ForeignCallbacks.jl

Julia 1.9+ supports foreign thread adoption this means we can now execute arbitrary code on foreign threads and ForeignCallbacks.jl is no longer needed.

Callbacks executing on a foreign (to Julia) thread are not allowed to interact with the Julia runtime. The one canonical exception is the use of Base.AsyncCondition and uv_async_send. This has worked for 1:1 cases where there is one event trigger mapped to one AsyncCondition. The problem with uv_async_send is that many triggers to the same handle can be coalesced into one invocation.

ForeignCallbacks implements a lock-free-queue that can be used to pass data from the foreign thread to a Julia callback. The data being passed must satisfy !Base.ismutabletype(T) && Base.datatype_pointerfree(T).

import ForeignCallbacks
struct Message
    id::Int
    data::Ptr{Cvoid}
end

barrier = Base.Event()
callback = ForeignCallbacks.ForeignCallback{Message}() do msg
    @info "Received message" id=msg.id
    notify(barrier)
    return
end

GC.@preserve callback begin
    token = ForeignCallbacks.ForeignToken(callback)
    ptr = @cfunction(ForeignCallbacks.notify!, Cvoid, (ForeignCallbacks.ForeignToken, Message))
    @sync Threads.@spawn begin
        msg = Message(1, C_NULL)
        ccall($ptr, Cvoid, (ForeignCallbacks.ForeignToken, Message), $token, msg)
    end
    wait(barrier)
end