process.jl

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

abstract type AbstractCmd end

# libuv process option flags
const UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS = UInt8(1 << 2)
const UV_PROCESS_DETACHED = UInt8(1 << 3)
const UV_PROCESS_WINDOWS_HIDE = UInt8(1 << 4)

struct Cmd <: AbstractCmd
    exec::Vector{String}
    ignorestatus::Bool
    flags::UInt32 # libuv process flags
    env::Union{Array{String},Nothing}
    dir::String
    Cmd(exec::Vector{String}) =
        new(exec, false, 0x00, nothing, "")
    Cmd(cmd::Cmd, ignorestatus, flags, env, dir) =
        new(cmd.exec, ignorestatus, flags, env,
            dir === cmd.dir ? dir : cstr(dir))
    function Cmd(cmd::Cmd; ignorestatus::Bool=cmd.ignorestatus, env=cmd.env, dir::AbstractString=cmd.dir,
                 detach::Bool = 0 != cmd.flags & UV_PROCESS_DETACHED,
                 windows_verbatim::Bool = 0 != cmd.flags & UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS,
                 windows_hide::Bool = 0 != cmd.flags & UV_PROCESS_WINDOWS_HIDE)
        flags = detach*UV_PROCESS_DETACHED |
                windows_verbatim*UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS |
                windows_hide*UV_PROCESS_WINDOWS_HIDE
        new(cmd.exec, ignorestatus, flags, byteenv(env),
            dir === cmd.dir ? dir : cstr(dir))
    end
end

"""
    Cmd(cmd::Cmd; ignorestatus, detach, windows_verbatim, windows_hide, env, dir)

Construct a new `Cmd` object, representing an external program and arguments, from `cmd`,
while changing the settings of the optional keyword arguments:

* `ignorestatus::Bool`: If `true` (defaults to `false`), then the `Cmd` will not throw an
  error if the return code is nonzero.
* `detach::Bool`: If `true` (defaults to `false`), then the `Cmd` will be run in a new
  process group, allowing it to outlive the `julia` process and not have Ctrl-C passed to
  it.
* `windows_verbatim::Bool`: If `true` (defaults to `false`), then on Windows the `Cmd` will
  send a command-line string to the process with no quoting or escaping of arguments, even
  arguments containing spaces. (On Windows, arguments are sent to a program as a single
  "command-line" string, and programs are responsible for parsing it into arguments. By
  default, empty arguments and arguments with spaces or tabs are quoted with double quotes
  `"` in the command line, and `\\` or `"` are preceded by backslashes.
  `windows_verbatim=true` is useful for launching programs that parse their command line in
  nonstandard ways.) Has no effect on non-Windows systems.
* `windows_hide::Bool`: If `true` (defaults to `false`), then on Windows no new console
  window is displayed when the `Cmd` is executed. This has no effect if a console is
  already open or on non-Windows systems.
* `env`: Set environment variables to use when running the `Cmd`. `env` is either a
  dictionary mapping strings to strings, an array of strings of the form `"var=val"`, an
  array or tuple of `"var"=>val` pairs, or `nothing`. In order to modify (rather than
  replace) the existing environment, create `env` by `copy(ENV)` and then set
  `env["var"]=val` as desired.
* `dir::AbstractString`: Specify a working directory for the command (instead
  of the current directory).

For any keywords that are not specified, the current settings from `cmd` are used. Normally,
to create a `Cmd` object in the first place, one uses backticks, e.g.

