Resolving async-stream + anyhow Issues: Understanding the Real Problem and Solutions

Discover why `async-stream` and `anyhow` combination in Rust isn't `Send`. Learn effective strategies for resolving this common issue in asynchronous programming.
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Tackling the async-stream + anyhow is not Send Challenge in Rust

As Rust developers, encountering errors is part of our journey. One such obstacle can arise when developing asynchronous applications using async-stream and anyhow. This guide will dissect the problem when these two components clash, highlighting how you can address it effectively while maintaining code ergonomics.

The Core Issue: What Went Wrong?

Consider a scenario where you have a function designed to return a stream of results, specifically Stream<Item=Result<..>>. You may have written the following function:

[[See Video to Reveal this Text or Code Snippet]]

When you try to box this stream, you might run into an error similar to:

[[See Video to Reveal this Text or Code Snippet]]

This error occurs because the mutex guard (_guard) is deemed active across an await point, preventing the future from being sent between threads.

Breakdown of the Problem: Why Is it Not Send?

Understanding the Error

The error message indicates that the MutexGuard used in your code is not Send. Here’s why this is happening:

The code you wrote ultimately expands to a match statement where an await occurs, holding onto the lock for longer than intended.

Specifically, since Err(anyhow::anyhow!("asdf"))? captures the lock at an await point, Rust's compiler believes that the MutexGuard could outlive it - thus marking the future as !Send.

An Illustrative Example

A simpler case might help illustrate the issue at hand. Consider the following function:

[[See Video to Reveal this Text or Code Snippet]]

In this code, similar use of locks results in the same !Send future error.

Effective Solutions: Making Your Stream Safe

To resolve the issue, encapsulate all locking within its own scope. This ensures the lock is dropped before yielding any stream items. Here’s the revised function to demonstrate this approach:

[[See Video to Reveal this Text or Code Snippet]]

Streamlining Error Handling

By using a scoped function, you can straightforwardly handle errors with the ? operator without unnecessary complications. This approach maintains the clean look of your asynchronous code while ensuring that locks aren't held around any await points.

Conclusion

When dealing with Rust's asynchronous programming, it’s crucial to understand how lifetimes and sends interact, particularly with async-stream and anyhow. This error, while initially frustrating, can be resolved efficiently by correctly scoping your locks. Keep these strategies in your toolkit, as they will help you maintain robust and maintainable asynchronous systems in Rust.

Keep coding smart and happy Rust programming!