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	diff --git a/README.md b/README.md
	index a10c933..18df93b 100644
	--- a/README.md
	+++ b/README.md
	@@ -1,77 +1,73 @@
	# Exile

	Exile is an alternative to beam [ports](https://hexdocs.pm/elixir/Port.html) for running external programs. It provides back-pressure, non-blocking io, and tries to fix issues with ports.

	Exile is built around the idea of having demand-driven, asynchronous interaction with external command. 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

	#### Port based solutions

	There are many port based libraries such as [Porcelain](https://github.com/alco/porcelain/), [Erlexec](https://github.com/saleyn/erlexec), [Rambo](https://github.com/jayjun/rambo), etc. These solve 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

	#### [ExCmd](https://github.com/akash-akya/ex_cmd)

	This is my other stab at solving back pressure on the external program issue. ExCmd also uses a middleware for the operation. But unlike the above libraries, it uses named pipes (FIFO) for io instead of port. Back-pressure created by the blocking system calls.

	__Issue__

	ExCmd uses blocking system calls for building back-pressure. For example, reading the output from the program internally resolves to a blocking [`read()`](http://man7.org/linux/man-pages/man2/read.2.html) system call. This blocks the dirty io scheduler indefinitely. Since the scheduler can not preempt system call, the scheduler will be blocked until `read()` returns. Worst case scenario: there are as many blocking read/write as there are dirty io schedulers. This can lead to starvation of other io operations, low throughput, and dreaded scheduler collapse.

	As of now, there are no non-blocking io file operations available to the user in beam. See ExCmd for possible workaround for this issue.

	## Exile

	Exile does non-blocking, asynchronous io system calls using NIF. Since it makes non-blocking system calls, schedulers are never blocked indefinitely. It does this in asynchronous fashion using `enif_select` so its efficient.

	Advantages over other approaches:

	* solves all three issues associated with port
	* it does not use any middleware
	* no additional os process. no performance/resource cost
	* no need to install any external command
	* can run many external programs in parallel without adversely affecting schedulers
	* stream abstraction for interacting with the external program
	* should be portable across POSIX compliant operating systems (not tested)

	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, spawned external processes are still completely isolated at OS level and the port issues it tries to solve are critical.

	-
	-### Usage
	If all you want is to run a command with no communication, then just sticking with `System.cmd` is a better option.
	-
	-For most of the use-cases using `Exile.stream!` abstraction should be enough. Use `Exile.Process` only if you need more control over the life-cycle of IO streams and OS process.
	diff --git a/c_src/exile.c b/c_src/exile.c
	index c985664..2eaf658 100644
	--- a/c_src/exile.c
	+++ b/c_src/exile.c
	@@ -1,683 +1,719 @@
	#ifndef _POSIX_C_SOURCE
	#define _POSIX_C_SOURCE 200809L
	#endif

	#include "erl_nif.h"
	#include <errno.h>
	#include <fcntl.h>
	#include <signal.h>
	#include <stdbool.h>
	#include <stdio.h>
	#include <string.h>
	#include <sys/types.h>
	#include <sys/wait.h>
	#include <unistd.h>

	#ifdef ERTS_DIRTY_SCHEDULERS
	#define USE_DIRTY_IO ERL_NIF_DIRTY_JOB_IO_BOUND
	#else
	#define USE_DIRTY_IO 0
	#endif

	//#define DEBUG

	#ifdef DEBUG
	#define debug(...) \
	do { \
	enif_fprintf(stderr, __VA_ARGS__); \
	enif_fprintf(stderr, "\n"); \
	} while (0)
	#define start_timing() ErlNifTime __start = enif_monotonic_time(ERL_NIF_USEC)
	#define elapsed_microseconds() (enif_monotonic_time(ERL_NIF_USEC) - __start)
	#else
	#define debug(...)
	#define start_timing()
	#define elapsed_microseconds() 0
	#endif

	#define error(...) \
	do { \
	enif_fprintf(stderr, __VA_ARGS__); \
	enif_fprintf(stderr, "\n"); \
	} while (0)

	#define GET_CTX(env, arg, ctx) \
	do { \
	ExilePriv *data = enif_priv_data(env); \
	if (enif_get_resource(env, arg, data->exec_ctx_rt, (void **)&ctx) == \
	false) { \
	return make_error(env, ATOM_INVALID_CTX); \
	} \
	} while (0);

	static const int PIPE_READ = 0;
	static const int PIPE_WRITE = 1;
	static const int PIPE_CLOSED = -1;
	static const int CMD_EXIT = -1;
	-static const int MAX_ARGUMENTS = 20;
	+static const int MAX_ARGUMENTS = 50;
	static const int MAX_ARGUMENT_LEN = 1024;
	+static const int MAX_ENV_LEN = 1024;
	static const int UNBUFFERED_READ = -1;
	static const int PIPE_BUF_SIZE = 65535;

	/* We are choosing an exit code which is not reserved see:
	* https://www.tldp.org/LDP/abs/html/exitcodes.html. */
	static const int FORK_EXEC_FAILURE = 125;

	static ERL_NIF_TERM ATOM_TRUE;
	static ERL_NIF_TERM ATOM_FALSE;
	static ERL_NIF_TERM ATOM_OK;
	static ERL_NIF_TERM ATOM_ERROR;
	static ERL_NIF_TERM ATOM_UNDEFINED;
	static ERL_NIF_TERM ATOM_INVALID_CTX;
	static ERL_NIF_TERM ATOM_PIPE_CLOSED;
	static ERL_NIF_TERM ATOM_EAGAIN;
	static ERL_NIF_TERM ATOM_ALLOC_FAILED;

	/* command exit types */
	static ERL_NIF_TERM ATOM_EXIT;
	static ERL_NIF_TERM ATOM_SIGNALED;
	static ERL_NIF_TERM ATOM_STOPPED;

	enum exec_status {
	SUCCESS,
	PIPE_CREATE_ERROR,
	PIPE_FLAG_ERROR,
	FORK_ERROR,
	PIPE_DUP_ERROR,
	NULL_DEV_OPEN_ERROR,
	};

	enum exit_type { NORMAL_EXIT, SIGNALED, STOPPED };

	typedef struct ExilePriv {
	ErlNifResourceType *exec_ctx_rt;
	ErlNifResourceType *io_rt;
	} ExilePriv;

	typedef struct ExecContext {
	int cmd_input_fd;
	int cmd_output_fd;
	int exit_status; // can be exit status or signal number depending on exit_type
	enum exit_type exit_type;
	pid_t pid;
	// these are to hold enif_select resource objects
	int *read_resource;
	int *write_resource;
	} ExecContext;

	typedef struct StartProcessResult {
	bool success;
	int err;
	ExecContext context;
	} StartProcessResult;

