@async error() isn't being printed

[robsmith11]

From searching, I thought there were fixes for this, but I'm still seeing no printed error on a current nightly:

julia> versioninfo()
Julia Version 1.6.0-DEV.551
Commit 470dfa6302 (2020-07-29 07:12 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: AMD Ryzen 7 4700U with Radeon Graphics
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, btver1)
Environment:
  JULIA_NUM_THREADS = 8

julia> function f()
        @async error()
        println("done")
       end
f (generic function with 1 method)

julia> f()
done

[StefanKarpinski]

By design. If you don't wait for the result, then it vanishes. Either wait on the result or wrap this in @sync.

julia> function f()
           @sync begin
               @async error()
               println("done")
           end
       end
f (generic function with 1 method)

julia> f()
done
ERROR: TaskFailedException:

See also #33248.

[robsmith11]

My actual code has two async threads that are long-running. Is there no way to get an exception from one thread when the other thread never finishes?

Example:

@sync begin
 @async error()
 @async while true
  sleep(1)
 end
end

[JeffBezanson]

I recommend wrapping the task in a "monitor" task for debugging:

macro monitored_async(expr)
    quote
        @async begin
            t = @async $(esc(expr))
            try
                wait(t)
            catch
                display(t)
            end
        end
    end
end

@sync begin
    @monitored_async error()
    @async while true
        sleep(1)
    end
end

[JeffBezanson]

Or, if you want the entire @sync block to propagate an exception as soon as one of the enclosed tasks errors, see Experimental.@sync.

[robsmith11]

Thanks! Experimental.@sync is exactly what I want.

Selecting on multiple tasks had also worked:
https://github.com/NHDaly/Select.jl