Using core.async and transducers to upload files from the browser to S3 (opens in new tab)

    > I still need to look into some things like
    > how to properly shut down the `go` blocks.

I have limited core.async experience, but one simple solution would be to replace `(go (while true ... (<! report-chan)))` with a `core.async/go-loop` that `alts!` from `[report-chan, (core.async/timeout 10000)]`. If the value comes from `report-chan`, then recur. If the value comes from the timeout channel (10 seconds of no activity) then don't thus ending the go block execution.

For more control, you can pass a poison channel in from the outside. In the go-loop, `alts!` from `[report-chan, poison-chan, (timeout 10000)]`. Like the timeout channel, don't recur if the value comes from poison-chan. And that's a chan that your exterior code will have access to.

I'm pretty sure that the alts! block is absolutely necessary otherwise you will deadlock after 10 files (your channel buffer size) because nothing else is taking from upload-chan or signing-chan.

Wouldn't a pipeline be a better solution to this? Taking and throwing away the values from a channel just to trigger a transducer that puts the value in another channel as a side effect is a strange way to do things.

Also a simple println test will show that the transducer function is triggered when you put on the channel rather than when you take.