	/* TODO: assert if the external process is exit (?) */
	static void exec_ctx_dtor(ErlNifEnv env, void obj) {
	ExecContext *ctx = obj;
	enif_release_resource(ctx->read_resource);
	enif_release_resource(ctx->write_resource);
	debug("Exile exec_ctx_dtor called");
	}

	static void exec_ctx_stop(ErlNifEnv env, void obj, int fd,
	int is_direct_call) {
	debug("Exile exec_ctx_stop called");
	}

	static void exec_ctx_down(ErlNifEnv env, void obj, ErlNifPid *pid,
	ErlNifMonitor *monitor) {
	debug("Exile exec_ctx_down called");
	}

	static ErlNifResourceTypeInit exec_ctx_rt_init = {exec_ctx_dtor, exec_ctx_stop,
	exec_ctx_down};

	static void io_resource_dtor(ErlNifEnv env, void obj) {
	debug("Exile io_resource_dtor called");
	}

	static void io_resource_stop(ErlNifEnv env, void obj, int fd,
	int is_direct_call) {
	debug("Exile io_resource_stop called %d", fd);
	}

	static void io_resource_down(ErlNifEnv env, void obj, ErlNifPid *pid,
	ErlNifMonitor *monitor) {
	debug("Exile io_resource_down called");
	}

	static ErlNifResourceTypeInit io_rt_init = {io_resource_dtor, io_resource_stop,
	io_resource_down};

	static inline ERL_NIF_TERM make_ok(ErlNifEnv *env, ERL_NIF_TERM term) {
	return enif_make_tuple2(env, ATOM_OK, term);
	}

	static inline ERL_NIF_TERM make_error(ErlNifEnv *env, ERL_NIF_TERM term) {
	return enif_make_tuple2(env, ATOM_ERROR, term);
	}

	static int set_flag(int fd, int flags) {
	return fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) \| flags);
	}

	static void close_all(int pipes[2][2]) {
	for (int i = 0; i < 2; i++) {
	if (pipes[i][PIPE_READ] > 0)
	close(pipes[i][PIPE_READ]);
	if (pipes[i][PIPE_WRITE] > 0)
	close(pipes[i][PIPE_WRITE]);
	}
	}

	/* time is assumed to be in microseconds */
	static void notify_consumed_timeslice(ErlNifEnv *env, ErlNifTime start,
	ErlNifTime stop) {
	ErlNifTime pct;

	pct = (ErlNifTime)((stop - start) / 10);
	if (pct > 100)
	pct = 100;
	else if (pct == 0)
	pct = 1;
	enif_consume_timeslice(env, pct);
	}

	/* This is not ideal, but as of now there is no portable way to do this */
	static void close_all_fds() {
	int fd_limit = (int)sysconf(_SC_OPEN_MAX);
	for (int i = STDERR_FILENO + 1; i < fd_limit; i++)
	close(i);
	}

	-static StartProcessResult start_process(char *args[], bool stderr_to_console,
	- const char dir[1024]) {
	+static StartProcessResult start_process(char *const args[],
	+ bool stderr_to_console,
	+ const char dir[],
	+ char *const exec_env[]) {
	StartProcessResult result = {.success = false};
	pid_t pid;
	int pipes[2][2] = {{0, 0}, {0, 0}};

	if (pipe(pipes[STDIN_FILENO]) == -1 \|\| pipe(pipes[STDOUT_FILENO]) == -1) {
	result.err = errno;
	perror("[exile] failed to create pipes");
	close_all(pipes);
	return result;
	}

	const int r_cmdin = pipes[STDIN_FILENO][PIPE_READ];
	const int w_cmdin = pipes[STDIN_FILENO][PIPE_WRITE];

	const int r_cmdout = pipes[STDOUT_FILENO][PIPE_READ];
	const int w_cmdout = pipes[STDOUT_FILENO][PIPE_WRITE];

	if (set_flag(r_cmdin, O_CLOEXEC) < 0 \|\| set_flag(w_cmdout, O_CLOEXEC) < 0 \|\|
	set_flag(w_cmdin, O_CLOEXEC \| O_NONBLOCK) < 0 \|\|
	set_flag(r_cmdout, O_CLOEXEC \| O_NONBLOCK) < 0) {
	result.err = errno;
	perror("[exile] failed to set flags for pipes");
	close_all(pipes);
	return result;
	}

	switch (pid = fork()) {

	case -1:
	result.err = errno;
	perror("[exile] failed to fork");
	close_all(pipes);
	return result;

	case 0: // child

	if (dir[0] && chdir(dir) != 0) {
	perror("[exile] failed to change directory");
	_exit(FORK_EXEC_FAILURE);
	}

	close(STDIN_FILENO);
	close(STDOUT_FILENO);

	if (dup2(r_cmdin, STDIN_FILENO) < 0) {
	perror("[exile] failed to dup to stdin");

	/* We are assuming FORK_EXEC_FAILURE exit code wont be used by the command
	* we are running. Technically we can not assume any exit code here. The
	* parent can not differentiate between exit before `exec` and the normal
	* command exit.
	* One correct way to solve this might be to have a separate
	* pipe shared between child and parent and signaling the parent by
	* closing it or writing to it. */
	_exit(FORK_EXEC_FAILURE);
	}
	if (dup2(w_cmdout, STDOUT_FILENO) < 0) {
	perror("[exile] failed to dup to stdout");
	_exit(FORK_EXEC_FAILURE);
	}

	if (stderr_to_console != true) {
	close(STDERR_FILENO);
	int dev_null = open("/dev/null", O_WRONLY);

	if (dev_null == -1) {
	perror("[exile] failed to open /dev/null");
	_exit(FORK_EXEC_FAILURE);
	}

	if (dup2(dev_null, STDERR_FILENO) < 0) {
	perror("[exile] failed to dup stderr");
	_exit(FORK_EXEC_FAILURE);
	}

	close(dev_null);
	}

	close_all_fds();