    Cmd(`echo "Hello world"`, ignorestatus=true, detach=false)
"""
Cmd

hash(x::Cmd, h::UInt) = hash(x.exec, hash(x.env, hash(x.ignorestatus, hash(x.dir, hash(x.flags, h)))))
==(x::Cmd, y::Cmd) = x.exec == y.exec && x.env == y.env && x.ignorestatus == y.ignorestatus &&
                     x.dir == y.dir && isequal(x.flags, y.flags)

struct OrCmds <: AbstractCmd
    a::AbstractCmd
    b::AbstractCmd
    OrCmds(a::AbstractCmd, b::AbstractCmd) = new(a, b)
end

struct ErrOrCmds <: AbstractCmd
    a::AbstractCmd
    b::AbstractCmd
    ErrOrCmds(a::AbstractCmd, b::AbstractCmd) = new(a, b)
end

struct AndCmds <: AbstractCmd
    a::AbstractCmd
    b::AbstractCmd
    AndCmds(a::AbstractCmd, b::AbstractCmd) = new(a, b)
end

hash(x::AndCmds, h::UInt) = hash(x.a, hash(x.b, h))
==(x::AndCmds, y::AndCmds) = x.a == y.a && x.b == y.b

shell_escape(cmd::Cmd; special::AbstractString="") =
    shell_escape(cmd.exec..., special=special)
shell_escape_posixly(cmd::Cmd) =
    shell_escape_posixly(cmd.exec...)

function show(io::IO, cmd::Cmd)
    print_env = cmd.env !== nothing
    print_dir = !isempty(cmd.dir)
    (print_env || print_dir) && print(io, "setenv(")
    print(io, '`')
    print(io, join(map(cmd.exec) do arg
        replace(sprint() do io
            with_output_color(:underline, io) do io
                print_shell_word(io, arg, shell_special)
            end
        end, '`' => "\\`")
    end, ' '))
    print(io, '`')
    print_env && (print(io, ","); show(io, cmd.env))
    print_dir && (print(io, "; dir="); show(io, cmd.dir))
    (print_dir || print_env) && print(io, ")")
end

function show(io::IO, cmds::Union{OrCmds,ErrOrCmds})
    print(io, "pipeline(")
    show(io, cmds.a)
    print(io, ", ")
    print(io, isa(cmds, ErrOrCmds) ? "stderr=" : "stdout=")
    show(io, cmds.b)
    print(io, ")")
end

function show(io::IO, cmds::AndCmds)
    show(io, cmds.a)
    print(io, " & ")
    show(io, cmds.b)
end

const STDIN_NO  = 0
const STDOUT_NO = 1
const STDERR_NO = 2

struct FileRedirect
    filename::AbstractString
    append::Bool
    function FileRedirect(filename, append)
        if lowercase(filename) == (@static Sys.iswindows() ? "nul" : "/dev/null")
            @warn "For portability use DevNull instead of a file redirect" maxlog=1
        end
        new(filename, append)
    end
end

uvhandle(::DevNullStream) = C_NULL
uvtype(::DevNullStream) = UV_STREAM

uvhandle(x::Ptr) = x
uvtype(::Ptr) = UV_STREAM

# Not actually a pointer, but that's how we pass it through the C API so it's fine
uvhandle(x::RawFD) = convert(Ptr{Cvoid}, x.fd % UInt)
uvtype(x::RawFD) = UV_RAW_FD

const Redirectable = Union{IO, FileRedirect, RawFD}
const StdIOSet = NTuple{3, Union{Redirectable, Ptr{Cvoid}}} # XXX: remove Ptr{Cvoid} once libuv is refactored to use upstream release

struct CmdRedirect <: AbstractCmd
    cmd::AbstractCmd
    handle::Redirectable
    stream_no::Int
end

function show(io::IO, cr::CmdRedirect)
    print(io, "pipeline(")
    show(io, cr.cmd)
    print(io, ", ")
    if cr.stream_no == STDOUT_NO
        print(io, "stdout=")
    elseif cr.stream_no == STDERR_NO
        print(io, "stderr=")
    elseif cr.stream_no == STDIN_NO
        print(io, "stdin=")
    end
    show(io, cr.handle)
    print(io, ")")
end

"""
    ignorestatus(command)

Mark a command object so that running it will not throw an error if the result code is non-zero.
"""
ignorestatus(cmd::Cmd) = Cmd(cmd, ignorestatus=true)
ignorestatus(cmd::Union{OrCmds,AndCmds}) =
    typeof(cmd)(ignorestatus(cmd.a), ignorestatus(cmd.b))

"""
    detach(command)

