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

"""
Support for sockets. Provides [`IPAddr`](@ref) and subtypes, [`TCPSocket`](@ref), and [`UDPSocket`](@ref).
"""
module Sockets

export
    accept,
    bind,
    connect,
    getaddrinfo,
    getalladdrinfo,
    getnameinfo,
    getipaddr,
    getipaddrs,
    islinklocaladdr,
    getpeername,
    getsockname,
    listen,
    listenany,
    recv,
    recvfrom,
    send,
    join_multicast_group,
    leave_multicast_group,
    TCPSocket,
    UDPSocket,
    @ip_str,
    IPAddr,
    IPv4,
    IPv6

import Base: isless, show, print, parse, bind, alloc_buf_hook, _uv_hook_close

using Base: LibuvStream, LibuvServer, PipeEndpoint, @handle_as, uv_error, associate_julia_struct, uvfinalize,
    notify_error, uv_req_data, uv_req_set_data, preserve_handle, unpreserve_handle, _UVError, IOError,
    eventloop, StatusUninit, StatusInit, StatusConnecting, StatusOpen, StatusClosing, StatusClosed, StatusActive,
    preserve_handle, unpreserve_handle, iolock_begin, iolock_end,
    uv_status_string, check_open, OS_HANDLE, RawFD,
    UV_EINVAL, UV_ENOMEM, UV_ENOBUFS, UV_EAGAIN, UV_ECONNABORTED, UV_EADDRINUSE, UV_EACCES, UV_EADDRNOTAVAIL,
    UV_EAI_ADDRFAMILY, UV_EAI_AGAIN, UV_EAI_BADFLAGS,
    UV_EAI_BADHINTS, UV_EAI_CANCELED, UV_EAI_FAIL,
    UV_EAI_FAMILY, UV_EAI_NODATA, UV_EAI_NONAME,
    UV_EAI_OVERFLOW, UV_EAI_PROTOCOL, UV_EAI_SERVICE,
    UV_EAI_SOCKTYPE, UV_EAI_MEMORY

include("IPAddr.jl")
include("addrinfo.jl")

"""
    TCPSocket(; delay=true)

Open a TCP socket using libuv. If `delay` is true, libuv delays creation of the
socket's file descriptor till the first [`bind`](@ref) call. `TCPSocket` has various
fields to denote the state of the socket as well as its send/receive buffers.
"""
mutable struct TCPSocket <: LibuvStream
    handle::Ptr{Cvoid}
    status::Int
    buffer::IOBuffer
    cond::Base.ThreadSynchronizer
    readerror::Any
    sendbuf::Union{IOBuffer, Nothing}
    lock::ReentrantLock # advisory lock
    throttle::Int

    function TCPSocket(handle::Ptr{Cvoid}, status)
        tcp = new(
                handle,
                status,
                PipeBuffer(),
                Base.ThreadSynchronizer(),
                nothing,
                nothing,
                ReentrantLock(),
                Base.DEFAULT_READ_BUFFER_SZ)
        associate_julia_struct(tcp.handle, tcp)
        finalizer(uvfinalize, tcp)
        return tcp
    end
end

# kw arg "delay": if true, libuv delays creation of the socket fd till the first bind call
function TCPSocket(; delay=true)
    tcp = TCPSocket(Libc.malloc(Base._sizeof_uv_tcp), StatusUninit)
    af_spec = delay ? 0 : 2   # AF_UNSPEC is 0, AF_INET is 2
    iolock_begin()
    err = ccall(:uv_tcp_init_ex, Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Cuint),
                eventloop(), tcp.handle, af_spec)
    uv_error("failed to create tcp socket", err)
    tcp.status = StatusInit
    iolock_end()
    return tcp
end

function TCPSocket(fd::OS_HANDLE)
    tcp = TCPSocket()
    iolock_begin()
    err = ccall(:uv_tcp_open, Int32, (Ptr{Cvoid}, OS_HANDLE), tcp.handle, fd)
    uv_error("tcp_open", err)
    tcp.status = StatusOpen
    iolock_end()
    return tcp
end
if OS_HANDLE != RawFD
    TCPSocket(fd::RawFD) = TCPSocket(Libc._get_osfhandle(fd))
end