	- execvp(args[0], args);
	+ execve(args[0], args, exec_env);
	perror("[exile] execvp(): failed");

	_exit(FORK_EXEC_FAILURE);

	default: // parent
	/* close file descriptors used by child */
	close(r_cmdin);
	close(w_cmdout);

	result.success = true;
	result.context.pid = pid;
	result.context.cmd_input_fd = w_cmdin;
	result.context.cmd_output_fd = r_cmdout;

	return result;
	}
	}

	/* TODO: return appropriate error instead returning generic "badarg" error */
	static ERL_NIF_TERM execute(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	- char tmp[MAX_ARGUMENTS][MAX_ARGUMENT_LEN + 1];
	- char *exec_args[MAX_ARGUMENTS + 1];
	char dir[1024] = {'\0'};
	ErlNifTime start;
	- unsigned int args_len;
	+ unsigned int args_len, env_len;
	+ ERL_NIF_TERM head, tail, list;

	start = enif_monotonic_time(ERL_NIF_USEC);

	- if (enif_get_list_length(env, argv[0], &args_len) != true)
	+ if (enif_get_list_length(env, argv[0], &args_len) != true) {
	+ error("invalid command with arguments param");
	return enif_make_badarg(env);
	+ }

	- if (args_len > MAX_ARGUMENTS)
	+ if (args_len > MAX_ARGUMENTS) {
	+ error("command argument size exceeds limit: %d", MAX_ARGUMENTS);
	return enif_make_badarg(env);
	+ }

	- ERL_NIF_TERM head, tail, list = argv[0];
	+ char _args_temp[MAX_ARGUMENTS][MAX_ARGUMENT_LEN + 1];
	+ char *exec_args[MAX_ARGUMENTS + 1];
	+
	+ list = argv[0];
	for (unsigned int i = 0; i < args_len; i++) {
	if (enif_get_list_cell(env, list, &head, &tail) != true)
	return enif_make_badarg(env);

	- if (enif_get_string(env, head, tmp[i], MAX_ARGUMENT_LEN, ERL_NIF_LATIN1) <
	- 1)
	+ if (enif_get_string(env, head, _args_temp[i], MAX_ARGUMENT_LEN,
	+ ERL_NIF_LATIN1) < 1)
	return enif_make_badarg(env);
	- exec_args[i] = tmp[i];
	+ exec_args[i] = _args_temp[i];
	list = tail;
	}
	exec_args[args_len] = NULL;

	+ if (enif_get_list_length(env, argv[1], &env_len) != true) {
	+ error("invalid env param");
	+ return enif_make_badarg(env);
	+ }
	+
	+ if (args_len > MAX_ARGUMENTS) {
	+ error("env size exceeds limit: %d", MAX_ARGUMENTS);
	+ return enif_make_badarg(env);
	+ }
	+
	+ char _env_temp[env_len][MAX_ENV_LEN];
	+ char *exec_env[env_len + 1];
	+
	+ list = argv[1];
	+ for (unsigned int i = 0; i < env_len; i++) {
	+ if (enif_get_list_cell(env, list, &head, &tail) != true)
	+ return enif_make_badarg(env);
	+
	+ if (enif_get_string(env, head, _env_temp[i], MAX_ENV_LEN, ERL_NIF_LATIN1) <
	+ 1)
	+ return enif_make_badarg(env);
	+ exec_env[i] = _env_temp[i];
	+ list = tail;
	+ }
	+ exec_env[env_len] = NULL;
	+
	bool stderr_to_console = true;
	int tmp_int;
	- if (enif_get_int(env, argv[1], &tmp_int) != true)
	+ if (enif_get_int(env, argv[3], &tmp_int) != true)
	return enif_make_badarg(env);
	stderr_to_console = tmp_int == 1 ? true : false;

	if (enif_get_string(env, argv[2], dir, 1024, ERL_NIF_LATIN1) < 0) {
	return enif_make_badarg(env);
	}

	struct ExilePriv *data = enif_priv_data(env);
	- StartProcessResult result = start_process(exec_args, stderr_to_console, dir);
	+ StartProcessResult result =
	+ start_process(exec_args, stderr_to_console, dir, exec_env);
	ExecContext *ctx = NULL;
	ERL_NIF_TERM term;

	if (result.success) {
	ctx = enif_alloc_resource(data->exec_ctx_rt, sizeof(ExecContext));
	ctx->cmd_input_fd = result.context.cmd_input_fd;
	ctx->cmd_output_fd = result.context.cmd_output_fd;
	ctx->read_resource = enif_alloc_resource(data->io_rt, sizeof(int));
	ctx->write_resource = enif_alloc_resource(data->io_rt, sizeof(int));
	ctx->pid = result.context.pid;

	debug("pid: %d cmd_in_fd: %d cmd_out_fd: %d", ctx->pid, ctx->cmd_input_fd,
	ctx->cmd_output_fd);

	term = enif_make_resource(env, ctx);

	/* resource should be collected beam GC when there are no more references */
	enif_release_resource(ctx);

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	return make_ok(env, term);
	} else {
	return make_error(env, enif_make_int(env, result.err));
	}
	}

	static int select_write(ErlNifEnv env, ExecContext ctx) {
	int retval = enif_select(env, ctx->cmd_input_fd, ERL_NIF_SELECT_WRITE,
	ctx->write_resource, NULL, ATOM_UNDEFINED);
	if (retval != 0)
	perror("select_write()");

	return retval;
	}

	static ERL_NIF_TERM sys_write(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	if (argc != 2)
	enif_make_badarg(env);

	ErlNifTime start;
	start = enif_monotonic_time(ERL_NIF_USEC);

	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->cmd_input_fd == PIPE_CLOSED)
	return make_error(env, ATOM_PIPE_CLOSED);

	ErlNifBinary bin;
	if (enif_inspect_binary(env, argv[1], &bin) != true)
	return enif_make_badarg(env);

	if (bin.size == 0)
	return enif_make_badarg(env);