Mark a command object so that it will be run in a new process group, allowing it to outlive the julia process, and not have Ctrl-C interrupts passed to it.
"""
detach(cmd::Cmd) = Cmd(cmd; detach=true)

# like String(s), but throw an error if s contains NUL, since
# libuv requires NUL-terminated strings
function cstr(s)
    if Base.containsnul(s)
        throw(ArgumentError("strings containing NUL cannot be passed to spawned processes"))
    end
    return String(s)
end

# convert various env representations into an array of "key=val" strings
byteenv(env::AbstractArray{<:AbstractString}) =
    String[cstr(x) for x in env]
byteenv(env::AbstractDict) =
    String[cstr(string(k)*"="*string(v)) for (k,v) in env]
byteenv(env::Nothing) = nothing
byteenv(env::Union{AbstractVector{Pair{T}}, Tuple{Vararg{Pair{T}}}}) where {T<:AbstractString} =
    String[cstr(k*"="*string(v)) for (k,v) in env]

"""
    setenv(command::Cmd, env; dir="")

Set environment variables to use when running the given `command`. `env` is either a
dictionary mapping strings to strings, an array of strings of the form `"var=val"`, or zero
or more `"var"=>val` pair arguments. In order to modify (rather than replace) the existing
environment, create `env` by `copy(ENV)` and then setting `env["var"]=val` as desired, or
use `withenv`.

The `dir` keyword argument can be used to specify a working directory for the command.
"""
setenv(cmd::Cmd, env; dir="") = Cmd(cmd; env=byteenv(env), dir=dir)
setenv(cmd::Cmd, env::Pair{<:AbstractString}...; dir="") =
    setenv(cmd, env; dir=dir)
setenv(cmd::Cmd; dir="") = Cmd(cmd; dir=dir)

(&)(left::AbstractCmd, right::AbstractCmd) = AndCmds(left, right)
redir_out(src::AbstractCmd, dest::AbstractCmd) = OrCmds(src, dest)
redir_err(src::AbstractCmd, dest::AbstractCmd) = ErrOrCmds(src, dest)

# Stream Redirects
redir_out(dest::Redirectable, src::AbstractCmd) = CmdRedirect(src, dest, STDIN_NO)
redir_out(src::AbstractCmd, dest::Redirectable) = CmdRedirect(src, dest, STDOUT_NO)
redir_err(src::AbstractCmd, dest::Redirectable) = CmdRedirect(src, dest, STDERR_NO)

# File redirects
redir_out(src::AbstractCmd, dest::AbstractString) = CmdRedirect(src, FileRedirect(dest, false), STDOUT_NO)
redir_out(src::AbstractString, dest::AbstractCmd) = CmdRedirect(dest, FileRedirect(src, false), STDIN_NO)
redir_err(src::AbstractCmd, dest::AbstractString) = CmdRedirect(src, FileRedirect(dest, false), STDERR_NO)
redir_out_append(src::AbstractCmd, dest::AbstractString) = CmdRedirect(src, FileRedirect(dest, true), STDOUT_NO)
redir_err_append(src::AbstractCmd, dest::AbstractString) = CmdRedirect(src, FileRedirect(dest, true), STDERR_NO)

"""
    pipeline(command; stdin, stdout, stderr, append=false)

Redirect I/O to or from the given `command`. Keyword arguments specify which of the
command's streams should be redirected. `append` controls whether file output appends to the
file. This is a more general version of the 2-argument `pipeline` function.
`pipeline(from, to)` is equivalent to `pipeline(from, stdout=to)` when `from` is a command,
and to `pipeline(to, stdin=from)` when `from` is another kind of data source.

