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diff --git a/README.md b/README.md
index 96e69c5..88ab046 100644
--- a/README.md
+++ b/README.md
@@ -1,75 +1,75 @@
# Exile
Exile is an alternative to [ports](https://hexdocs.pm/elixir/Port.html) for running external programs. It provides back-pressure, non-blocking io, and tries to fix ports issues.
Exile is built around the idea of having demand-driven, asynchronous interaction with external process. Think of streaming a video through `ffmpeg` to serve a web request. Exile internally uses NIF. See [Rationale](#rationale) for details. It also provides stream abstraction for interacting with an external program. For example, getting audio out of a stream is as simple as
``` elixir
Exile.stream!(~w(ffmpeg -i pipe:0 -f mp3 pipe:1), input: File.stream!("music_video.mkv", [], 65535))
|> Stream.into(File.stream!("music.mp3"))
|> Stream.run()
```
`Exile.stream!` is a convenience wrapper around `Exile.Process`. If you want more control over stdin, stdout, and os process use `Exile.Process` directly.
**Note: Exile is experimental and it is still work-in-progress. Exile is based on NIF, please know the implications of it before using it**
## Rationale
Approaches, and issues
#### Port
Port is the default way of executing external commands. This is okay when you have control over the external program's implementation and the interaction is minimal. Port has several important issues.
* it can end up creating [zombie process](https://hexdocs.pm/elixir/Port.html#module-zombie-operating-system-processes)
* cannot selectively close stdin. This is required when the external programs act on EOF from stdin
* it sends command output as a message to the beam process. This does not put back pressure on the external program and leads exhausting VM memory
#### Middleware based solutions
Libraries such as [Porcelain](https://github.com/alco/porcelain/), [Erlexec](https://github.com/saleyn/erlexec), [Rambo](https://github.com/jayjun/rambo), etc. solves the first two issues associated with ports - zombie process and selectively closing STDIN. But not the third issue - having back-pressure. At a high level, these libraries solve port issues by spawning an external middleware program which in turn spawns the program we want to run. Internally uses port for reading the output and writing input. Note that these libraries are solving a different subset of issues and have different functionality, please check the relevant project page for details.
* no back-pressure
* additional os process (middleware) for every execution of your program
* in few cases such as porcelain user has to install this external program explicitly
* might not be suitable when the program requires constant communication between beam process and external program
On the plus side, unlike Exile, bugs in the implementation does not bring down whole beam VM.
#### [ExCmd](https://github.com/akash-akya/ex_cmd)
This is my other stab at solving back pressure on the external program issue. It implements a demand-driven protocol using [odu](https://github.com/akash-akya/odu) to solve this. Since ExCmd is also a port based solution, concerns previously mentioned applies to ExCmd too.
## Exile
-Internally Exile uses non-blocking asynchronous system calls to interact with the external process. It does not use port's message based communication instead does raw stdio using NIF. Uses `select()` based API for asynchronous IO. Most of the system calls are non-blocking, so it should not block the beam schedulers. Makes use of dirty-schedulers for IO.
+Internally Exile uses non-blocking asynchronous system calls to interact with the external process. It does not use port's message based communication instead does raw stdio using NIF. Uses asynchronous system calls for IO. Most of the system calls are non-blocking, so it should not block the beam schedulers. Makes use of dirty-schedulers for IO.
**Highlights**
* Back pressure
* it does not use any middleware program
* no additional os process. no performance/resource cost
* no need to install any external command
* tries to handle zombie process by attempting to cleanup external process. *But* as there is no middleware involved with exile so it is still possbile to endup with zombie process
* stream abstraction
If you are running executing huge number of external programs **concurrently** (more than few hundred) you might have to increase open file descriptors limit (`ulimit -n`)
Non-blocking io can be used for other interesting things. Such as reading named pipe (FIFO) files. `Exile.stream!(~w(cat data.pipe))` does not block schedulers so you can open hundreds of fifo files unlike default `file` based io.