	/* should we limit the bin.size here? */
	ssize_t result = write(ctx->cmd_input_fd, bin.data, bin.size);
	int write_errono = errno;

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	/* TODO: branching is ugly, cleanup required */
	if (result >= (ssize_t)bin.size) { // request completely satisfied
	return make_ok(env, enif_make_int(env, result));
	} else if (result >= 0) { // request partially satisfied
	int retval = select_write(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_ok(env, enif_make_int(env, result));
	} else if (write_errono == EAGAIN) { // busy
	int retval = select_write(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_error(env, ATOM_EAGAIN);
	} else { // Error
	perror("write()");
	return make_error(env, enif_make_int(env, write_errono));
	}
	}

	static ERL_NIF_TERM sys_close(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	int kind;
	enif_get_int(env, argv[1], &kind);

	int result;
	switch (kind) {
	case 0:
	if (ctx->cmd_input_fd == PIPE_CLOSED) {
	return ATOM_OK;
	} else {
	enif_select(env, ctx->cmd_input_fd, ERL_NIF_SELECT_STOP,
	ctx->write_resource, NULL, ATOM_UNDEFINED);
	result = close(ctx->cmd_input_fd);
	if (result == 0) {
	ctx->cmd_input_fd = PIPE_CLOSED;
	return ATOM_OK;
	} else {
	perror("cmd_input_fd close()");
	return make_error(env, enif_make_int(env, errno));
	}
	}
	case 1:
	if (ctx->cmd_output_fd == PIPE_CLOSED) {
	return ATOM_OK;
	} else {
	enif_select(env, ctx->cmd_output_fd, ERL_NIF_SELECT_STOP,
	ctx->read_resource, NULL, ATOM_UNDEFINED);
	result = close(ctx->cmd_output_fd);
	if (result == 0) {
	ctx->cmd_output_fd = PIPE_CLOSED;
	return ATOM_OK;
	} else {
	perror("cmd_output_fd close()");
	return make_error(env, enif_make_int(env, errno));
	}
	}
	default:
	debug("invalid file descriptor type");
	return enif_make_badarg(env);
	}
	}

	static int select_read(ErlNifEnv env, ExecContext ctx) {
	int retval = enif_select(env, ctx->cmd_output_fd, ERL_NIF_SELECT_READ,
	ctx->read_resource, NULL, ATOM_UNDEFINED);
	if (retval != 0)
	perror("select_read()");
	return retval;
	}

	static ERL_NIF_TERM sys_read(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	if (argc != 2)
	enif_make_badarg(env);

	ErlNifTime start;
	start = enif_monotonic_time(ERL_NIF_USEC);

	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->cmd_output_fd == PIPE_CLOSED)
	return make_error(env, ATOM_PIPE_CLOSED);

	int size, request;

	enif_get_int(env, argv[1], &request);
	size = request;

	if (request == UNBUFFERED_READ) {
	size = PIPE_BUF_SIZE;
	} else if (request < 1) {
	enif_make_badarg(env);
	} else if (request > PIPE_BUF_SIZE) {
	size = PIPE_BUF_SIZE;
	}

	unsigned char buf[size];
	ssize_t result = read(ctx->cmd_output_fd, buf, size);
	int read_errno = errno;

	ERL_NIF_TERM bin_term = 0;
	if (result >= 0) {
	/* no need to release this binary */
	unsigned char *temp = enif_make_new_binary(env, result, &bin_term);
	memcpy(temp, buf, result);
	}

	notify_consumed_timeslice(env, start, enif_monotonic_time(ERL_NIF_USEC));

	if (result >= 0) {
	/* we do not 'select' if request completely satisfied OR EOF OR its
	* UNBUFFERED_READ */
	if (result == request \|\| result == 0 \|\| request == UNBUFFERED_READ) {
	return make_ok(env, bin_term);
	} else { // request partially satisfied
	int retval = select_read(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_ok(env, bin_term);
	}
	} else {
	if (read_errno == EAGAIN) { // busy
	int retval = select_read(env, ctx);
	if (retval != 0)
	return make_error(env, enif_make_int(env, retval));
	return make_error(env, ATOM_EAGAIN);
	} else { // Error
	perror("read()");
	return make_error(env, enif_make_int(env, read_errno));
	}
	}
	}

	static ERL_NIF_TERM is_alive(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->pid == CMD_EXIT)
	return make_ok(env, ATOM_TRUE);

	int result = kill(ctx->pid, 0);

	if (result == 0) {
	return make_ok(env, ATOM_TRUE);
	} else {
	return make_ok(env, ATOM_FALSE);
	}
	}

	static ERL_NIF_TERM sys_terminate(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, kill(ctx->pid, SIGTERM)));
	}

	static ERL_NIF_TERM sys_kill(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, kill(ctx->pid, SIGKILL)));
	}

	static ERL_NIF_TERM make_exit_term(ErlNifEnv env, ExecContext ctx) {
	switch (ctx->exit_type) {
	case NORMAL_EXIT:
	return make_ok(env, enif_make_tuple2(env, ATOM_EXIT,
	enif_make_int(env, ctx->exit_status)));
	case SIGNALED:
	/* exit_status here points to signal number */
	return make_ok(env, enif_make_tuple2(env, ATOM_SIGNALED,
	enif_make_int(env, ctx->exit_status)));
	case STOPPED:
	return make_ok(env, enif_make_tuple2(env, ATOM_STOPPED,
	enif_make_int(env, ctx->exit_status)));
	default:
	error("Invalid wait status");
	return make_error(env, ATOM_UNDEFINED);
	}
	}

	static ERL_NIF_TERM sys_wait(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);

	if (ctx->pid == CMD_EXIT)
	return make_exit_term(env, ctx);

	int status;
	int wpid = waitpid(ctx->pid, &status, WNOHANG);

	if (wpid == ctx->pid) {
	ctx->pid = CMD_EXIT;

	if (WIFEXITED(status)) {
	ctx->exit_type = NORMAL_EXIT;
	ctx->exit_status = WEXITSTATUS(status);
	} else if (WIFSIGNALED(status)) {
	ctx->exit_type = SIGNALED;
	ctx->exit_status = WTERMSIG(status);
	} else if (WIFSTOPPED(status)) {
	ctx->exit_type = STOPPED;
	ctx->exit_status = 0;
	}

