AgentBuilder should be clonable

[lefuturiste]

AgentBuilder should be clonable

[jsha]

One possible way: rather than implementing Clone (which is infallible), we could have a try_clone() which only succeeds if there is no Middleware.

[algesten]

Hi @lefuturiste and welcome to ureq!

While I don't object in principle to have AgentBuilder implement the Clone trait, there is a hurdle which makes it hard in the short term, and that is Middleware.

The Agent/builder contains: this:

    middleware: Vec<Box<dyn Middleware>>,

There is currently no requirement of Middleware to implement Clone, which means this Vec of middlewares are not clonable either. This leads to one of two solutions:

• Require Middleware to be Clone. This would be a breaking change and force a major version. We have no appetite for that right now, but we've started a FUTURE.md doc in the root where we can add this as a potential consideration for a future major version.
• Arc/Mutex lock the middleware chain. This would not be great for performance because Agent manages a pool of connections with the intention of being able to handle many parallel requests. Arc/mutex in that mix would work against those intentions.

[algesten]

That's a nice idea.

Or clone_without_middleware if we want to be very specific.