cbtest.c

// Build with gcc --shared -o cbtest.so

#include <pthread.h>
#include <unistd.h>
#include <stdio.h>

typedef void *(*callback_t)(void *);
pthread_t thread;

callback_t cb = NULL;
void *cbdata = NULL;

void *threadtask(void *data) {
    while(1) {
        if(cb != NULL) {
            cb(cbdata);
        }
        sleep(1);
    }
    NULL;
}

void registercb(callback_t c, void *data) {
    if(cb != NULL) {
        printf("there can be only one");
        return;
    }
    cb = c;
    cbdata = data;
    pthread_create(&thread, NULL, threadtask, NULL);
}

cbtest.jl

# run this as a script after building cbtest.so

registercb(cb, data) = ccall((:registercb, "./cbtest.so"), Void,
                             (Ptr{Void}, Ptr{Void}), cb, data)

function threadtask(userdata)
    ccall(:uv_async_send, Void, (Ptr{Void},), userdata)
    C_NULL
end

waiters = Condition[]
maincond = Condition()

function jltask()
    if length(waiters) > 0
        println("waiter found")
        notify(waiters[1])
        yield() # this behaves as expected
        # wait(maincond) # the addwaiter task never wakes up if we wait
    end

    print("T")
end

work = Base.SingleAsyncWork(_ -> jltask())
threadtask_c = cfunction(threadtask, Ptr{Void}, (Ptr{Void},))
registercb(threadtask_c, work.handle)

function addwaiter()
    c = Condition()
    push!(waiters, c)
    for _ in 1:5
        wait(c)
        println("waiter awoken")
        notify(maincond)
    end
    shift!(waiters)
end

sleep(3)
addwaiter()

# Expected output:

# TTTTwaiter found
# Twaiter awoken
# waiter found
# Twaiter awoken
# waiter found
# Twaiter awoken
# ...

# output with `wait`

# TTTTwaiter found
# Twaiter found
# waiter found
# waiter found
# waiter found