#### TODO
* add benchmarks results
### 🚨 Obligatory NIF warning
As with any NIF based solution, bugs or issues in Exile implementation **can bring down the beam VM**. But NIF implementation is comparatively small and mostly uses POSIX system calls. Also, spawned external processes are still completely isolated at OS level.
If all you want is to run a command with no communication, then just sticking with `System.cmd` is a better option.
### License
Copyright (c) 2020 Akash Hiremath.
Exile source code is released under Apache License 2.0. Check [LICENSE](LICENSE.md) for more information.
diff --git a/lib/exile/process.ex b/lib/exile/process.ex
index a4bde41..ee338b3 100644
--- a/lib/exile/process.ex
+++ b/lib/exile/process.ex
@@ -1,510 +1,510 @@
defmodule Exile.Process do
@moduledoc """
GenServer which wraps spawned external command.
`Exile.stream!/1` should be preferred over this. Use this only if you need more control over the life-cycle of IO streams and OS process.
## Comparison with Port
* it is demand driven. User explicitly has to `read` the command output, and the progress of the external command is controlled using OS pipes. Exile never load more output than we can consume, so we should never experience memory issues
* it can close stdin while consuming stdout
* tries to handle zombie process by attempting to cleanup external process. Note that there is no middleware involved with exile so it is still possbile to endup with zombie process.
- Internally Exile uses non-blocking asynchronous system calls to interact with the external process. It does not use port's message based communication, instead uses raw stdio and NIF. Uses `select()` based API for asynchronous IO. Most of the system calls are non-blocking, so it should not block the beam schedulers. Make use of dirty-schedulers for IO
+ Internally Exile uses non-blocking asynchronous system calls to interact with the external process. It does not use port's message based communication, instead uses raw stdio and NIF. Uses asynchronous system calls for IO. Most of the system calls are non-blocking, so it should not block the beam schedulers. Make use of dirty-schedulers for IO
"""
alias Exile.ProcessNif, as: Nif
require Logger
use GenServer
defmodule Error do
defexception [:message]
end
alias Exile.Process.Error
@default_opts [env: []]
@doc """
Starts `Exile.ProcessServer`
Starts external program using `cmd_with_args` with options `opts`
`cmd_with_args` must be a list containing command with arguments. example: `["cat", "file.txt"]`.
### Options
* `cd` - the directory to run the command in
* `env` - a list of tuples containing environment key-value. These can be accessed in the external program
"""
@type process :: pid
@spec start_link(nonempty_list(String.t()),
cd: String.t(),
env: [{String.t(), String.t()}]
) :: {:ok, process} | {:error, any()}
def start_link(cmd_with_args, opts \\ []) do
opts = Keyword.merge(@default_opts, opts)
with {:ok, args} <- normalize_args(cmd_with_args, opts) do
GenServer.start(__MODULE__, args)
end
end
@doc """
Closes external program's input stream
"""
@spec close_stdin(process) :: :ok | {:error, any()}
def close_stdin(process) do
GenServer.call(process, :close_stdin, :infinity)
end
@doc """
Writes iodata `data` to program's input streams
This blocks when the pipe is full
"""
@spec write(process, binary) :: :ok | {:error, any()}
def write(process, iodata) do
GenServer.call(process, {:write, IO.iodata_to_binary(iodata)}, :infinity)
end
@doc """
Return bytes written by the program to output stream.
This blocks until the programs write and flush the output depending on the `size`
"""
@spec read(process, pos_integer()) ::
{:ok, iodata} | {:eof, iodata} | {:error, any()}
def read(process, size) when (is_integer(size) and size > 0) or size == :unbuffered do
GenServer.call(process, {:read, size}, :infinity)
end
def read(process) do
GenServer.call(process, {:read, :unbuffered}, :infinity)
end
@doc """
Sends signal to external program
"""
@spec kill(process, :sigkill | :sigterm) :: :ok
def kill(process, signal) when signal in [:sigkill, :sigterm] do
GenServer.call(process, {:kill, signal}, :infinity)
end
@doc """
Waits for the program to terminate.