Base.fd(sock::TCPSocket) = Base._fd(sock)


mutable struct TCPServer <: LibuvServer
    handle::Ptr{Cvoid}
    status::Int
    cond::Base.ThreadSynchronizer

    function TCPServer(handle::Ptr{Cvoid}, status)
        tcp = new(
            handle,
            status,
            Base.ThreadSynchronizer())
        associate_julia_struct(tcp.handle, tcp)
        finalizer(uvfinalize, tcp)
        return tcp
    end
end

# Keyword arg "delay": if true, libuv delays creation of socket fd till bind.
# It can be set to false if there is a need to set socket options before
# further calls to `bind` and `listen`, e.g. `SO_REUSEPORT`.
function TCPServer(; delay=true)
    tcp = TCPServer(Libc.malloc(Base._sizeof_uv_tcp), StatusUninit)
    af_spec = delay ? 0 : 2   # AF_UNSPEC is 0, AF_INET is 2
    iolock_begin()
    err = ccall(:uv_tcp_init_ex, Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Cuint),
                eventloop(), tcp.handle, af_spec)
    uv_error("failed to create tcp server", err)
    tcp.status = StatusInit
    iolock_end()
    return tcp
end

Base.fd(server::TCPServer) = Base._fd(server)

"""
    accept(server[, client])

Accepts a connection on the given server and returns a connection to the client. An
uninitialized client stream may be provided, in which case it will be used instead of
creating a new stream.
"""
accept(server::TCPServer) = accept(server, TCPSocket())

function accept(callback, server::LibuvServer)
    task = @async try
            while true
                client = accept(server)
                callback(client)
            end
        catch ex
            # accept below may explicitly throw UV_ECONNABORTED:
            # filter that out since we expect that error
            if !(ex isa IOError && ex.code == UV_ECONNABORTED) || isopen(server)
                rethrow()
            end
        end
    return task # caller is responsible for checking for errors
end


# UDP
"""
    UDPSocket()

Open a UDP socket using libuv. `UDPSocket` has various
fields to denote the state of the socket.
"""
mutable struct UDPSocket <: LibuvStream
    handle::Ptr{Cvoid}
    status::Int
    recvnotify::Base.ThreadSynchronizer
    cond::Base.ThreadSynchronizer

    function UDPSocket(handle::Ptr{Cvoid}, status)
        cond = Base.ThreadSynchronizer()
        udp = new(handle, status, Base.ThreadSynchronizer(cond.lock), cond)
        associate_julia_struct(udp.handle, udp)
        finalizer(uvfinalize, udp)
        return udp
    end
end
function UDPSocket()
    this = UDPSocket(Libc.malloc(Base._sizeof_uv_udp), StatusUninit)
    iolock_begin()
    err = ccall(:uv_udp_init, Cint, (Ptr{Cvoid}, Ptr{Cvoid}),
                eventloop(), this.handle)
    uv_error("failed to create udp socket", err)
    this.status = StatusInit
    iolock_end()
    return this
end

show(io::IO, stream::UDPSocket) = print(io, typeof(stream), "(", uv_status_string(stream), ")")

Base.fd(sock::UDPSocket) = Base._fd(sock)

function _uv_hook_close(sock::UDPSocket)
    lock(sock.cond)
    try
        sock.status = StatusClosed
        notify(sock.cond)
        notify_error(sock.recvnotify, EOFError())
    finally
        unlock(sock.cond)
    end
    nothing
end

# Disables dual stack mode.
const UV_TCP_IPV6ONLY = 1

# Disables dual stack mode. Only available when using ipv6 binf
const UV_UDP_IPV6ONLY = 1

# Indicates message was truncated because read buffer was too small. The
# remainder was discarded by the OS.
const UV_UDP_PARTIAL = 2

# Indicates if SO_REUSEADDR will be set when binding the handle in uv_udp_bind. This sets
# the SO_REUSEPORT socket flag on the BSDs and OS X. On other Unix platforms, it sets the
# SO_REUSEADDR flag. What that means is that multiple threads or processes can bind to the
# same address without error (provided they all set the flag) but only the last one to bind
# will receive any traffic, in effect "stealing" the port from the previous listener.
const UV_UDP_REUSEADDR = 4