	return make_exit_term(env, ctx);
	} else if (wpid != 0) {
	perror("waitpid()");
	}
	ERL_NIF_TERM term = enif_make_tuple2(env, enif_make_int(env, wpid),
	enif_make_int(env, status));
	return make_error(env, term);
	}

	static ERL_NIF_TERM os_pid(ErlNifEnv *env, int argc,
	const ERL_NIF_TERM argv[]) {
	ExecContext *ctx = NULL;
	GET_CTX(env, argv[0], ctx);
	if (ctx->pid == CMD_EXIT)
	return make_ok(env, enif_make_int(env, 0));

	return make_ok(env, enif_make_int(env, ctx->pid));
	}

	static int on_load(ErlNifEnv env, void *priv, ERL_NIF_TERM load_info) {
	struct ExilePriv *data = enif_alloc(sizeof(struct ExilePriv));
	if (!data)
	return 1;

	data->exec_ctx_rt =
	enif_open_resource_type_x(env, "exile_resource", &exec_ctx_rt_init,
	ERL_NIF_RT_CREATE \| ERL_NIF_RT_TAKEOVER, NULL);
	data->io_rt =
	enif_open_resource_type_x(env, "exile_resource", &io_rt_init,
	ERL_NIF_RT_CREATE \| ERL_NIF_RT_TAKEOVER, NULL);

	ATOM_TRUE = enif_make_atom(env, "true");
	ATOM_FALSE = enif_make_atom(env, "false");
	ATOM_OK = enif_make_atom(env, "ok");
	ATOM_ERROR = enif_make_atom(env, "error");
	ATOM_UNDEFINED = enif_make_atom(env, "undefined");
	ATOM_INVALID_CTX = enif_make_atom(env, "invalid_exile_exec_ctx");
	ATOM_PIPE_CLOSED = enif_make_atom(env, "closed_pipe");
	ATOM_EXIT = enif_make_atom(env, "exit");
	ATOM_SIGNALED = enif_make_atom(env, "signaled");
	ATOM_STOPPED = enif_make_atom(env, "stopped");
	ATOM_EAGAIN = enif_make_atom(env, "eagain");
	ATOM_ALLOC_FAILED = enif_make_atom(env, "alloc_failed");

	priv = (void )data;

	return 0;
	}

	static void on_unload(ErlNifEnv env, void priv) {
	debug("exile unload");
	enif_free(priv);
	}

	static ErlNifFunc nif_funcs[] = {
	- {"execute", 3, execute, USE_DIRTY_IO},
	+ {"execute", 4, execute, USE_DIRTY_IO},
	{"sys_write", 2, sys_write, USE_DIRTY_IO},
	{"sys_read", 2, sys_read, USE_DIRTY_IO},
	{"sys_close", 2, sys_close, USE_DIRTY_IO},
	{"sys_terminate", 1, sys_terminate, USE_DIRTY_IO},
	{"sys_wait", 1, sys_wait, USE_DIRTY_IO},
	{"sys_kill", 1, sys_kill, USE_DIRTY_IO},
	{"alive?", 1, is_alive, USE_DIRTY_IO},
	{"os_pid", 1, os_pid, USE_DIRTY_IO},
	};

	ERL_NIF_INIT(Elixir.Exile.ProcessNif, nif_funcs, &on_load, NULL, NULL,
	&on_unload)
	diff --git a/lib/exile/process.ex b/lib/exile/process.ex
	index 54bf51e..d390e9b 100644
	--- a/lib/exile/process.ex
	+++ b/lib/exile/process.ex
	@@ -1,386 +1,395 @@
	defmodule Exile.Process do
	@moduledoc """
	GenServer which wraps spawned external command.

	One should use `Exile.stream!` over `Exile.Process`. stream internally manages this server for you. Use this only if you need more control over the life-cycle OS process.

	## Overview
	`Exile.Process` is an alternative primitive for Port. It has different interface and approach to running external programs to solve the issues associated with the ports.

	### When compared to Port
	* it is demand driven. User explicitly has to `read` output of the command and the progress of the external command is controlled using OS pipes. so unlike Port, this never cause memory issues in beam by loading more than we can consume
	* it can close stdin of the program explicitly
	* does not create zombie process. It always tries to cleanup resources

	At high level it makes non-blocking asynchronous system calls to execute and interact with the external program. It completely bypasses beam implementation for the same using NIF. It uses `select()` system call for asynchronous IO. Most of the system calls are non-blocking, so it does not has adverse effect on scheduler. Issues such as "scheduler collapse".

	### 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, spawned external processes are still completely isolated at OS level and the port issues it tries to solve are critical.
	"""

	alias Exile.ProcessNif
	require Logger
	use GenServer

	defmacro fork_exec_failure(), do: 125

	# delay between exit_check when io is busy (in milliseconds)
	- @default_opts [io_exit_check_delay: 1, stderr_to_console: false]
	+ @exit_check_timeout 5
	+
	+ @default_opts [stderr_to_console: false, env: []]

	def start_link(cmd_with_args, opts \\ []) do
	opts = Keyword.merge(@default_opts, opts)
	GenServer.start(__MODULE__, %{cmd_with_args: cmd_with_args, opts: opts})
	end

	def close_stdin(process) do
	GenServer.call(process, :close_stdin, :infinity)
	end

	def write(process, iodata) do
	GenServer.call(process, {:write, IO.iodata_to_binary(iodata)}, :infinity)
	end

	def read(process, size) when is_integer(size) or size == :unbuffered do
	GenServer.call(process, {:read, size}, :infinity)
	end

	def read(process) do
	GenServer.call(process, {:read, :unbuffered}, :infinity)
	end

	def kill(process, signal) when signal in [:sigkill, :sigterm] do
	GenServer.call(process, {:kill, signal}, :infinity)
	end

	def await_exit(process, timeout \\ :infinity) do
	GenServer.call(process, {:await_exit, timeout}, :infinity)
	end

	def os_pid(process) do
	GenServer.call(process, :os_pid, :infinity)
	end

	def stop(process), do: GenServer.call(process, :stop, :infinity)