If the program terminates before timeout, it returns `{:ok, exit_status}` else returns `:timeout`
"""
@spec await_exit(process, timeout: timeout()) :: {:ok, integer()} | :timeout
def await_exit(process, timeout \\ :infinity) do
GenServer.call(process, {:await_exit, timeout}, :infinity)
end
@doc """
Returns os pid of the command
"""
@spec os_pid(process) :: pos_integer()
def os_pid(process) do
GenServer.call(process, :os_pid, :infinity)
end
@doc """
Stops the exile process, external program will be terminated in the background
"""
@spec stop(process) :: :ok
def stop(process), do: GenServer.call(process, :stop, :infinity)
## Server
defmodule Pending do
@moduledoc false
defstruct bin: [], remaining: 0, client_pid: nil
end
defstruct [
:args,
:errno,
:port,
:socket_path,
:stdin,
:stdout,
:context,
:status,
await: %{},
pending_read: nil,
pending_write: nil
]
alias __MODULE__
def init(args) do
state = %__MODULE__{
args: args,
errno: nil,
status: :init,
await: %{},
pending_read: %Pending{},
pending_write: %Pending{}
}
{:ok, state, {:continue, nil}}
end
def handle_continue(nil, state) do
Elixir.Process.flag(:trap_exit, true)
{:noreply, start_process(state)}
end
def handle_call(:stop, _from, state) do
# TODO: pending write and read should receive "stopped" return
# value instead of exit signal
case state.status do
{:exit, _} -> :ok
_ -> Port.close(state.port)
end
{:stop, :normal, :ok, state}
end
def handle_call(:close_stdin, _from, state) do
case state.status do
{:exit, _} -> {:reply, :ok, state}
_ -> do_close(state, :stdin)
end
end
def handle_call({:await_exit, _}, _from, %{status: {:exit, status}} = state) do
{:reply, {:ok, {:exit, status}}, state}
end
def handle_call({:await_exit, timeout}, from, %{status: :start} = state) do
tref =
if timeout != :infinity do
Elixir.Process.send_after(self(), {:await_exit_timeout, from}, timeout)
else
nil
end
{:noreply, %Process{state | await: Map.put(state.await, from, tref)}}
end
+ def handle_call({:read, size}, from, state) do
+ if state.pending_read.client_pid do
+ {:reply, {:error, :pending_read}, state}
+ else
+ pending = %Pending{remaining: size, client_pid: from}
+ do_read(%Process{state | pending_read: pending})
+ end
+ end
+
def handle_call(_, _from, %{status: {:exit, status}} = state) do
{:reply, {:error, {:exit, status}}, state}
end
def handle_call({:write, binary}, from, state) do
cond do
!is_binary(binary) ->
{:reply, {:error, :not_binary}, state}
state.pending_write.client_pid ->
{:reply, {:error, :pending_write}, state}
true ->
pending = %Pending{bin: binary, client_pid: from}
do_write(%Process{state | pending_write: pending})
end
end
- def handle_call({:read, size}, from, state) do
- if state.pending_read.client_pid do
- {:reply, {:error, :pending_read}, state}
- else
- pending = %Pending{remaining: size, client_pid: from}
- do_read(%Process{state | pending_read: pending})
- end
- end
-
def handle_call(:os_pid, _from, state) do
case Port.info(state.port, :os_pid) do
{:os_pid, os_pid} ->
{:reply, {:ok, os_pid}, state}
:undefined ->
Logger.debug("Process not alive")
{:reply, :undefined, state}
end
end
def handle_call({:kill, signal}, _from, state) do
{:reply, signal(state.port, signal), state}
end
def handle_info({:await_exit_timeout, from}, state) do
GenServer.reply(from, :timeout)