##

function _bind(sock::Union{TCPServer, TCPSocket}, host::Union{IPv4, IPv6}, port::UInt16, flags::UInt32=UInt32(0))
    host_in = Ref(hton(host.host))
    return ccall(:jl_tcp_bind, Int32, (Ptr{Cvoid}, UInt16, Ptr{Cvoid}, Cuint, Cint),
            sock, hton(port), host_in, flags, host isa IPv6)
end

function _bind(sock::UDPSocket, host::Union{IPv4, IPv6}, port::UInt16, flags::UInt32=UInt32(0))
    host_in = Ref(hton(host.host))
    return ccall(:jl_udp_bind, Int32, (Ptr{Cvoid}, UInt16, Ptr{Cvoid}, Cuint, Cint),
            sock, hton(port), host_in, flags, host isa IPv6)
end

"""
    bind(socket::Union{TCPServer, UDPSocket, TCPSocket}, host::IPAddr, port::Integer; ipv6only=false, reuseaddr=false, kws...)

Bind `socket` to the given `host:port`. Note that `0.0.0.0` will listen on all devices.

* The `ipv6only` parameter disables dual stack mode. If `ipv6only=true`, only an IPv6 stack is created.
* If `reuseaddr=true`, multiple threads or processes can bind to the same address without error
  if they all set `reuseaddr=true`, but only the last to bind will receive any traffic.
"""
function bind(sock::Union{TCPServer, UDPSocket, TCPSocket}, host::IPAddr, port::Integer; ipv6only = false, reuseaddr = false, kws...)
    if sock.status != StatusInit
        error("$(typeof(sock)) is not in initialization state")
    end
    flags = 0
    if isa(host, IPv6) && ipv6only
        flags |= isa(sock, UDPSocket) ? UV_UDP_IPV6ONLY : UV_TCP_IPV6ONLY
    end
    if isa(sock, UDPSocket) && reuseaddr
        flags |= UV_UDP_REUSEADDR
    end
    iolock_begin()
    err = _bind(sock, host, UInt16(port), UInt32(flags))
    if err < 0
        iolock_end()
        if err != UV_EADDRINUSE && err != UV_EACCES && err != UV_EADDRNOTAVAIL
            #TODO: this codepath is not currently tested
            throw(_UVError("bind", err))
        else
            return false
        end
    end
    if isa(sock, TCPServer) || isa(sock, UDPSocket)
        sock.status = StatusOpen
    end
    isa(sock, UDPSocket) && setopt(sock; kws...)
    iolock_end()
    return true
end

bind(sock::TCPServer, addr::InetAddr) = bind(sock, addr.host, addr.port)

"""
    setopt(sock::UDPSocket; multicast_loop=nothing, multicast_ttl=nothing, enable_broadcast=nothing, ttl=nothing)

Set UDP socket options.

* `multicast_loop`: loopback for multicast packets (default: `true`).
* `multicast_ttl`: TTL for multicast packets (default: `nothing`).
* `enable_broadcast`: flag must be set to `true` if socket will be used for broadcast
  messages, or else the UDP system will return an access error (default: `false`).
* `ttl`: Time-to-live of packets sent on the socket (default: `nothing`).
"""
function setopt(sock::UDPSocket; multicast_loop=nothing, multicast_ttl=nothing, enable_broadcast=nothing, ttl=nothing)
    iolock_begin()
    if sock.status == StatusUninit
        error("Cannot set options on uninitialized socket")
    end
    if multicast_loop !== nothing
        uv_error("multicast_loop", ccall(:uv_udp_set_multicast_loop, Cint, (Ptr{Cvoid}, Cint), sock.handle, multicast_loop) < 0)
    end
    if multicast_ttl !== nothing
        uv_error("multicast_ttl", ccall(:uv_udp_set_multicast_ttl, Cint, (Ptr{Cvoid}, Cint), sock.handle, multicast_ttl))
    end
    if enable_broadcast !== nothing
        uv_error("enable_broadcast", ccall(:uv_udp_set_broadcast, Cint, (Ptr{Cvoid}, Cint), sock.handle, enable_broadcast))
    end
    if ttl !== nothing
        uv_error("ttl", ccall(:uv_udp_set_ttl, Cint, (Ptr{Cvoid}, Cint), sock.handle, ttl))
    end
    iolock_end()
    nothing
end

"""
    recv(socket::UDPSocket)

Read a UDP packet from the specified socket, and return the bytes received. This call blocks.
"""
function recv(sock::UDPSocket)
    addr, data = recvfrom(sock)
    return data
end

function uv_recvcb end

"""
    recvfrom(socket::UDPSocket) -> (host_port, data)

Read a UDP packet from the specified socket, returning a tuple of `(host_port, data)`, where
`host_port` will be an InetAddr{IPv4} or InetAddr{IPv6}, as appropriate.

