Question: should IMAPClientHandler be Sendable?

[odrobnik]

Hi, we ran into Swift concurrency warnings when using IMAPClientHandler with SwiftNIO async pipeline APIs from tests in SwiftMail.

The pattern is:

try await channel.pipeline.addHandler(IMAPClientHandler())

ChannelPipeline.addHandler requires ChannelHandler & Sendable, but IMAPClientHandler is not currently declared Sendable, so the compiler warns that IMAPClientHandler does not conform to Sendable. For now we are using a test-target workaround:

@preconcurrency import NIOIMAP

extension IMAPClientHandler: @retroactive @unchecked Sendable {}

That works for our event-loop-confined usage, but it would be nicer not to need a downstream retroactive conformance. How would the maintainers prefer this to be addressed?

Options that came to mind:

• mark IMAPClientHandler as @unchecked Sendable upstream, matching NIO event-loop confinement expectations
• provide a recommended wrapper or factory for async pipeline installation
• use another model if the handler should not be treated as Sendable, possibly actor-based or otherwise isolated

Happy to open a PR if there is an intended direction.

[danieleggert]

This is related to #750 — I’ve done some work for this, and will look at this more soon and open a PR.