{:noreply, %Process{state | await: Map.delete(state.await, from)}}
end
def handle_info({:select, _write_resource, _ref, :ready_output}, state), do: do_write(state)
def handle_info({:select, _read_resource, _ref, :ready_input}, state), do: do_read(state)
def handle_info({port, {:exit_status, exit_status}}, %{port: port} = state),
do: handle_port_exit(exit_status, state)
def handle_info({:EXIT, port, :normal}, %{port: port} = state), do: {:noreply, state}
def handle_info({:EXIT, _, reason}, state), do: {:stop, reason, state}
defp handle_port_exit(exit_status, state) do
Enum.each(state.await, fn {from, tref} ->
GenServer.reply(from, {:ok, {:exit, exit_status}})
if tref do
Elixir.Process.cancel_timer(tref)
end
end)
{:noreply, %Process{state | status: {:exit, exit_status}}, await: %{}}
end
defp do_write(%Process{pending_write: %Pending{bin: <<>>}} = state) do
GenServer.reply(state.pending_write.client_pid, :ok)
{:noreply, %{state | pending_write: %Pending{}}}
end
defp do_write(%Process{pending_write: pending} = state) do
case Nif.nif_write(state.stdin, pending.bin) do
{:ok, size} ->
if size < byte_size(pending.bin) do
binary = binary_part(pending.bin, size, byte_size(pending.bin) - size)
{:noreply, %{state | pending_write: %Pending{pending | bin: binary}}}
else
GenServer.reply(pending.client_pid, :ok)
{:noreply, %{state | pending_write: %Pending{}}}
end
{:error, :eagain} ->
{:noreply, state}
{:error, errno} ->
GenServer.reply(pending.client_pid, {:error, errno})
{:noreply, %{state | errno: errno}}
end
end
defp do_read(%Process{pending_read: %Pending{remaining: :unbuffered} = pending} = state) do
case Nif.nif_read(state.stdout, -1) do
{:ok, <<>>} ->
GenServer.reply(pending.client_pid, {:eof, []})
{:noreply, %Process{state | pending_read: %Pending{}}}
{:ok, binary} ->
GenServer.reply(pending.client_pid, {:ok, binary})
{:noreply, %Process{state | pending_read: %Pending{}}}
{:error, :eagain} ->
{:noreply, state}
{:error, errno} ->
GenServer.reply(pending.client_pid, {:error, errno})
{:noreply, %Process{state | pending_read: %Pending{}, errno: errno}}
end
end
defp do_read(%Process{pending_read: pending} = state) do
case Nif.nif_read(state.stdout, pending.remaining) do
{:ok, <<>>} ->
GenServer.reply(pending.client_pid, {:eof, pending.bin})
{:noreply, %Process{state | pending_read: %Pending{}}}
{:ok, binary} ->
if byte_size(binary) < pending.remaining do
pending = %Pending{
pending
| bin: [pending.bin | binary],
remaining: pending.remaining - byte_size(binary)
}
{:noreply, %Process{state | pending_read: pending}}
else
GenServer.reply(pending.client_pid, {:ok, [state.pending_read.bin | binary]})
{:noreply, %Process{state | pending_read: %Pending{}}}
end
{:error, :eagain} ->
{:noreply, state}
{:error, errno} ->
GenServer.reply(pending.client_pid, {:error, errno})
{:noreply, %{state | pending_read: %Pending{}, errno: errno}}
end
end
defp do_close(state, type) do
fd =
if type == :stdin do
state.stdin
else
state.stdout
end
case Nif.nif_close(fd) do
:ok ->
{:reply, :ok, state}
{:error, errno} ->
# FIXME: correct
raise errno
{:reply, {:error, errno}, %Process{state | errno: errno}}
end
end
defp normalize_cmd([cmd | _]) when is_binary(cmd) do
path = System.find_executable(cmd)
if path do
{:ok, to_charlist(path)}
else