!!! compat "Julia 1.3"
    Prior to Julia version 1.3, the first returned value was an address (`IPAddr`).
    In version 1.3 it was changed to an `InetAddr`.
"""
function recvfrom(sock::UDPSocket)
    iolock_begin()
    # If the socket has not been bound, it will be bound implicitly to ::0 and a random port
    if sock.status != StatusInit && sock.status != StatusOpen && sock.status != StatusActive
        error("UDPSocket is not initialized and open")
    end
    if ccall(:uv_is_active, Cint, (Ptr{Cvoid},), sock.handle) == 0
        err = ccall(:uv_udp_recv_start, Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}),
                    sock,
                    @cfunction(Base.uv_alloc_buf, Cvoid, (Ptr{Cvoid}, Csize_t, Ptr{Cvoid})),
                    @cfunction(uv_recvcb, Cvoid, (Ptr{Cvoid}, Cssize_t, Ptr{Cvoid}, Ptr{Cvoid}, Cuint)))
        uv_error("recv_start", err)
    end
    sock.status = StatusActive
    lock(sock.recvnotify)
    iolock_end()
    try
        From = Union{InetAddr{IPv4}, InetAddr{IPv6}}
        Data = Vector{UInt8}
        from, data = wait(sock.recvnotify)::Tuple{From, Data}
        return (from, data)
    finally
        unlock(sock.recvnotify)
    end
end

alloc_buf_hook(sock::UDPSocket, size::UInt) = (Libc.malloc(size), Int(size)) # size is always 64k from libuv

function uv_recvcb(handle::Ptr{Cvoid}, nread::Cssize_t, buf::Ptr{Cvoid}, addr::Ptr{Cvoid}, flags::Cuint)
    sock = @handle_as handle UDPSocket
    lock(sock.recvnotify)
    try
        buf_addr = ccall(:jl_uv_buf_base, Ptr{UInt8}, (Ptr{Cvoid},), buf)
        if nread == 0 && addr == C_NULL
            Libc.free(buf_addr)
        elseif nread < 0
            Libc.free(buf_addr)
            notify_error(sock.recvnotify, _UVError("recv", nread))
        elseif flags & UV_UDP_PARTIAL > 0
            Libc.free(buf_addr)
            notify_error(sock.recvnotify, "Partial message received")
        else
            buf_size = Int(ccall(:jl_uv_buf_len, Csize_t, (Ptr{Cvoid},), buf))
            if buf_size - nread < 16384 # waste at most 16k (note: buf_size is currently always 64k)
                buf = unsafe_wrap(Array, buf_addr, nread, own=true)
            else
                buf = Vector{UInt8}(undef, nread)
                GC.@preserve buf unsafe_copyto!(pointer(buf), buf_addr, nread)
                Libc.free(buf_addr)
            end
            # need to check the address type in order to convert to a Julia IPAddr
            host = IPv4(0)
            port = UInt16(0)
            if ccall(:jl_sockaddr_is_ip4, Cint, (Ptr{Cvoid},), addr) == 1
                host = IPv4(ntoh(ccall(:jl_sockaddr_host4, UInt32, (Ptr{Cvoid},), addr)))
                port = ntoh(ccall(:jl_sockaddr_port4, UInt16, (Ptr{Cvoid},), addr))
            elseif ccall(:jl_sockaddr_is_ip6, Cint, (Ptr{Cvoid},), addr) == 1
                tmp = Ref{UInt128}(0)
                scope_id = ccall(:jl_sockaddr_host6, UInt32, (Ptr{Cvoid}, Ptr{UInt128}), addr, tmp)
                host = IPv6(ntoh(tmp[]))
                port = ntoh(ccall(:jl_sockaddr_port6, UInt16, (Ptr{Cvoid},), addr))
            end
            from = InetAddr(host, port)
            notify(sock.recvnotify, (from, buf), all=false)
        end
        if sock.status == StatusActive && isempty(sock.recvnotify)
            sock.status = StatusOpen
            ccall(:uv_udp_recv_stop, Cint, (Ptr{Cvoid},), sock)
        end
    finally
        unlock(sock.recvnotify)
    end
    nothing
end

