Prevent state from being rolled back during an error (transactional)

Question

We're writing a proxy extrinsic that dispatches a child call and consumes a 'free call'. We want to return the error of the child call to the user, but we want to consume a free call regardless.

Since I believe transactions are now transactional by default (ie rollback on error), what's the best way to return the results of the call to the user?

Answer 1

In general you cannot prevent the state from being rolled back on an error.

Did you have a look at the pallet-utility pallet?
The force_batch call has a similar semantic where errors are reported through events. It does not abort on an error but rather emits them as events. The respective event definitions look like this:

/// A single item within a Batch of dispatches has completed with no error.
ItemCompleted,

/// A single item within a Batch of dispatches has completed with error.
ItemFailed { error: DispatchError },

It then emits these events depending on the result of a previous dispatch:

if let Err(e) = result {
    Self::deposit_event(Event::ItemFailed { error: e.error });
} else {
    Self::deposit_event(Event::ItemCompleted);
}

So you could probably define similar events for FreeCallConsumed and ChildCallFailed.