Wrap remote exceptions in Malt.RemoteException

[fonsp]

Fix #31

New type:

struct RemoteException <: Exception
    worker::AbstractWorker
    message::String
end

Example:

julia> Malt.remote_eval_fetch(m1, :(sqrt(-1)))
ERROR: Remote exception from Malt.Worker on port 9115:

DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
 [1] throw_complex_domainerror(f::Symbol, x::Float64)
   @ Base.Math ./math.jl:33
 [2] sqrt
   @ ./math.jl:591 [inlined]
 [3] sqrt(x::Int64)
   @ Base.Math ./math.jl:1372
 [4] top-level scope
   @ none:1
 [5] eval
   @ ./boot.jl:368 [inlined]
 [6] macro expansion
   @ ~/Documents/Malt.jl/src/worker.jl:118 [inlined]
 [7] (::var"#1#3"{Sockets.TCPSocket, UInt64, Bool, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, Tuple{Module, Expr}, typeof(Core.eval)})()
   @ Main ./task.jl:484
Stacktrace:
 [1] unwrap_worker_result(worker::Malt.Worker, result::Malt.WorkerResult)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:69
 [2] _wait_for_response(worker::Malt.Worker, msg_id::UInt64)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:304
 [3] _send_receive
   @ ~/Documents/Malt.jl/src/Malt.jl:315 [inlined]
 [4] #remotecall_fetch#41
   @ ~/Documents/Malt.jl/src/Malt.jl:385 [inlined]
 [5] remotecall_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:384 [inlined]
 [6] remote_eval_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
 [7] remote_eval_fetch(w::Malt.Worker, expr::Expr)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [8] top-level scope
   @ REPL[5]:1

This also helps to normalize the exception behaviour of the three backends.

No original exception, only String

I decided to use a different API from Distributed. Distributed tries to serialize the original exception object and stacktrace, and it has a special error-failed-to-deserialize-flow for when this goes wrong (but then you don't see any info).

I think that this will occur too often with Malt use-cases (because Malt is for non-homogenous computing, so types are non shared), and I think a Maltonian program would write a specific try-catch handler on the worker end, and uncaught remote exceptions are only meant to be consumed by humans (so a string is fine).

So in Malt, we call showerror on the worker end to get the error message + stacktrace as a String, and this is what is returned to the host. This means that the error will always be visible to the host.

New behaviour

Of all three worker types:

julia> import Malt

julia> m1 = Malt.Worker();

julia> m2 = Malt.InProcessWorker();

julia> m3 = Malt.DistributedStdlibWorker();

julia> Malt.remote_eval_fetch(m1, :(sqrt(-1)))
ERROR: Remote exception from Malt.Worker on port 9115:

DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
 [1] throw_complex_domainerror(f::Symbol, x::Float64)
   @ Base.Math ./math.jl:33
 [2] sqrt
   @ ./math.jl:591 [inlined]
 [3] sqrt(x::Int64)
   @ Base.Math ./math.jl:1372
 [4] top-level scope
   @ none:1
 [5] eval
   @ ./boot.jl:368 [inlined]
 [6] macro expansion
   @ ~/Documents/Malt.jl/src/worker.jl:118 [inlined]
 [7] (::var"#1#3"{Sockets.TCPSocket, UInt64, Bool, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, Tuple{Module, Expr}, typeof(Core.eval)})()
   @ Main ./task.jl:484
Stacktrace:
 [1] unwrap_worker_result(worker::Malt.Worker, result::Malt.WorkerResult)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:69
 [2] _wait_for_response(worker::Malt.Worker, msg_id::UInt64)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:304
 [3] _send_receive
   @ ~/Documents/Malt.jl/src/Malt.jl:315 [inlined]
 [4] #remotecall_fetch#41
   @ ~/Documents/Malt.jl/src/Malt.jl:385 [inlined]
 [5] remotecall_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:384 [inlined]
 [6] remote_eval_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
 [7] remote_eval_fetch(w::Malt.Worker, expr::Expr)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [8] top-level scope
   @ REPL[5]:1

julia> Malt.remote_eval_fetch(m2, :(sqrt(-1)))
ERROR: Remote exception from Malt.InProcessWorker in module Main:

DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
  [1] throw_complex_domainerror(f::Symbol, x::Float64)
    @ Base.Math ./math.jl:33
  [2] sqrt
    @ ./math.jl:591 [inlined]
  [3] sqrt(x::Int64)
    @ Base.Math ./math.jl:1372
  [4] top-level scope
    @ none:1
  [5] eval(m::Module, e::Any)
    @ Core ./boot.jl:368
  [6] remotecall_fetch(::typeof(Core.eval), ::Malt.InProcessWorker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Malt ~/Documents/Malt.jl/src/Malt.jl:361
  [7] remotecall_fetch
    @ ~/Documents/Malt.jl/src/Malt.jl:359 [inlined]
  [8] remote_eval_fetch
    @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
  [9] remote_eval_fetch(w::Malt.InProcessWorker, expr::Expr)
    @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [10] top-level scope
    @ REPL[6]:1
 [11] eval
    @ ./boot.jl:368 [inlined]
 [12] eval_user_input(ast::Any, backend::REPL.REPLBackend)
    @ REPL /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/REPL/src/REPL.jl:151
 [13] repl_backend_loop(backend::REPL.REPLBackend)
    @ REPL /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/REPL/src/REPL.jl:247
 [14] start_repl_backend(backend::REPL.REPLBackend, consumer::Any)
    @ REPL /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/REPL/src/REPL.jl:232
 [15] run_repl(repl::REPL.AbstractREPL, consumer::Any; backend_on_current_task::Bool)
    @ REPL /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/REPL/src/REPL.jl:369
 [16] run_repl(repl::REPL.AbstractREPL, consumer::Any)
    @ REPL /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/REPL/src/REPL.jl:355
 [17] (::Base.var"#967#969"{Bool, Bool, Bool})(REPL::Module)
    @ Base ./client.jl:419
 [18] #invokelatest#2
    @ ./essentials.jl:729 [inlined]
 [19] invokelatest
    @ ./essentials.jl:726 [inlined]
 [20] run_main_repl(interactive::Bool, quiet::Bool, banner::Bool, history_file::Bool, color_set::Bool)
    @ Base ./client.jl:404
 [21] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:318
 [22] _start()
    @ Base ./client.jl:522
Stacktrace:
 [1] remotecall_fetch(::typeof(Core.eval), ::Malt.InProcessWorker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:363
 [2] remotecall_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:359 [inlined]
 [3] remote_eval_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
 [4] remote_eval_fetch(w::Malt.InProcessWorker, expr::Expr)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [5] top-level scope
   @ REPL[6]:1

caused by: DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
  [1] throw_complex_domainerror(f::Symbol, x::Float64)
    @ Base.Math ./math.jl:33
  [2] sqrt
    @ ./math.jl:591 [inlined]
  [3] sqrt(x::Int64)
    @ Base.Math ./math.jl:1372
  [4] top-level scope
    @ none:1
  [5] eval(m::Module, e::Any)
    @ Core ./boot.jl:368
  [6] remotecall_fetch(::typeof(Core.eval), ::Malt.InProcessWorker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Malt ~/Documents/Malt.jl/src/Malt.jl:361
  [7] remotecall_fetch
    @ ~/Documents/Malt.jl/src/Malt.jl:359 [inlined]
  [8] remote_eval_fetch
    @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
  [9] remote_eval_fetch(w::Malt.InProcessWorker, expr::Expr)
    @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [10] top-level scope
    @ REPL[6]:1

julia> Malt.remote_eval_fetch(m3, :(sqrt(-1)))
ERROR: Remote exception from Malt.DistributedStdlibWorker with pid 2:

DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
  [1] throw_complex_domainerror
    @ ./math.jl:33
  [2] sqrt
    @ ./math.jl:591 [inlined]
  [3] sqrt
    @ ./math.jl:1372
  [4] top-level scope
    @ none:1
  [5] eval
    @ ./boot.jl:368
  [6] #invokelatest#2
    @ ./essentials.jl:729
  [7] invokelatest
    @ ./essentials.jl:726
  [8] #110
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:285
  [9] run_work_thunk
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:70
 [10] macro expansion
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:285 [inlined]
 [11] #109
    @ ./task.jl:484
Stacktrace:
 [1] remotecall_fetch(::Function, ::Malt.DistributedStdlibWorker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Malt ~/Documents/Malt.jl/src/DistributedStdlibWorker.jl:47
 [2] remotecall_fetch
   @ ~/Documents/Malt.jl/src/DistributedStdlibWorker.jl:58 [inlined]
 [3] remote_eval_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
 [4] remote_eval_fetch(w::Malt.DistributedStdlibWorker, expr::Expr)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [5] top-level scope
   @ REPL[7]:1

caused by: On worker 2:
DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
  [1] throw_complex_domainerror
    @ ./math.jl:33
  [2] sqrt
    @ ./math.jl:591 [inlined]
  [3] sqrt
    @ ./math.jl:1372
  [4] top-level scope
    @ none:1
  [5] eval
    @ ./boot.jl:368
  [6] #invokelatest#2
    @ ./essentials.jl:729
  [7] invokelatest
    @ ./essentials.jl:726
  [8] #110
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:285
  [9] run_work_thunk
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:70
 [10] macro expansion
    @ /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/process_messages.jl:285 [inlined]
 [11] #109
    @ ./task.jl:484
Stacktrace:
 [1] remotecall_fetch(::Function, ::Distributed.Worker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Distributed /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/remotecall.jl:465
 [2] remotecall_fetch(::Function, ::Distributed.Worker, ::Module, ::Vararg{Any})
   @ Distributed /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/remotecall.jl:454
 [3] remotecall_fetch(::Function, ::Int64, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Distributed /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/remotecall.jl:492
 [4] remotecall_fetch(::Function, ::Int64, ::Module, ::Vararg{Any})
   @ Distributed /Applications/Julia-1.8 ARM.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Distributed/src/remotecall.jl:492
 [5] remotecall_fetch(::Function, ::Malt.DistributedStdlibWorker, ::Module, ::Vararg{Any}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Malt ~/Documents/Malt.jl/src/DistributedStdlibWorker.jl:44
 [6] remotecall_fetch
   @ ~/Documents/Malt.jl/src/DistributedStdlibWorker.jl:58 [inlined]
 [7] remote_eval_fetch
   @ ~/Documents/Malt.jl/src/Malt.jl:463 [inlined]
 [8] remote_eval_fetch(w::Malt.DistributedStdlibWorker, expr::Expr)
   @ Malt ~/Documents/Malt.jl/src/Malt.jl:464
 [9] top-level scope
   @ REPL[7]:1