function _send_async(sock::UDPSocket, ipaddr::Union{IPv4, IPv6}, port::UInt16, buf)
    req = Libc.malloc(Base._sizeof_uv_udp_send)
    uv_req_set_data(req, C_NULL) # in case we get interrupted before arriving at the wait call
    host_in = Ref(hton(ipaddr.host))
    err = ccall(:jl_udp_send, Cint, (Ptr{Cvoid}, Ptr{Cvoid}, UInt16, Ptr{Cvoid}, Ptr{UInt8}, Csize_t, Ptr{Cvoid}, Cint),
                req, sock, hton(port), host_in, buf, sizeof(buf),
                @cfunction(Base.uv_writecb_task, Cvoid, (Ptr{Cvoid}, Cint)),
                ipaddr isa IPv6)
    if err < 0
        Libc.free(req)
        uv_error("send", err)
    end
    return req
end

"""
    send(socket::UDPSocket, host::IPAddr, port::Integer, msg)

Send `msg` over `socket` to `host:port`.
"""
function send(sock::UDPSocket, ipaddr::IPAddr, port::Integer, msg)
    # If the socket has not been bound, it will be bound implicitly to ::0 and a random port
    iolock_begin()
    if sock.status != StatusInit && sock.status != StatusOpen && sock.status != StatusActive
        error("UDPSocket is not initialized and open")
    end
    uvw = _send_async(sock, ipaddr, UInt16(port), msg)
    ct = current_task()
    preserve_handle(ct)
    Base.sigatomic_begin()
    uv_req_set_data(uvw, ct)
    iolock_end()
    status = try
        Base.sigatomic_end()
        wait()::Cint
    finally
        Base.sigatomic_end()
        iolock_begin()
        q = ct.queue; q === nothing || Base.list_deletefirst!(q::Base.IntrusiveLinkedList{Task}, ct)
        if uv_req_data(uvw) != C_NULL
            # uvw is still alive,
            # so make sure we won't get spurious notifications later
            uv_req_set_data(uvw, C_NULL)
        else
            # done with uvw
            Libc.free(uvw)
        end
        iolock_end()
        unpreserve_handle(ct)
    end
    uv_error("send", status)
    nothing
end


#from `connect`
function uv_connectcb_tcp(conn::Ptr{Cvoid}, status::Cint)
    hand = ccall(:jl_uv_connect_handle, Ptr{Cvoid}, (Ptr{Cvoid},), conn)
    sock = @handle_as hand TCPSocket
    connectcb(conn, status, hand, sock)
end

function uv_connectcb_pipe(conn::Ptr{Cvoid}, status::Cint)
    hand = ccall(:jl_uv_connect_handle, Ptr{Cvoid}, (Ptr{Cvoid},), conn)
    sock = @handle_as hand PipeEndpoint
    connectcb(conn, status, hand, sock)
end

function connectcb(conn::Ptr{Cvoid}, status::Cint, hand::Ptr{Cvoid}, sock::LibuvStream)
    lock(sock.cond)
    try
        if status >= 0 # success
            if !(sock.status == StatusClosed || sock.status == StatusClosing)
                sock.status = StatusOpen
            end
        else
            sock.readerror = _UVError("connect", status) # TODO: perhaps we should not reuse readerror for this
            if !(sock.status == StatusClosed || sock.status == StatusClosing)
                ccall(:jl_forceclose_uv, Cvoid, (Ptr{Cvoid},), hand)
                sock.status = StatusClosing
            end
        end
        notify(sock.cond)
    finally
        unlock(sock.cond)
    end
    Libc.free(conn)
    nothing
end