	## Server

	defmodule Pending do
	defstruct bin: [], remaining: 0, client_pid: nil
	end

	defstruct [
	:cmd_with_args,
	:opts,
	:errno,
	:context,
	:status,
	await: %{},
	pending_read: nil,
	pending_write: nil
	]

	alias __MODULE__

	def init(args) do
	%{cmd_with_args: [cmd \| args], opts: opts} = args
	path = System.find_executable(cmd)

	unless path do
	raise "Command not found: #{cmd}"
	end

	if opts[:cd] do
	if !File.exists?(opts[:cd]) \|\| !File.dir?(opts[:cd]) do
	raise ":dir is not a valid path"
	end
	end

	state = %__MODULE__{
	cmd_with_args: [path \| args],
	opts: opts,
	errno: nil,
	status: :init,
	await: %{},
	pending_read: %Pending{},
	pending_write: %Pending{}
	}

	{:ok, state, {:continue, nil}}
	end

	def handle_continue(nil, state) do
	exec_args = Enum.map(state.cmd_with_args, &to_charlist/1)
	stderr_to_console = if state.opts[:stderr_to_console], do: 1, else: 0
	cd = to_charlist(state.opts[:cd] \|\| "")
	+ env = normalize_env(state.opts[:env] \|\| %{})

	- case ProcessNif.execute(exec_args, stderr_to_console, cd) do
	+ case ProcessNif.execute(exec_args, env, cd, stderr_to_console) do
	{:ok, context} ->
	start_watcher(context)
	{:noreply, %Process{state \| context: context, status: :start}}

	{:error, errno} ->
	raise "Failed to start command: #{state.cmd_with_args}, errno: #{errno}"
	end
	end

	def handle_call(:stop, _from, state) do
	# watcher will take care of termination of external process

	# TODO: pending write and read should receive "stopped" return
	# value instead of exit signal
	{:stop, :normal, :ok, state}
	end

	def handle_call(_, _from, %{status: {:exit, status}}), do: {:reply, {:error, {:exit, status}}}

	def handle_call({:await_exit, timeout}, from, state) do
	tref =
	if timeout != :infinity do
	Elixir.Process.send_after(self(), {:await_exit_timeout, from}, timeout)
	else
	nil
	end

	state = put_timer(state, from, :timeout, tref)
	check_exit(state, from)
	end

	def handle_call({:write, binary}, from, state) when is_binary(binary) do
	pending = %Pending{bin: binary, client_pid: from}
	do_write(%Process{state \| pending_write: pending})
	end

	def handle_call({:read, size}, from, state) do
	pending = %Pending{remaining: size, client_pid: from}
	do_read(%Process{state \| pending_read: pending})
	end

	def handle_call(:close_stdin, _from, state), do: do_close(state, :stdin)

	def handle_call(:os_pid, _from, state), do: {:reply, ProcessNif.os_pid(state.context), state}

	def handle_call({:kill, signal}, _from, state) do
	do_kill(state.context, signal)
	{:reply, :ok, %{state \| status: {:exit, :killed}}}
	end

	def handle_info({:check_exit, from}, state), do: check_exit(state, from)

	def handle_info({:await_exit_timeout, from}, state) do
	cancel_timer(state, from, :check)

	receive do
	{:check_exit, ^from} -> :ok
	after
	0 -> :ok
	end

	GenServer.reply(from, :timeout)
	{:noreply, clear_await(state, 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(msg, _state), do: raise(msg)

	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 ProcessNif.sys_write(state.context, 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 ProcessNif.sys_read(state.context, -1) do
	{:ok, <<>>} ->
	GenServer.reply(pending.client_pid, {:eof, []})
	{:noreply, state}

	{:ok, binary} ->
	GenServer.reply(pending.client_pid, {:ok, binary})
	{:noreply, state}

	{: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} = state) do
	case ProcessNif.sys_read(state.context, 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 check_exit(state, from) do
	case ProcessNif.sys_wait(state.context) do
	{:ok, {:exit, fork_exec_failure()}} ->
	GenServer.reply(from, {:error, :failed_to_execute})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}

	{:ok, status} ->
	GenServer.reply(from, {:ok, status})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}

	{:error, {0, _}} ->
	# Ideally we should not poll and we should handle this with SIGCHLD signal
	- tref =
	- Elixir.Process.send_after(self(), {:check_exit, from}, state.opts[:io_exit_check_delay])
	+ tref = Elixir.Process.send_after(self(), {:check_exit, from}, @exit_check_timeout)

	{:noreply, put_timer(state, from, :check, tref)}

	{:error, {-1, status}} ->
	GenServer.reply(from, {:error, status})
	cancel_timer(state, from, :timeout)
	{:noreply, clear_await(state, from)}
	end
	end

	defp do_kill(context, :sigkill), do: ProcessNif.sys_kill(context)

	defp do_kill(context, :sigterm), do: ProcessNif.sys_terminate(context)

	defp do_close(state, type) do
	case ProcessNif.sys_close(state.context, stream_type(type)) do
	:ok ->
	{:reply, :ok, state}

	{:error, errno} ->
	raise errno
	{:reply, {:error, errno}, %Process{state \| errno: errno}}
	end
	end

	defp clear_await(state, from) do
	%Process{state \| await: Map.delete(state.await, from)}
	end

	defp cancel_timer(state, from, key) do
	case get_timer(state, from, key) do
	nil -> :ok
	tref -> Elixir.Process.cancel_timer(tref)
	end
	end

	defp put_timer(state, from, key, timer) do
	if Map.has_key?(state.await, from) do
	await = put_in(state.await, [from, key], timer)
	%Process{state \| await: await}
	else
	%Process{state \| await: %{from => %{key => timer}}}
	end
	end

	defp get_timer(state, from, key), do: get_in(state.await, [from, key])

	# Try to gracefully terminate external process if the genserver associated with the process is killed
	defp start_watcher(context) do
	process_server = self()
	watcher_pid = spawn(fn -> watcher(process_server, context) end)

	receive do
	{^watcher_pid, :done} -> :ok
	end
	end

	defp stream_type(:stdin), do: 0
	defp stream_type(:stdout), do: 1

	defp process_exit?(context) do
	match?({:ok, _}, ProcessNif.sys_wait(context))
	end

	defp process_exit?(context, timeout) do
	if process_exit?(context) do
	true
	else
	:timer.sleep(timeout)
	process_exit?(context)
	end
	end

