[swift-users] [Swift3][Linux][URLSession]: Core dump when trying to make simple HTTP request

[Steven Harms]

All,

Thanks for all your help thus far. I'll have some time this weekend to try
things out but wanted to let you know that I'm only using Linux-native
tools: vim, swift binary from the command line.

To this end: meaningful error messages, friendly API documentation,
comparability to Python / Node / Ruby for interacting with the web etc. are
all the things I'm investigating. Apparently a task that's come up is
friendliness around coding around the asynchrony model ;)

Thanks for the suggestions, I'll work through them soon and report back.

On Thu, Oct 27, 2016 at 6:32 PM, Dave Abrahams via swift-users <
swift-users at swift.org> wrote:

>
> Pasting a reply on behalf of Matt Wright (cc'd):
>
> > On Oct 27, 2016, at 1:57 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> >
> >
> > From: Steven Harms via swift-users <swift-users at swift.org>
> > Subject: [Swift3][Linux][URLSession]: Core dump when trying to make
> simple HTTP request
> ...<schnipp 40>...
> > Date: October 27, 2016 at 5:01:58 AM PDT
> > To: swift-users at swift.org
> > Reply-To: Steven Harms <sgharms at stevengharms.com>
> >
> >
> > Hello Swift,
> >
> > I've been trying out Swift as a general utility language on the server
> (replacing Ruby or Node). I'm trying to write a simple image retriever via
> HTTP as a learning project.
> >
> > Inspired by this [gist][], I've written the following:
> >
> > let queue = DispatchQueue.global(qos: .background)
> >
> > let sessionConfiguration = URLSessionConfiguration.default
> > let session = URLSession(configuration: sessionConfiguration)
> >
> > print("staring sync")
> > queue.async(execute: {
> >     print(">>> [\(queue.label)]: At the queue start")
> >     print("x")
> >     let task = session.dataTask(with: URL(string: "http://google.com")!,
> completionHandler: {
> >         (data, response, error) in
> >         print("Task ran!")
> >     })
> >     print("y")
> >     task.resume()
> >     print("z")
> > })
> > print("ending sync")
> >
> > With output:
> >
> > staring sync
> > ending sync
> >
> > or...
> >
> > staring sync
> > ending sync
> > >>> [com.apple.root.background-qos]: At the queue start
> > x
> > y
> > y
> > z
> >
> > Whoa.
> >
> > At this point I'm going to have to confess that I need some help
> clarifying my model.
> >
> > 1. How did point "y" get fired twice? Or how did it happen not at all?
>
> The double-fire is something I can’t immediately come up with an
> explanation. It might be specific to the Linux dispatch
> implementation, which I’m not well versed on but it’s very strange
> that it would print twice. Not firing at all is likely because his
> snippet of code exited before the work was run on another thread. The
> “main” thread here either has to wait for the session work to finish
> explicitly, or call dispatch_main() to cede control of the main thread
> to the system (and subsequently exit out of the tool somewhere else).
>
> > 2. How did my callback for dataTask *never* fire? Even if the connection
> task *failed* the handler
> ought have fired, no?
>
> Likely similar to (1), though if URLSession uses runloops (Linux?)
> then it could also be because the runloop was never run anywhere. On
> Darwin I’d suspect the runloop for URLSession wasn’t running, I don’t
> know how it works on Linux.
>
> > 3. I didn't want to bring this whole queue business into the
> > picture, but it appears that without it the program exits before the
> > handler has a chance to fire.
>
> Probably similar to the issues in (1) and (2). If the snippet exits after
> running then a lot of this
> asynchronously scheduled work will never happen.
>
> > 4. Changing to a queue.sync from queue.async consistently produces the
> output I expect, but does
> not fire my completionHandler still.
>
> This forces the queue.sync to run in-line (on the current thread) but
> the snippet will then likely still exit before anything happens to
> cause the URLSession to finish.
>
> --
> -Dave
>
> _______________________________________________
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> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
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