**Examples**:

```julia
run(pipeline(`dothings`, stdout="out.txt", stderr="errs.txt"))
run(pipeline(`update`, stdout="log.txt", append=true))
```
"""
function pipeline(cmd::AbstractCmd; stdin=nothing, stdout=nothing, stderr=nothing, append::Bool=false)
    if append && stdout === nothing && stderr === nothing
        error("append set to true, but no output redirections specified")
    end
    if stdin !== nothing
        cmd = redir_out(stdin, cmd)
    end
    if stdout !== nothing
        cmd = append ? redir_out_append(cmd, stdout) : redir_out(cmd, stdout)
    end
    if stderr !== nothing
        cmd = append ? redir_err_append(cmd, stderr) : redir_err(cmd, stderr)
    end
    return cmd
end

pipeline(cmd::AbstractCmd, dest) = pipeline(cmd, stdout=dest)
pipeline(src::Union{Redirectable,AbstractString}, cmd::AbstractCmd) = pipeline(cmd, stdin=src)

"""
    pipeline(from, to, ...)

Create a pipeline from a data source to a destination. The source and destination can be
commands, I/O streams, strings, or results of other `pipeline` calls. At least one argument
must be a command. Strings refer to filenames. When called with more than two arguments,
they are chained together from left to right. For example, `pipeline(a,b,c)` is equivalent to
`pipeline(pipeline(a,b),c)`. This provides a more concise way to specify multi-stage
pipelines.

**Examples**:

```julia
run(pipeline(`ls`, `grep xyz`))
run(pipeline(`ls`, "out.txt"))
run(pipeline("out.txt", `grep xyz`))
```
"""
pipeline(a, b, c, d...) = pipeline(pipeline(a,b), c, d...)

mutable struct Process <: AbstractPipe
    cmd::Cmd
    handle::Ptr{Cvoid}
    in::IO
    out::IO
    err::IO
    exitcode::Int64
    termsignal::Int32
    exitnotify::Condition
    closenotify::Condition
    openstream::Symbol # for open(cmd) deprecation
    function Process(cmd::Cmd, handle::Ptr{Cvoid},
                     in::Union{Redirectable, Ptr{Cvoid}},
                     out::Union{Redirectable, Ptr{Cvoid}},
                     err::Union{Redirectable, Ptr{Cvoid}})
        if !isa(in, IO)
            in = DevNull
        end
        if !isa(out, IO)
            out = DevNull
        end
        if !isa(err, IO)
            err = DevNull
        end
        this = new(cmd, handle, in, out, err,
                   typemin(fieldtype(Process, :exitcode)),
                   typemin(fieldtype(Process, :termsignal)),
                   Condition(), Condition())
        finalizer(uvfinalize, this)
        return this
    end
end
pipe_reader(p::Process) = p.out
pipe_writer(p::Process) = p.in

struct ProcessChain <: AbstractPipe
    processes::Vector{Process}
    in::Redirectable
    out::Redirectable
    err::Redirectable
    openstream::Symbol # for open(cmd) deprecation
    ProcessChain(stdios::StdIOSet) = new(Process[], stdios[1], stdios[2], stdios[3])
    ProcessChain(chain::ProcessChain, openstream::Symbol) = new(chain.processes, chain.in, chain.out, chain.err, openstream) # for open(cmd) deprecation
end
pipe_reader(p::ProcessChain) = p.out
pipe_writer(p::ProcessChain) = p.in

function _jl_spawn(cmd, argv, loop::Ptr{Cvoid}, pp::Process,
                   in, out, err)
    proc = Libc.malloc(_sizeof_uv_process)
    disassociate_julia_struct(proc)
    error = ccall(:jl_spawn, Int32,
        (Cstring, Ptr{Cstring}, Ptr{Cvoid}, Ptr{Cvoid}, Any, Int32,
         Ptr{Cvoid}, Int32, Ptr{Cvoid}, Int32, Ptr{Cvoid}, Int32, Ptr{Cstring}, Cstring, Ptr{Cvoid}),
        cmd, argv, loop, proc, pp, uvtype(in),
        uvhandle(in), uvtype(out), uvhandle(out), uvtype(err), uvhandle(err),
        pp.cmd.flags, pp.cmd.env === nothing ? C_NULL : pp.cmd.env, isempty(pp.cmd.dir) ? C_NULL : pp.cmd.dir,
        uv_jl_return_spawn::Ptr{Cvoid})
    if error != 0
        ccall(:jl_forceclose_uv, Cvoid, (Ptr{Cvoid},), proc)
        throw(UVError("could not spawn "*string(pp.cmd), error))
    end
    associate_julia_struct(proc, pp)
    return proc
end