	+ defp normalize_env(env) do
	+ Enum.map(env, fn {key, value} ->
	+ (String.trim(key) <> "=" <> String.trim(value))
	+ \|> to_charlist()
	+ end)
	+ end
	+
	# for proper process exit parent of the child must wait() for
	# child processes termination exit and "pickup" after the exit
	# (receive child exit_status). Resources acquired by child such as
	# file descriptors won't be released even if the child process
	# itself is terminated.
	defp watcher(process_server, context) do
	ref = Elixir.Process.monitor(process_server)
	send(process_server, {self(), :done})

	receive do
	{:DOWN, ^ref, :process, ^process_server, _reason} ->
	try do
	Logger.debug(fn -> "Stopping external program" end)

	# sys_close is idempotent, calling it multiple times is okay
	ProcessNif.sys_close(context, stream_type(:stdin))
	ProcessNif.sys_close(context, stream_type(:stdout))

	# at max we wait for 100ms for program to exit
	process_exit?(context, 100) && throw(:done)

	Logger.debug("Failed to stop external program gracefully. attempting SIGTERM")
	ProcessNif.sys_terminate(context)
	process_exit?(context, 100) && throw(:done)

	Logger.debug("Failed to stop external program with SIGTERM. attempting SIGKILL")
	ProcessNif.sys_kill(context)
	process_exit?(context, 1000) && throw(:done)

	Logger.error("[exile] failed to kill external process")
	raise "Failed to kill external process"
	catch
	:done -> Logger.debug(fn -> "External program exited successfully" end)
	end
	end
	end
	end
	diff --git a/lib/exile/process_nif.ex b/lib/exile/process_nif.ex
	index 5db610e..92d158f 100644
	--- a/lib/exile/process_nif.ex
	+++ b/lib/exile/process_nif.ex
	@@ -1,28 +1,29 @@
	defmodule Exile.ProcessNif do
	@moduledoc false

	@on_load :load_nifs

	def load_nifs do
	nif_path = :filename.join(:code.priv_dir(:exile), "exile")
	:erlang.load_nif(nif_path, 0)
	end

	- def execute(_cmd, _stderr_to_console, _dir), do: :erlang.nif_error(:nif_library_not_loaded)
	+ def execute(_cmd, _dir, _env, _stderr_to_console),
	+ do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_write(_context, _bin), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_read(_context, _bytes), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_close(_context, _pipe), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_kill(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_terminate(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def sys_wait(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def os_pid(_context), do: :erlang.nif_error(:nif_library_not_loaded)

	def alive?(_context), do: :erlang.nif_error(:nif_library_not_loaded)
	end
	diff --git a/test/exile/process_nif_test.exs b/test/exile/process_nif_test.exs
	index 5be7845..2358c0d 100644
	--- a/test/exile/process_nif_test.exs
	+++ b/test/exile/process_nif_test.exs
	@@ -1,10 +1,10 @@
	defmodule Exile.ProcessNifTest do
	use ExUnit.Case, async: false
	alias Exile.ProcessNif

	test "exit before exec" do
	- {:ok, ctx} = ProcessNif.execute(['invalid'], 0, '')
	+ {:ok, ctx} = ProcessNif.execute(['invalid'], [], '', 0)
	:timer.sleep(500)
	assert {:ok, {:exit, 125}} = ProcessNif.sys_wait(ctx)
	end
	end
	diff --git a/test/exile/process_test.exs b/test/exile/process_test.exs
	index 92f3699..16767d6 100644
	--- a/test/exile/process_test.exs
	+++ b/test/exile/process_test.exs
	@@ -1,293 +1,300 @@
	defmodule Exile.ProcessTest do
	use ExUnit.Case, async: true
	alias Exile.Process

	test "read" do
	{:ok, s} = Process.start_link(~w(echo test))
	assert {:eof, iodata} = Process.read(s, 100)
	assert IO.iodata_to_binary(iodata) == "test\n"
	assert :ok == Process.close_stdin(s)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	end

	test "write" do
	{:ok, s} = Process.start_link(~w(cat))
	assert :ok == Process.write(s, "hello")
	assert {:ok, iodata} = Process.read(s, 5)
	assert IO.iodata_to_binary(iodata) == "hello"

	assert :ok == Process.write(s, "world")
	assert {:ok, iodata} = Process.read(s, 5)
	assert IO.iodata_to_binary(iodata) == "world"

	assert :ok == Process.close_stdin(s)
	assert {:eof, []} == Process.read(s)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 100)
	end

	test "stdin close" do
	logger = start_events_collector()

	# base64 produces output only after getting EOF from stdin. we
	# collect events in order and assert that we can still read from
	# stdout even after closing stdin
	{:ok, s} = Process.start_link(~w(base64))

	# parallel reader should be blocked till we close stdin
	start_parallel_reader(s, logger)
	:timer.sleep(100)

	assert :ok == Process.write(s, "hello")
	add_event(logger, {:write, "hello"})
	assert :ok == Process.write(s, "world")
	add_event(logger, {:write, "world"})
	:timer.sleep(100)

	assert :ok == Process.close_stdin(s)
	add_event(logger, :input_close)
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 50)

	assert [
	{:write, "hello"},
	{:write, "world"},
	:input_close,
	{:read, "aGVsbG93b3JsZA==\n"},
	:eof
	] == get_events(logger)
	end

	test "external command termination on stop" do
	{:ok, s} = Process.start_link(~w(cat))
	{:ok, os_pid} = Process.os_pid(s)
	assert os_process_alive?(os_pid)

	Process.stop(s)
	:timer.sleep(100)

	refute os_process_alive?(os_pid)
	end

	test "external command kill on stop" do
	# cat command hangs waiting for EOF
	{:ok, s} = Process.start_link([fixture("ignore_sigterm.sh")])