function connect!(sock::TCPSocket, host::Union{IPv4, IPv6}, port::Integer)
    iolock_begin()
    if sock.status != StatusInit
        error("TCPSocket is not in initialization state")
    end
    if !(0 <= port <= typemax(UInt16))
        throw(ArgumentError("port out of range, must be 0 ≤ port ≤ 65535, got $port"))
    end
    host_in = Ref(hton(host.host))
    uv_error("connect", ccall(:jl_tcp_connect, Int32, (Ptr{Cvoid}, Ptr{Cvoid}, UInt16, Ptr{Cvoid}, Cint),
                              sock, host_in, hton(UInt16(port)), @cfunction(uv_connectcb_tcp, Cvoid, (Ptr{Cvoid}, Cint)),
                              host isa IPv6))
    sock.status = StatusConnecting
    iolock_end()
    nothing
end

connect!(sock::TCPSocket, addr::InetAddr) = connect!(sock, addr.host, addr.port)

function wait_connected(x::LibuvStream)
    iolock_begin()
    check_open(x)
    isopen(x) || x.readerror === nothing || throw(x.readerror)
    preserve_handle(x)
    lock(x.cond)
    try
        while x.status == StatusConnecting
            iolock_end()
            wait(x.cond)
            unlock(x.cond)
            iolock_begin()
            lock(x.cond)
        end
        isopen(x) || x.readerror === nothing || throw(x.readerror)
    finally
        unlock(x.cond)
        unpreserve_handle(x)
    end
    iolock_end()
    nothing
end

# Default Host to localhost

"""
    connect([host], port::Integer) -> TCPSocket

Connect to the host `host` on port `port`.
"""
connect(sock::TCPSocket, port::Integer) = connect(sock, localhost, port)
connect(port::Integer) = connect(localhost, port)

# Valid connect signatures for TCP
connect(host::AbstractString, port::Integer) = connect(TCPSocket(), host, port)
connect(addr::IPAddr, port::Integer) = connect(TCPSocket(), addr, port)
connect(addr::InetAddr) = connect(TCPSocket(), addr)

function connect!(sock::TCPSocket, host::AbstractString, port::Integer)
    if sock.status != StatusInit
        error("TCPSocket is not in initialization state")
    end
    ipaddr = getaddrinfo(host)
    connect!(sock, ipaddr, port)
    return sock
end

function connect(sock::LibuvStream, args...)
    connect!(sock, args...)
    wait_connected(sock)
    return sock
end

"""
    nagle(socket::Union{TCPServer, TCPSocket}, enable::Bool)

Nagle's algorithm batches multiple small TCP packets into larger
ones. This can improve throughput but worsen latency. Nagle's algorithm
is enabled by default. This function sets whether Nagle's algorithm is
active on a given TCP server or socket. The opposite option is called
`TCP_NODELAY` in other languages.

!!! compat "Julia 1.3"
    This function requires Julia 1.3 or later.
"""
function nagle(sock::Union{TCPServer, TCPSocket}, enable::Bool)
    # disable or enable Nagle's algorithm on all OSes
    iolock_begin()
    check_open(sock)
    err = ccall(:uv_tcp_nodelay, Cint, (Ptr{Cvoid}, Cint), sock.handle, Cint(!enable))
    # TODO: check err
    iolock_end()
    return err
end

"""
    quickack(socket::Union{TCPServer, TCPSocket}, enable::Bool)

On Linux systems, the TCP_QUICKACK is disabled or enabled on `socket`.
"""
function quickack(sock::Union{TCPServer, TCPSocket}, enable::Bool)
    iolock_begin()
    check_open(sock)
    @static if Sys.islinux()
        # tcp_quickack is a linux only option
        if ccall(:jl_tcp_quickack, Cint, (Ptr{Cvoid}, Cint), sock.handle, Cint(enable)) < 0
            @warn "Networking unoptimized ( Error enabling TCP_QUICKACK : $(Libc.strerror(Libc.errno())) )" maxlog=1
        end
    end
    iolock_end()
    nothing
end


##

const BACKLOG_DEFAULT = 511

"""
    listen([addr, ]port::Integer; backlog::Integer=BACKLOG_DEFAULT) -> TCPServer