function uvfinalize(proc::Process)
    if proc.handle != C_NULL
        disassociate_julia_struct(proc.handle)
        ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), proc.handle)
        proc.handle = C_NULL
    end
    nothing
end

function uv_return_spawn(p::Ptr{Cvoid}, exit_status::Int64, termsignal::Int32)
    data = ccall(:jl_uv_process_data, Ptr{Cvoid}, (Ptr{Cvoid},), p)
    data == C_NULL && return
    proc = unsafe_pointer_to_objref(data)::Process
    proc.exitcode = exit_status
    proc.termsignal = termsignal
    ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), proc.handle)
    notify(proc.exitnotify)
    nothing
end

function _uv_hook_close(proc::Process)
    proc.handle = C_NULL
    notify(proc.closenotify)
end

function spawn(redirect::CmdRedirect, stdios::StdIOSet; chain::Union{ProcessChain, Nothing}=nothing)
    spawn(redirect.cmd,
          (redirect.stream_no == STDIN_NO  ? redirect.handle : stdios[1],
           redirect.stream_no == STDOUT_NO ? redirect.handle : stdios[2],
           redirect.stream_no == STDERR_NO ? redirect.handle : stdios[3]),
           chain=chain)
end

function spawn(cmds::OrCmds, stdios::StdIOSet; chain::Union{ProcessChain, Nothing}=nothing)
    out_pipe = Libc.malloc(_sizeof_uv_named_pipe)
    in_pipe = Libc.malloc(_sizeof_uv_named_pipe)
    link_pipe(in_pipe, false, out_pipe, false)
    if chain === nothing
        chain = ProcessChain(stdios)
    end
    try
        spawn(cmds.a, (stdios[1], out_pipe, stdios[3]), chain=chain)
        spawn(cmds.b, (in_pipe, stdios[2], stdios[3]), chain=chain)
    finally
        close_pipe_sync(out_pipe)
        close_pipe_sync(in_pipe)
        Libc.free(out_pipe)
        Libc.free(in_pipe)
    end
    chain
end

function spawn(cmds::ErrOrCmds, stdios::StdIOSet; chain::Union{ProcessChain, Nothing}=nothing)
    out_pipe = Libc.malloc(_sizeof_uv_named_pipe)
    in_pipe = Libc.malloc(_sizeof_uv_named_pipe)
    link_pipe(in_pipe, false, out_pipe, false)
    if chain === nothing
        chain = ProcessChain(stdios)
    end
    try
        spawn(cmds.a, (stdios[1], stdios[2], out_pipe), chain=chain)
        spawn(cmds.b, (in_pipe, stdios[2], stdios[3]), chain=chain)
    finally
        close_pipe_sync(out_pipe)
        close_pipe_sync(in_pipe)
        Libc.free(out_pipe)
        Libc.free(in_pipe)
    end
    chain
end

function setup_stdio(stdio::PipeEndpoint, readable::Bool)
    closeafter = false
    if stdio.status == StatusUninit
        if readable
            link_pipe(io, false, stdio, true)
        else
            link_pipe(stdio, true, io, false)
        end
        closeafter = true
    end
    return (stdio.handle, closeafter)
end

function setup_stdio(stdio::Pipe, readable::Bool)
    if stdio.in.status == StatusUninit && stdio.out.status == StatusUninit
        link_pipe(stdio)
    end
    io = readable ? stdio.out : stdio.in
    return (io, false)
end

setup_stdio(stdio::AbstractPipe, readable::Bool) =
    setup_stdio(readable ? pipe_reader(stdio) : pipe_writer(stdio), readable)

function setup_stdio(stdio::IOStream, readable::Bool)
    io = Filesystem.File(RawFD(fd(stdio)))
    return (io, false)
end