	{:ok, os_pid} = Process.os_pid(s)
	assert os_process_alive?(os_pid)
	Process.stop(s)

	if os_process_alive?(os_pid) do
	:timer.sleep(3000)
	refute os_process_alive?(os_pid)
	else
	:ok
	end
	end

	test "exit status" do
	{:ok, s} = Process.start_link(~w(sh -c "exit 2"))
	assert {:ok, {:exit, 2}} == Process.await_exit(s, 500)
	end

	test "writing binary larger than pipe buffer size" do
	large_bin = generate_binary(5 * 65535)
	{:ok, s} = Process.start_link(~w(cat))

	writer =
	Task.async(fn ->
	Process.write(s, large_bin)
	Process.close_stdin(s)
	end)

	:timer.sleep(100)

	{_, iodata} = Process.read(s, 5 * 65535)
	Task.await(writer)

	assert IO.iodata_length(iodata) == 5 * 65535
	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)
	end

	test "back-pressure" do
	logger = start_events_collector()

	# we test backpressure by testing if `write` is delayed when we delay read
	{:ok, s} = Process.start_link(~w(cat))

	large_bin = generate_binary(65535 * 5)

	writer =
	Task.async(fn ->
	Enum.each(1..10, fn i ->
	Process.write(s, large_bin)
	add_event(logger, {:write, i})
	end)

	Process.close_stdin(s)
	end)

	:timer.sleep(50)

	reader =
	Task.async(fn ->
	Enum.each(1..10, fn i ->
	Process.read(s, 5 * 65535)
	add_event(logger, {:read, i})
	# delay in reading should delay writes
	:timer.sleep(10)
	end)
	end)

	Task.await(writer)
	Task.await(reader)

	assert {:ok, {:exit, 0}} == Process.await_exit(s, 500)

	assert [
	write: 1,
	read: 1,
	write: 2,
	read: 2,
	write: 3,
	read: 3,
	write: 4,
	read: 4,
	write: 5,
	read: 5,
	write: 6,
	read: 6,
	write: 7,
	read: 7,
	write: 8,
	read: 8,
	write: 9,
	read: 9,
	write: 10,
	read: 10
	] == get_events(logger)
	end

	# this test does not work properly in linux
	@tag :skip
	test "if we are leaking file descriptor" do
	{:ok, s} = Process.start_link(~w(sleep 60))
	{:ok, os_pid} = Process.os_pid(s)

	# we are only printing FD, TYPE, NAME with respective prefix
	{bin, 0} = System.cmd("lsof", ["-F", "ftn", "-p", to_string(os_pid)])

	Process.stop(s)

	open_files = parse_lsof(bin)
	assert [%{fd: "0", name: _, type: "PIPE"}, %{type: "PIPE", fd: "1", name: _}] = open_files
	end

	test "process kill with pending write" do
	{:ok, s} = Process.start_link(~w(cat))
	{:ok, os_pid} = Process.os_pid(s)

	large_data =
	Stream.cycle(["test"]) \|> Stream.take(500_000) \|> Enum.to_list() \|> IO.iodata_to_binary()

	task =
	Task.async(fn ->
	try do
	Process.write(s, large_data)
	catch
	:exit, reason -> reason
	end
	end)

	:timer.sleep(200)
	Process.stop(s)
	:timer.sleep(3000)

	refute os_process_alive?(os_pid)
	assert {:normal, _} = Task.await(task)
	end

	test "cd" do
	parent = Path.expand("..", File.cwd!())
	{:ok, s} = Process.start_link(~w(sh -c pwd), cd: parent)
	{:ok, dir} = Process.read(s)
	assert String.trim(dir) == parent
	assert {:ok, {:exit, 0}} = Process.await_exit(s)
	end

	test "invalid path" do
	assert {:error, _} = Process.start_link(~w(sh -c pwd), cd: "invalid")
	end

	+ test "env" do
	+ assert {:ok, s} = Process.start_link([fixture("env.sh")], env: %{"TEST_ENV" => "test"})
	+
	+ assert {:ok, "test"} = Process.read(s)
	+ assert {:ok, {:exit, 0}} = Process.await_exit(s)
	+ end
	+
	def start_parallel_reader(proc_server, logger) do
	spawn_link(fn -> reader_loop(proc_server, logger) end)
	end

	def reader_loop(proc_server, logger) do
	case Process.read(proc_server) do
	{:ok, data} ->
	add_event(logger, {:read, data})
	reader_loop(proc_server, logger)

	{:eof, []} ->
	add_event(logger, :eof)
	end
	end

	def start_events_collector do
	{:ok, ordered_events} = Agent.start(fn -> [] end)
	ordered_events
	end

	def add_event(agent, event) do
	:ok = Agent.update(agent, fn events -> events ++ [event] end)
	end

	def get_events(agent) do
	Agent.get(agent, & &1)
	end

	defp os_process_alive?(pid) do
	match?({_, 0}, System.cmd("ps", ["-p", to_string(pid)]))
	end

	defp fixture(script) do
	Path.join([__DIR__, "../scripts", script])
	end

	defp parse_lsof(iodata) do
	String.split(IO.iodata_to_binary(iodata), "\n", trim: true)
	\|> Enum.reduce([], fn
	"f" <> fd, acc -> [%{fd: fd} \| acc]
	"t" <> type, [h \| acc] -> [Map.put(h, :type, type) \| acc]
	"n" <> name, [h \| acc] -> [Map.put(h, :name, name) \| acc]
	_, acc -> acc
	end)
	\|> Enum.reverse()
	\|> Enum.reject(fn
	%{fd: fd} when fd in ["255", "cwd", "txt"] ->
	true

	%{fd: "rtd", name: "/", type: "DIR"} ->
	true

	# filter libc and friends
	%{fd: "mem", type: "REG", name: "/lib/x86_64-linux-gnu/" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/locale/C.UTF-8/" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/locale/locale-archive" <> _} ->
	true

	%{fd: "mem", type: "REG", name: "/usr/lib/x86_64-linux-gnu/gconv" <> _} ->
	true

	_ ->
	false
	end)
	end

	defp generate_binary(size) do
	Stream.repeatedly(fn -> "A" end) \|> Enum.take(size) \|> IO.iodata_to_binary()
	end
	end
	diff --git a/test/scripts/env.sh b/test/scripts/env.sh
	new file mode 100755
	index 0000000..e684d9e
	--- /dev/null
	+++ b/test/scripts/env.sh
	@@ -0,0 +1,3 @@
	+#!/bin/bash
	+
	+printf ${TEST_ENV}

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