Tasks and exceptions

[Keno]

Working with exceptions in the presence of tasks can be a little frustrating. Two concrete issues I encountered:

julia> t = @schedule try
       while true; sleep(1); end
       catch e
       isa(e,InterruptException) || rethrow(e)
       end
Task (waiting) @0x00000001116c6cb0

julia> try
       error("You will never see me")
       catch e
       rethrow(e)
       finally
       Base.throwto(t,InterruptException)
       end
ERROR: InterruptException
 in throwto at ./task.jl:29
 in anonymous at ./no file:6

This is because exception in transit is not task local.

The other problem is that stack traces are pretty bad when waiting on a task:

julia> foo() = error("Foo")
foo (generic function with 1 method)

julia> foo()
ERROR: Foo
 in foo at ./none:1

julia> wait(@schedule foo())
ERROR: Foo
 in yieldto at ./task.jl:24
 in wait at ./task.jl:313
 in wait at ./task.jl:228
 in wait at ./task.jl:61

The backtrace from the task is not shown anywhere. Ideally it would be displayed somehow.

[amitmurthy]

In the second case, "handled" tasks do not print their backtraces. However, it can still be accessed if you have a reference to the task object.

t=@schedule foo(); wait(t);
Base.show_backtrace(STDOUT, t.backtrace)

[amitmurthy]

In the first case, backtrace of t can be accessed by

julia> Base.show_backtrace(STDOUT, t.backtrace)

 in yieldto at ./task.jl:75
 in wait at ./task.jl:371
 in wait at ./task.jl:286
 in sleep at stream.jl:616
 in anonymous at task.jl:2

The Base.throwto seems to hide the local rethrown exception.

 try
              error("You will never see me")
              catch e
              rethrow(e)
              finally
              1
              end
ERROR: You will never see me
 in anonymous at no file:2

[amitmurthy]

Probably indirectly related - #1784

I have also seen a segfault triggered from code within a finally block - trying to get a reduced case for the same.

[amitmurthy]

Also, show of a task in error now prints its backtrace.

julia> t=@schedule foo(); wait(t);
ERROR: Foo
 in yieldto at ./task.jl:75
 in wait at ./task.jl:371
 in wait at ./task.jl:286
 in wait at ./task.jl:112

julia> t
Task (failed) @0x00007f36d4160010
ErrorException("Foo")
 in foo at none:1
 in anonymous at task.jl:67

[vtjnash]

possible solution idea https://gist.github.com/vtjnash/9cd53434cc7f7e2f7a29

[JeffBezanson]

Yes, something like that should work. It can mostly be fixed in the front end by saving the exception data around the finally block. jl_exception_in_transit was intended to be very short-lived, and shouldn't have to be task local.

[JeffBezanson]

Also, need parens at the end of Base.throwto(t,InterruptException()).

[sbchisholm]

Is this similar? I found that rethrow() (no arguments) will throw the last thrown exception even if it is not the exception that was caught in the catch statement:

type IgnoredException <: Exception end
type HandledException <: Exception end

try
    @async begin
        sleep(1.0)
        try 
            throw(IgnoredException())
        catch ex
            println("Task threw and caught $ex")
            !isa(ex, IgnoredException) && rethrow()
        end 
    end 

    println("Throwing HandledException")
    throw(HandledException())
catch ex
    if isa(ex, HandledException)
        println("Caught: $ex")
    end 
    sleep(2.0)
    println("Rethrowing...")
    rethrow(ex) # throws HandledException()
    # rethrow()  # throws IgnoredException()
end

If I don't pass ex to rethrow, it will throw the IgnoredException() that was caught and ignored by the asynchronous task. I was expecting that rethrow() would throw the exception that was caught by the catch statement.

[JeffBezanson]

Yes that looks like the same issue to me.

[sbchisholm]

Thanks @JeffBezanson.

[c42f]

Experimental PR which fixes this: #28878