Listen on port on the address specified by `addr`.
By default this listens on `localhost` only.
To listen on all interfaces pass `IPv4(0)` or `IPv6(0)` as appropriate.
`backlog` determines how many connections can be pending (not having
called [`accept`](@ref)) before the server will begin to
reject them. The default value of `backlog` is 511.
"""
function listen(addr; backlog::Integer=BACKLOG_DEFAULT)
    sock = TCPServer()
    bind(sock, addr) || error("cannot bind to port; may already be in use or access denied")
    listen(sock; backlog=backlog)
    return sock
end
listen(port::Integer; backlog::Integer=BACKLOG_DEFAULT) = listen(localhost, port; backlog=backlog)
listen(host::IPAddr, port::Integer; backlog::Integer=BACKLOG_DEFAULT) = listen(InetAddr(host, port); backlog=backlog)

function listen(sock::LibuvServer; backlog::Integer=BACKLOG_DEFAULT)
    uv_error("listen", trylisten(sock; backlog=backlog))
    return sock
end

# from `listen`
function uv_connectioncb(stream::Ptr{Cvoid}, status::Cint)
    sock = @handle_as stream LibuvServer
    lock(sock.cond)
    try
        if status >= 0
            notify(sock.cond)
        else
            notify_error(sock.cond, _UVError("connection", status))
        end
    finally
        unlock(sock.cond)
    end
    nothing
end

function trylisten(sock::LibuvServer; backlog::Integer=BACKLOG_DEFAULT)
    iolock_begin()
    check_open(sock)
    err = ccall(:uv_listen, Cint, (Ptr{Cvoid}, Cint, Ptr{Cvoid}),
                sock, backlog, @cfunction(uv_connectioncb, Cvoid, (Ptr{Cvoid}, Cint)))
    sock.status = StatusActive
    iolock_end()
    return err
end

##

function accept_nonblock(server::TCPServer, client::TCPSocket)
    iolock_begin()
    if client.status != StatusInit
        error("client TCPSocket is not in initialization state")
    end
    err = ccall(:uv_accept, Int32, (Ptr{Cvoid}, Ptr{Cvoid}), server.handle, client.handle)
    if err == 0
        client.status = StatusOpen
    end
    iolock_end()
    return err
end

function accept_nonblock(server::TCPServer)
    client = TCPSocket()
    uv_error("accept", accept_nonblock(server, client))
    return client
end

function accept(server::LibuvServer, client::LibuvStream)
    iolock_begin()
    if server.status != StatusActive && server.status != StatusClosing && server.status != StatusClosed
        throw(ArgumentError("server not connected, make sure \"listen\" has been called"))
    end
    while isopen(server)
        err = accept_nonblock(server, client)
        if err == 0
            iolock_end()
            return client
        elseif err != UV_EAGAIN
            uv_error("accept", err)
        end
        preserve_handle(server)
        lock(server.cond)
        iolock_end()
        try
            wait(server.cond)
        finally
            unlock(server.cond)
            unpreserve_handle(server)
        end
        iolock_begin()
    end
    uv_error("accept", UV_ECONNABORTED)
    nothing
end

## Utility functions

const localhost = ip"127.0.0.1"

"""
    listenany([host::IPAddr,] port_hint; backlog::Integer=BACKLOG_DEFAULT) -> (UInt16, TCPServer)

Create a `TCPServer` on any port, using hint as a starting point. Returns a tuple of the
actual port that the server was created on and the server itself.
The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow.
"""
function listenany(host::IPAddr, default_port; backlog::Integer=BACKLOG_DEFAULT)
    addr = InetAddr(host, default_port)
    while true
        sock = TCPServer()
        if bind(sock, addr) && trylisten(sock; backlog) == 0
            if default_port == 0
                _addr, port = getsockname(sock)
                return (port, sock)
            end
            return (addr.port, sock)
        end
        close(sock)
        addr = InetAddr(addr.host, addr.port + UInt16(1))
        if addr.port == default_port
            error("no ports available")
        end
    end
end

listenany(default_port; backlog::Integer=BACKLOG_DEFAULT) = listenany(localhost, default_port; backlog)

function udp_set_membership(sock::UDPSocket, group_addr::String,
                            interface_addr::Union{Nothing, String}, operation)
    if interface_addr === nothing
        interface_addr = C_NULL
    end
    r = ccall(:uv_udp_set_membership, Cint,
              (Ptr{Cvoid}, Cstring, Cstring, Cint),
              sock.handle, group_addr, interface_addr, operation)
    uv_error("uv_udp_set_membership", r)
    return
end

