# This file is a part of Julia. License is MIT: https://julialang.org/license

const max_ccall_threads = parse(Int, get(ENV, "UV_THREADPOOL_SIZE", "4"))
const thread_notifiers = [Nullable{Condition}() for i in 1:max_ccall_threads]
const threadcall_restrictor = Semaphore(max_ccall_threads)

function notify_fun(idx)
    global thread_notifiers
    notify(get(thread_notifiers[idx]))
    return
end

function init_threadcall()
    global c_notify_fun = cfunction(notify_fun, Void, Tuple{Cint})
end

"""
    @threadcall((cfunc, clib), rettype, (argtypes...), argvals...)

The `@threadcall` macro is called in the same way as [`ccall`](@ref) but does the work
in a different thread. This is useful when you want to call a blocking C
function without causing the main `julia` thread to become blocked. Concurrency
is limited by size of the libuv thread pool, which defaults to 4 threads but
can be increased by setting the `UV_THREADPOOL_SIZE` environment variable and
restarting the `julia` process.

Note that the called function should never call back into Julia.
"""
macro threadcall(f, rettype, argtypes, argvals...)
    # check for usage errors
    isa(argtypes,Expr) && argtypes.head == :tuple ||
        error("threadcall: argument types must be a tuple")
    length(argtypes.args) == length(argvals) ||
        error("threadcall: wrong number of arguments to C function")

    # hygiene escape arguments
    f = esc(f)
    rettype = esc(rettype)
    argtypes = map(esc, argtypes.args)
    argvals = map(esc, argvals)

    # construct non-allocating wrapper to call C function
    wrapper = :(function wrapper(args_ptr::Ptr{Void}, retval_ptr::Ptr{Void})
        p = args_ptr
    end)
    body = wrapper.args[2].args
    args = Symbol[]
    for (i,T) in enumerate(argtypes)
        arg = Symbol("arg", i)
        push!(body, :($arg = unsafe_load(convert(Ptr{$T}, p))))
        push!(body, :(p += sizeof($T)))
        push!(args, arg)
    end
    push!(body, :(ret = ccall($f, $rettype, ($(argtypes...),), $(args...))))
    push!(body, :(unsafe_store!(convert(Ptr{$rettype}, retval_ptr), ret)))
    push!(body, :(return sizeof($rettype)))

    # return code to generate wrapper function and send work request thread queue
    :(let
        $wrapper
        do_threadcall(wrapper, $rettype, Any[$(argtypes...)], Any[$(argvals...)])
    end)
end

function do_threadcall(wrapper::Function, rettype::Type, argtypes::Vector, argvals::Vector)
    # generate function pointer
    fun_ptr = cfunction(wrapper, Int, Tuple{Ptr{Void}, Ptr{Void}})

    # cconvert, root and unsafe_convert arguments
    roots = Any[]
    args_size = isempty(argtypes) ? 0 : sum(sizeof, argtypes)
    args_arr = Vector{UInt8}(args_size)
    ptr = pointer(args_arr)
    for (T, x) in zip(argtypes, argvals)
        y = cconvert(T, x)
        push!(roots, y)
        unsafe_store!(convert(Ptr{T}, ptr), unsafe_convert(T, y))
        ptr += sizeof(T)
    end

    # create return buffer
    ret_arr = Vector{UInt8}(sizeof(rettype))

    # wait for a worker thread to be available
    acquire(threadcall_restrictor)
    idx = findfirst(isnull, thread_notifiers)
    thread_notifiers[idx] = Nullable{Condition}(Condition())

    # queue up the work to be done
    ccall(:jl_queue_work, Void,
        (Ptr{Void}, Ptr{UInt8}, Ptr{UInt8}, Ptr{Void}, Cint),
        fun_ptr, args_arr, ret_arr, c_notify_fun, idx)

    # wait for a result & return it
    wait(get(thread_notifiers[idx]))
    thread_notifiers[idx] = Nullable{Condition}()
    release(threadcall_restrictor)

    unsafe_load(convert(Ptr{rettype}, pointer(ret_arr)))
end