blocks anymore.