{:error, "command not found: #{inspect(cmd)}"}
end
end
defp normalize_cmd(_cmd_with_args) do
{:error, "`cmd_with_args` must be a list of strings, Please check the documentation"}
end
defp normalize_cmd_args([_ | args]) do
if is_list(args) && Enum.all?(args, &is_binary/1) do
{:ok, Enum.map(args, &to_charlist/1)}
else
{:error, "command arguments must be list of strings. #{inspect(args)}"}
end
end
defp normalize_cd(nil), do: {:ok, ''}
defp normalize_cd(cd) do
if File.exists?(cd) && File.dir?(cd) do
{:ok, to_charlist(cd)}
else
{:error, "`:cd` must be valid directory path"}
end
end
defp normalize_env(nil), do: {:ok, []}
defp normalize_env(env) do
env =
Enum.map(env, fn {key, value} ->
{to_charlist(key), to_charlist(value)}
end)
{:ok, env}
end
defp validate_opts_fields(opts) do
{_, additional_opts} = Keyword.split(opts, [:cd, :env])
if Enum.empty?(additional_opts) do
:ok
else
{:error, "invalid opts: #{inspect(additional_opts)}"}
end
end
defp normalize_args(cmd_with_args, opts) do
with {:ok, cmd} <- normalize_cmd(cmd_with_args),
{:ok, args} <- normalize_cmd_args(cmd_with_args),
:ok <- validate_opts_fields(opts),
{:ok, cd} <- normalize_cd(opts[:cd]),
{:ok, env} <- normalize_env(opts[:env]) do
{:ok, %{cmd_with_args: [cmd | args], cd: cd, env: env}}
end
end
@spawner_path :filename.join(:code.priv_dir(:exile), "spawner")
defp exec(cmd_with_args, socket_path, env, cd) do
opts = []
opts = if cd, do: [{:cd, cd} | opts], else: []
opts = if env, do: [{:env, env} | opts], else: opts
opts =
[
:nouse_stdio,
:exit_status,
:binary,
args: [socket_path | cmd_with_args]
] ++ opts
Port.open({:spawn_executable, @spawner_path}, opts)
end
defp socket_path do
str = :crypto.strong_rand_bytes(16) |> Base.url_encode64() |> binary_part(0, 16)
path = Path.join(System.tmp_dir!(), str)
_ = :file.delete(path)
path
end
defp signal(port, sig) when sig in [:sigkill, :sigterm] do
case Port.info(port, :os_pid) do
{:os_pid, os_pid} -> Nif.nif_kill(os_pid, sig)
:undefined -> {:error, :process_not_alive}
end
end
defp start_process(%{args: %{cmd_with_args: cmd_with_args, cd: cd, env: env}} = state) do
path = socket_path()
{:ok, sock} = :socket.open(:local, :stream, :default)
try do
{:ok, _} = :socket.bind(sock, %{family: :local, path: path})
:ok = :socket.listen(sock)
port = exec(cmd_with_args, path, env, cd)
{:os_pid, os_pid} = Port.info(port, :os_pid)
Exile.Watcher.watch(self(), os_pid, path)
{stdout, stdin} = receive_fds(sock)
%Process{
state
| port: port,
status: :start,
socket_path: path,
stdin: stdin,
stdout: stdout
}
after
:socket.close(sock)
end
end
@socket_timeout 2000
defp receive_fds(lsock) do
{:ok, sock} = :socket.accept(lsock, @socket_timeout)
try do
case :socket.recvmsg(sock, @socket_timeout) do
{:ok, msg} ->
%{
ctrl: [
%{
data: <<stdin_fd_int::native-32, stdout_fd_int::native-32, _::binary>>,
level: :socket,
type: :rights
}
]
} = msg
with {:ok, stdout} <- Nif.nif_create_fd(stdout_fd_int),
{:ok, stdin} <- Nif.nif_create_fd(stdin_fd_int) do
{stdout, stdin}
else
error ->
raise Error,
message: "Failed to create fd resources\n error: #{inspect(error)}"
end
{:error, reason} ->
raise Error,
message:
"Failed to receive stdin and stdout file descriptors\n error: #{inspect(reason)}"
end
after
:socket.close(sock)
end
end
end

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