function setup_stdio(stdio::FileRedirect, readable::Bool)
    if readable
        attr = JL_O_RDONLY
        perm = zero(S_IRUSR)
    else
        attr = JL_O_WRONLY | JL_O_CREAT
        attr |= stdio.append ? JL_O_APPEND : JL_O_TRUNC
        perm = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
    end
    io = Filesystem.open(stdio.filename, attr, perm)
    return (io, true)
end

function setup_stdio(io, readable::Bool)
    # if there is no specialization,
    # assume that uvhandle and uvtype are defined for it
    return io, false
end

function setup_stdio(stdio::Ptr{Cvoid}, readable::Bool)
    return (stdio, false)
end

function close_stdio(stdio::Ptr{Cvoid})
    close_pipe_sync(stdio)
    Libc.free(stdio)
end

function close_stdio(stdio)
    close(stdio)
end

function setup_stdio(anon::Function, stdio::StdIOSet)
    in, close_in = setup_stdio(stdio[1], true)
    out, close_out = setup_stdio(stdio[2], false)
    err, close_err = setup_stdio(stdio[3], false)
    anon(in, out, err)
    close_in  && close_stdio(in)
    close_out && close_stdio(out)
    close_err && close_stdio(err)
end

function spawn(cmd::Cmd, stdios::StdIOSet; chain::Union{ProcessChain, Nothing}=nothing)
    if isempty(cmd.exec)
        throw(ArgumentError("cannot spawn empty command"))
    end
    loop = eventloop()
    pp = Process(cmd, C_NULL, stdios[1], stdios[2], stdios[3])
    setup_stdio(stdios) do in, out, err
        pp.handle = _jl_spawn(cmd.exec[1], cmd.exec, loop, pp,
                              in, out, err)
    end
    if chain !== nothing
        push!(chain.processes, pp)
    end
    pp
end

function spawn(cmds::AndCmds, stdios::StdIOSet; chain::Union{ProcessChain, Nothing}=nothing)
    if chain === nothing
        chain = ProcessChain(stdios)
    end
    setup_stdio(stdios) do in, out, err
        spawn(cmds.a, (in,out,err), chain=chain)
        spawn(cmds.b, (in,out,err), chain=chain)
    end
    chain
end

# INTERNAL
# returns stdios:
# A set of up to 256 stdio instructions, where each entry can be either:
#   | - An IO to be passed to the child
#   | - DevNull to pass /dev/null
#   | - An Filesystem.File object to redirect the output to
#   \ - A string specifying a filename to be opened

spawn_opts_swallow(stdios::StdIOSet) = (stdios,)
spawn_opts_swallow(in::Redirectable=DevNull, out::Redirectable=DevNull, err::Redirectable=DevNull, args...) =
    ((in, out, err), args...)
spawn_opts_inherit(stdios::StdIOSet) = (stdios,)
# pass original descriptors to child processes by default, because we might
# have already exhausted and closed the libuv object for our standard streams.
# this caused issue #8529.
spawn_opts_inherit(in::Redirectable=RawFD(0), out::Redirectable=RawFD(1), err::Redirectable=RawFD(2), args...) =
    ((in, out, err), args...)

"""
    spawn(command)

Run a command object asynchronously, returning the resulting `Process` object.
"""
spawn(cmds::AbstractCmd, args...; chain::Union{ProcessChain, Nothing}=nothing) =
    spawn(cmds, spawn_opts_swallow(args...)...; chain=chain)

function eachline(cmd::AbstractCmd; chomp=nothing, keep::Bool=false)
    if chomp !== nothing
        keep = !chomp
        depwarn("The `chomp=$chomp` argument to `eachline` is deprecated in favor of `keep=$keep`.", :eachline)
    end
    stdout = Pipe()
    processes = spawn(cmd, (DevNull,stdout,STDERR))
    close(stdout.in)
    out = stdout.out
    # implicitly close after reading lines, since we opened
    return EachLine(out, keep=keep,
        ondone=()->(close(out); success(processes) || pipeline_error(processes)))::EachLine
end

# return a Process object to read-to/write-from the pipeline
"""
    open(command, mode::AbstractString="r", stdio=DevNull)

