thoughts on Sendable and swift 5.10 concurrency warnings?

[jaredh159]

Hi there! Really appreciate the library, and have been using it in production for a couple of years.

I'm working on getting some of my codebases ready for Swift 6 by running 5.10 and turning up as many concurrency warning and strictness settings as I can.

I'm getting a decent number of warnings from this library, stuff like:

/my/api/.build/checkouts/swift-url-routing/Sources/URLRouting/Method.swift:27:21: warning: static property 'post' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6
  public static let post = Self("POST")

I would have just opened a PR adding some Sendable conformances, but I'm not really sure how to think about Parser/Printers and Sendability. Are these straightforward to conform to Sendable? or straightforward that they can not be conformed to Sendable? are there plans to do an update addressing concurrency issues? if not, would you guys be willing to give a little guidance on how you'd like to see it done and then accept a PR?

thanks again!

[mbrandonw]

Hi @jaredh159, we definitely can (and will) look into updating the library for sendability. For things like the Method warning, that is very low hanging fruit and so feel free to PR things like that at anytime.

However, I am curious how exactly you are getting warnings for our library inside your project? When I start a fresh Xcode project with max concurrency warnings, and then import Parsing, it builds with no warnings. And I think that's how it's intended to work. Are you doing something special to also get warnings for your dependencies?

[jaredh159]

yeah, i'm a bit of an outlier -- i develop almost exclusively in Neovim, and keep a terminal window open with a file watcher running swift build or swift run. for my server apps i basically never even open xcode, and for my app apps i just use it as a build tool. so, i'm seeing the warnings in the terminal, which maybe surfaces more than the xcode ui does.

the thing about Method and sendability is that to make the static let not a concurrency warning, i'd have to conform OneOf from parser-printers to Sendable, which is what was making me wonder if the conformance was straightforward, since that's a highly generic struct from a different package. or am i missing something dumb?

[mbrandonw]

i develop almost exclusively in Neovim, and keep a terminal window open with a file watcher running swift build or swift run.

I'm actually still struggling to reproduce this. I created an SPM package, added the library, enabled "StrictConcurrency" in Package.swift, and build from the command line, and there are no warnings. I even added a var x = 1 to the library to make sure that any warning shows up, and that one does.

Could you create a simple project that shows off this behavior with swift build and share it with us?

the thing about Method and sendability

Oh yes, I didn't look at the code and had thought Method was just a simple data type with some statics. I didn't remember it was a parser-printer on its own.

[jaredh159]

sure, here's a repro:

https://github.com/jaredh159/url-routing-sendable-repro

in making that, i realized that it's really a transitive dependency through vapor-routing (the repro is a nearly empty vapor app). not sure how much that matters, but running swift build from the terminal here produces the warning (plus a warning for the static let router pattern, which is really the same issue of OneOf not being Sendable)

[mbrandonw]

Hi @jaredh159, thanks for that. I see now. I think the warning happens if there is a symbol in a dependency with a warning, and then the main library/app target uses that symbol. And I'm guessing doing @preconcurrency import doesn't help either does it?

We will look into making this library (and swift-parsing) more sendable friendly soon.