feat(ethexe-rpc): Support subscription for all promises

[ecol-master]

@grishasobol @breathx

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
ethexe/rpc/src/lib.rs	74% smaller
ethexe/runtime/common/src/transitions.rs	25% smaller
ethexe/rpc/src/apis/injected.rs	13% smaller
Cargo.lock	Unsupported file format
Cargo.toml	Unsupported file format
ethexe/rpc/Cargo.toml	Unsupported file format
ethexe/rpc/src/errors.rs	0% smaller

[gemini-code-assist]

Summary of Changes

Hello @ecol-master, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the ethexe-rpc module by introducing a new RPC subscription endpoint that allows clients to subscribe to a stream of all SignedPromise events. This change refactors the existing promise handling mechanism, centralizing it within a new PromiseManager to efficiently manage both individual transaction promise waiters and a broadcast channel for general subscribers. The update also incorporates new Rust crates for improved error handling and asynchronous stream processing, alongside minor dependency and formatting adjustments.

Highlights

• New RPC Subscription: Introduced a new RPC subscription method, injected_subscribePromises, allowing clients to subscribe to a stream of all SignedPromise events, providing a broader view of transaction promises.
• Promise Management Refactor: Centralized the logic for handling and distributing promises into a new PromiseManager struct. This separates the concerns of managing specific transaction waiters from broadcasting promises to general subscribers.
• Timeout for Injected Transactions: Implemented a timeout mechanism for waiting on promises for specific injected transactions, preventing indefinite waits and improving robustness.
• Dependency Updates: Added new dependencies including alloy, derive_more, thiserror, and tokio-stream to support the new asynchronous stream processing, improved error handling, and other functionalities.

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[gemini-code-assist]

Code Review

This pull request introduces a new RPC subscription injected_subscribePromises to allow clients to subscribe to all signed promises, and refactors the promise handling logic into a new PromiseManager struct. The changes are a good step towards improving the RPC capabilities.

However, I've identified a few issues that need to be addressed:

• There is a critical race condition in how promise waiters are registered, which could lead to incorrect behavior under concurrent requests.
• The implementation of the new subscribe_promises endpoint is incomplete and contains a todo!.
• I've also found opportunities to improve performance by removing an unnecessary clone and to enhance logging for better observability.

Please see the detailed comments for suggestions on how to resolve these issues.

[gemini-code-assist]

As you've noted in your TODO, there is a concurrency safety issue here. The check for an existing waiter on line 245 and this unsafe registration create a time-of-check-to-time-of-use (TOCTOU) race condition.

You can resolve this by making the registration atomic. First, refactor PromiseManager::register_waiter to use DashMap's entry API, which makes it safe and allows removing register_waiter_unchecked and the unsafe block.

// In PromiseManager
pub(crate) fn register_waiter(
    &self,
    tx_hash: HashOf<InjectedTransaction>,
) -> Result<oneshot::Receiver<SignedPromise>, SubscriberAlreadyExistsError> {
    use dashmap::mapref::entry::Entry;

    let (sender, receiver) = oneshot::channel();
    match self.promise_waiters.entry(tx_hash) {
        Entry::Occupied(_) => Err(SubscriberAlreadyExistsError(tx_hash)),
        Entry::Vacant(entry) => {
            entry.insert(sender);
            Ok(receiver)
        }
    }
}

Then, you can remove the check on lines 244-250 and replace this unsafe block with a call to the new safe register_waiter.

        let promise_receiver = match self.promise_manager.register_waiter(tx_hash) {
            Ok(receiver) => receiver,
            Err(err) => {
                tracing::trace!(tx_hash = ?tx_hash, "rejecting subscription: subscriber already exists");
                pending.reject(errors::bad_request(err.to_string())).await;
                return Ok(());
            }
        };