Could `@everywhere` support using macros from packages that are loaded in the same block?

[oxinabox]

Consider the following (with TimeZones.jl installed in global enviroment),

using Distributed
addprocs(1)
@everywhere begin
    using TimeZones
    foo() = tz"America/New_York"
end

This should be valid as far as I know.

This failed in julia 1.6, but i think i first noticed this many many versiions ago

julia> using Distributed

julia> addprocs(1)
1-element Vector{Int64}:
 2

julia> @everywhere begin
           using TimeZones
           foo() = tz"America/New_York"
       end
ERROR: On worker 2:
LoadError: UndefVarError: @tz_str not defined
Stacktrace:
 [1] top-level scope
   @ :0
 [2] eval
   @ ./boot.jl:360
 [3] #103
   @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:274
 [4] run_work_thunk
   @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:63
 [5] run_work_thunk
   @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:72
 [6] #96
   @ ./task.jl:411
in expression starting at REPL[3]:3
Stacktrace:
 [1] sync_end(c::Channel{Any})
   @ Base ./task.jl:369
 [2] macro expansion
   @ ./task.jl:388 [inlined]
 [3] remotecall_eval(m::Module, procs::Vector{Int64}, ex::Expr)
   @ Distributed /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Distributed/src/macros.jl:223
 [4] top-level scope
   @ /usr/local/src/julia/julia-1.6/usr/share/julia/stdlib/v1.6/Distributed/src/macros.jl:207

In contrast

@everywhere using TimeZones
@everywhere foo() = tz"America/New_York"

works just fine.

Similar failure occurs with normal macros, e.g.

@everywhere begin
       using JuMP
       @variable(Model(), x)
end

[KristofferC]

I think this is just how macros work, they get expanded before any code is executed. So if code in the block defines the macros then expanding the macro itself in that block can't be done. Not sure what can be done about this.

[fredrikekre]

This is not related to Distributed either

julia> begin
           using TimeZones
           foo() = tz"America/New_York"
       end
ERROR: LoadError: UndefVarError: @tz_str not defined
in expression starting at REPL[1]:3

[oxinabox]

Not sure what can be done about this.

It might be we just need to document this. but maybe we can do better.

--

I would kind argue that this remains related to Distributed. Because people look at the begin block as being passed to the everywhere macro, which will do macro magic so it is as if they loaded it on each process at global scope (not in a begin block) Rather than thinking of it as a begin block that happens to run everywhere.

I know @everywhere already pulls out using statements from within begin blocks; to ensure they are loaded on main process before elsewhere. So we could just also execute those using statements on the worker process. So effectively we expand it into effectively two @everywhere statements? One that just has the using lines and one that is the original?

Alternatively, could we just remove the begin block on what gets run on the workers so it truely runs at global scope?

[mbauman]

Yes — obviously there's not much we can do about the general case without a huge reworking. But inside the @everywhere macro there could definitely be special support for this... and it's not terribly unreasonable to think that it would work differently. I think we should refocus the issue back on that particular use-case.

[oxinabox]

So there is a link i think we can basically take imports from https://github.com/JuliaLang/julia/blob/master/stdlib/Distributed/src/macros.jl#L198 and just generate the using from there to run first -- which it already does on the main process, just make it also for users