Start running `command` asynchronously, and return a tuple `(stream,process)`.  If `mode` is
`"r"`, then `stream` reads from the process's standard output and `stdio` optionally
specifies the process's standard input stream.  If `mode` is `"w"`, then `stream` writes to
the process's standard input and `stdio` optionally specifies the process's standard output
stream.
"""
function open(cmds::AbstractCmd, mode::AbstractString="r", other::Redirectable=DevNull)
    if mode == "r+" || mode == "w+"
        other === DevNull || throw(ArgumentError("no other stream for mode rw+"))
        in = Pipe()
        out = Pipe()
        processes = spawn(cmds, (in,out,STDERR))
        close(in.out)
        close(out.in)
    elseif mode == "r"
        in = other
        out = Pipe()
        processes = spawn(cmds, (in,out,STDERR))
        close(out.in)
        if isa(processes, ProcessChain) # for open(cmd) deprecation
            processes = ProcessChain(processes, :out)
        else
            processes.openstream = :out
        end
    elseif mode == "w"
        in = Pipe()
        out = other
        processes = spawn(cmds, (in,out,STDERR))
        close(in.out)
        if isa(processes, ProcessChain) # for open(cmd) deprecation
            processes = ProcessChain(processes, :in)
        else
            processes.openstream = :in
        end
    else
        throw(ArgumentError("mode must be \"r\" or \"w\", not \"$mode\""))
    end
    return processes
end

"""
    open(f::Function, command, mode::AbstractString="r", stdio=DevNull)

Similar to `open(command, mode, stdio)`, but calls `f(stream)` on the resulting process
stream, then closes the input stream and waits for the process to complete.
Returns the value returned by `f`.
"""
function open(f::Function, cmds::AbstractCmd, args...)
    P = open(cmds, args...)
    ret = try
        f(P)
    catch e
        kill(P)
        rethrow(e)
    finally
        close(P.in)
    end
    success(P) || pipeline_error(P)
    return ret
end

function read(cmd::AbstractCmd)
    procs = open(cmd, "r", DevNull)
    bytes = read(procs.out)
    success(procs) || pipeline_error(procs)
    return bytes
end

read(cmd::AbstractCmd, ::Type{String}) = String(read(cmd))

"""
    run(command, args...)

Run a command object, constructed with backticks. Throws an error if anything goes wrong,
including the process exiting with a non-zero status.
"""
function run(cmds::AbstractCmd, args...)
    ps = spawn(cmds, spawn_opts_inherit(args...)...)
    success(ps) ? nothing : pipeline_error(ps)
end

# some common signal numbers that are usually available on all platforms
# and might be useful as arguments to `kill` or testing against `Process.termsignal`
const SIGHUP  = 1
const SIGINT  = 2
const SIGQUIT = 3 # !windows
const SIGKILL = 9
const SIGPIPE = 13 # !windows
const SIGTERM = 15

function test_success(proc::Process)
    @assert process_exited(proc)
    if proc.exitcode < 0
        #TODO: this codepath is not currently tested
        throw(UVError("could not start process $(string(proc.cmd))", proc.exitcode))
    end
    proc.exitcode == 0 && (proc.termsignal == 0 || proc.termsignal == SIGPIPE)
end

function success(x::Process)
    wait(x)
    return test_success(x)
end
success(procs::Vector{Process}) = mapreduce(success, &, procs)
success(procs::ProcessChain) = success(procs.processes)

"""
    success(command)

Run a command object, constructed with backticks, and tell whether it was successful (exited
with a code of 0). An exception is raised if the process cannot be started.
"""
success(cmd::AbstractCmd) = success(spawn(cmd))

function pipeline_error(proc::Process)
    if !proc.cmd.ignorestatus
        error("failed process: ", proc, " [", proc.exitcode, "]")
    end
    nothing
end

function pipeline_error(procs::ProcessChain)
    failed = Process[]
    for p = procs.processes
        if !test_success(p) && !p.cmd.ignorestatus
            push!(failed, p)
        end
    end
    isempty(failed) && return nothing
    length(failed) == 1 && pipeline_error(failed[1])
    msg = "failed processes:"
    for proc in failed
        msg = string(msg, "\n  ", proc, " [", proc.exitcode, "]")
    end
    error(msg)
end