"""
    join_multicast_group(sock::UDPSocket, group_addr, interface_addr = nothing)

Join a socket to a particular multicast group defined by `group_addr`.
If `interface_addr` is given, specifies a particular interface for multi-homed
systems.  Use `leave_multicast_group()` to disable reception of a group.
"""
function join_multicast_group(sock::UDPSocket, group_addr::String,
                              interface_addr::Union{Nothing, String} = nothing)
    return udp_set_membership(sock, group_addr, interface_addr, 1)
end
function join_multicast_group(sock::UDPSocket, group_addr::IPAddr,
                              interface_addr::Union{Nothing, IPAddr} = nothing)
    if interface_addr !== nothing
        interface_addr = string(interface_addr)
    end
    return join_multicast_group(sock, string(group_addr), interface_addr)
end

"""
    leave_multicast_group(sock::UDPSocket, group_addr, interface_addr = nothing)

Remove a socket from  a particular multicast group defined by `group_addr`.
If `interface_addr` is given, specifies a particular interface for multi-homed
systems.  Use `join_multicast_group()` to enable reception of a group.
"""
function leave_multicast_group(sock::UDPSocket, group_addr::String,
                               interface_addr::Union{Nothing, String} = nothing)
    return udp_set_membership(sock, group_addr, interface_addr, 0)
end
function leave_multicast_group(sock::UDPSocket, group_addr::IPAddr,
                               interface_addr::Union{Nothing, IPAddr} = nothing)
    if interface_addr !== nothing
        interface_addr = string(interface_addr)
    end
    return leave_multicast_group(sock, string(group_addr), interface_addr)
end

"""
    getsockname(sock::Union{TCPServer, TCPSocket}) -> (IPAddr, UInt16)

Get the IP address and port that the given socket is bound to.
"""
getsockname(sock::Union{TCPSocket, TCPServer}) = _sockname(sock, true)


"""
    getpeername(sock::TCPSocket) -> (IPAddr, UInt16)

Get the IP address and port of the remote endpoint that the given
socket is connected to. Valid only for connected TCP sockets.
"""
getpeername(sock::TCPSocket) = _sockname(sock, false)

function _sockname(sock, self=true)
    sock.status == StatusInit || check_open(sock)
    rport = Ref{Cushort}(0)
    raddress = zeros(UInt8, 16)
    rfamily = Ref{Cuint}(0)

    iolock_begin()
    if self
        r = ccall(:jl_tcp_getsockname, Int32,
                (Ptr{Cvoid}, Ref{Cushort}, Ptr{Cvoid}, Ref{Cuint}),
                sock.handle, rport, raddress, rfamily)
    else
        r = ccall(:jl_tcp_getpeername, Int32,
                (Ptr{Cvoid}, Ref{Cushort}, Ptr{Cvoid}, Ref{Cuint}),
                sock.handle, rport, raddress, rfamily)
    end
    iolock_end()
    uv_error("cannot obtain socket name", r)
    port = ntoh(rport[])
    af_inet6 = @static if Sys.iswindows() # AF_INET6 in <sys/socket.h>
        23
    elseif Sys.isapple()
        30
    elseif Sys.KERNEL ∈ (:FreeBSD, :DragonFly)
        28
    elseif Sys.KERNEL ∈ (:NetBSD, :OpenBSD)
        24
    else
        10
    end

    if rfamily[] == 2 # AF_INET
        addrv4 = raddress[1:4]
        naddr = ntoh(unsafe_load(Ptr{Cuint}(pointer(addrv4)), 1))
        addr = IPv4(naddr)
    elseif rfamily[] == af_inet6
        naddr = ntoh(unsafe_load(Ptr{UInt128}(pointer(raddress)), 1))
        addr = IPv6(naddr)
    else
        error(string("unsupported address family: ", rfamily[]))
    end
    return addr, port
end

# domain sockets

include("PipeServer.jl")

end