"""
    kill(p::Process, signum=SIGTERM)

Send a signal to a process. The default is to terminate the process.
Returns successfully if the process has already exited, but throws an
error if killing the process failed for other reasons (e.g. insufficient
permissions).
"""
function kill(p::Process, signum::Integer)
    if process_running(p)
        @assert p.handle != C_NULL
        err = ccall(:uv_process_kill, Int32, (Ptr{Cvoid}, Int32), p.handle, signum)
        if err != 0 && err != UV_ESRCH
            throw(UVError("kill", err))
        end
    end
end
kill(ps::Vector{Process}) = foreach(kill, ps)
kill(ps::ProcessChain) = foreach(kill, ps.processes)
kill(p::Process) = kill(p, SIGTERM)

function _contains_newline(bufptr::Ptr{Cvoid}, len::Int32)
    return (ccall(:memchr, Ptr{Cvoid}, (Ptr{Cvoid},Int32,Csize_t), bufptr, '\n', len) != C_NULL)
end

## process status ##

"""
    process_running(p::Process)

Determine whether a process is currently running.
"""
process_running(s::Process) = s.exitcode == typemin(fieldtype(Process, :exitcode))
process_running(s::Vector{Process}) = any(process_running, s)
process_running(s::ProcessChain) = process_running(s.processes)

"""
    process_exited(p::Process)

Determine whether a process has exited.
"""
process_exited(s::Process) = !process_running(s)
process_exited(s::Vector{Process}) = all(process_exited, s)
process_exited(s::ProcessChain) = process_exited(s.processes)

process_signaled(s::Process) = (s.termsignal > 0)

#process_stopped (s::Process) = false #not supported by libuv. Do we need this?
#process_stop_signal(s::Process) = false #not supported by libuv. Do we need this?

function process_status(s::Process)
    process_running(s) ? "ProcessRunning" :
    process_signaled(s) ? "ProcessSignaled("*string(s.termsignal)*")" :
    #process_stopped(s) ? "ProcessStopped("*string(process_stop_signal(s))*")" :
    process_exited(s) ? "ProcessExited("*string(s.exitcode)*")" :
    error("process status error")
end

## implementation of `cmd` syntax ##

arg_gen()          = String[]
arg_gen(x::AbstractString) = String[cstr(x)]
arg_gen(cmd::Cmd)  = cmd.exec

function arg_gen(head)
    if applicable(start, head)
        vals = String[]
        for x in head
            push!(vals, cstr(string(x)))
        end
        return vals
    else
        return String[cstr(string(head))]
    end
end

function arg_gen(head, tail...)
    head = arg_gen(head)
    tail = arg_gen(tail...)
    vals = String[]
    for h = head, t = tail
        push!(vals, cstr(string(h,t)))
    end
    return vals
end

function cmd_gen(parsed)
    args = String[]
    for arg in parsed
        append!(args, arg_gen(arg...))
    end
    return Cmd(args)
end

macro cmd(str)
    return :(cmd_gen($(esc(shell_parse(str, special=shell_special)[1]))))
end

wait(x::Process)      = if !process_exited(x); stream_wait(x, x.exitnotify); end
wait(x::ProcessChain) = for p in x.processes; wait(p); end

show(io::IO, p::Process) = print(io, "Process(", p.cmd, ", ", process_status(p), ")")

# allow the elements of the Cmd to be accessed as an array or iterator
for f in (:length, :firstindex, :lastindex, :start, :keys, :first, :last)
    @eval $f(cmd::Cmd) = $f(cmd.exec)
end
eltype(::Type{Cmd}) = eltype(fieldtype(Cmd, :exec))
for f in (:next, :done, :getindex)
    @eval $f(cmd::Cmd, i) = $f(cmd.exec, i)
end