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	diff --git a/c_src/exile.c b/c_src/exile.c
	index e4af4f7..9b7e529 100644
	--- a/c_src/exile.c
	+++ b/c_src/exile.c
	@@ -1,716 +1,776 @@
	#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 = 50;
	-static const int MAX_ARGUMENT_LEN = 1024;
	-static const int MAX_ENV_VAR_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 void free_array_of_cstr(char ***arr) {
	+ int i;
	+
	+ if (*arr) {
	+ for(i=0; (*arr)[i] != NULL; i++) {
	+ free((*arr)[i]);
	+ (*arr)[i] = NULL;
	+ }
	+
	+ free(*arr);
	+ }
	+ *arr = NULL;
	+}
	+
	+static bool read_string(ErlNifEnv env, ERL_NIF_TERM term, char *str) {
	+ unsigned int length;
	+ *str = NULL;
	+
	+ if (enif_get_list_length(env, term, &length) != true) {
	+ error("failed to get string length");
	+ return false;
	+ }
	+
	+ str = (char )malloc((length + 1) * sizeof(char));
	+
	+ if (enif_get_string(env, term, *str, length + 1, ERL_NIF_LATIN1) < 1) {
	+ error("failed to get string");
	+
	+ free(str);
	+ *str = NULL;
	+
	+ return false;
	+ }
	+
	+ return true;
	+}
	+
	+static bool erl_list_to_array_of_cstr(ErlNifEnv *env, ERL_NIF_TERM list,
	+ char ***arr) {
	+ ERL_NIF_TERM head, tail;
	+ unsigned int length, i;
	+
	+ *arr = NULL;
	+
	+ if (enif_is_list(env, list) != true) {
	+ error("erl term is not a list");
	+ goto error;
	+ }
	+
	+ if (enif_get_list_length(env, list, &length) != true) {
	+ error("erl term is not a proper list");
	+ goto error;
	+ }
	+
	+ arr = (char )malloc((length + 1) sizeof(char *));
	+
	+ for (i = 0; i < length + 1; i++)
	+ (*arr)[i] = NULL;
	+
	+ tail = list;
	+
	+ for (i = 0; i < length; i++) {
	+ if (enif_get_list_cell(env, tail, &head, &tail) != true) {
	+ error("failed to get cell from list");
	+ goto error;
	+ }
	+
	+ if (read_string(env, head, &(*arr)[i]) != true)
	+ goto error;
	+ }
	+
	+ return true;
	+
	+error:
	+ free_array_of_cstr(arr);
	+ return false;
	+}
	+
	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 *const args[],
	- bool stderr_to_console,
	- const char dir[],
	- char *const exec_env[]) {
	+static StartProcessResult start_process(char **args, bool stderr_to_console,
	+ 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();

	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 dir[1024] = {'\0'};
	ErlNifTime start;
	- unsigned int args_len, env_len;
	- ERL_NIF_TERM head, tail, list;
	+ int stderr_to_console_int;
	+ ERL_NIF_TERM term;
	+ char exec_args, exec_env;
	+ char *dir;
	+ bool stderr_to_console = true;
	+ struct ExilePriv *data;
	+ StartProcessResult result;
	+ ExecContext *ctx = NULL;
	+
	+ exec_args = NULL;
	+ exec_env = NULL;
	+ dir = NULL;

	start = enif_monotonic_time(ERL_NIF_USEC);

	- if (enif_get_list_length(env, argv[0], &args_len) != true) {
	- error("invalid command with arguments param");
	- return enif_make_badarg(env);
	+ if (argc != 4) {
	+ error("number of arguments for `execute` must be 4");
	+ term = enif_make_badarg(env);
	+ goto exit;
	}

	- if (args_len > MAX_ARGUMENTS) {
	- error("command argument size exceeds limit: %d", MAX_ARGUMENTS);
	- return enif_make_badarg(env);
	+ if (erl_list_to_array_of_cstr(env, argv[0], &exec_args) != true) {
	+ error("failed to read command arguments");
	+ term = enif_make_badarg(env);
	+ goto exit;
	}

	- char _args_temp[args_len][MAX_ARGUMENT_LEN + 1];
	- char *exec_args[args_len + 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, _args_temp[i], MAX_ARGUMENT_LEN,
	- ERL_NIF_LATIN1) < 1)
	- return enif_make_badarg(env);
	- exec_args[i] = _args_temp[i];
	- list = tail;
	+ if (erl_list_to_array_of_cstr(env, argv[1], &exec_env) != true) {
	+ error("failed to read env list");
	+ term = enif_make_badarg(env);
	+ goto exit;
	}
	- 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 (read_string(env, argv[2], &dir) != true) {
	+ error("failed to get `dir`");
	+ term = enif_make_badarg(env);
	+ goto exit;
	}

	- debug("env size: %d", env_len);
	-
	- char _env_temp[env_len][MAX_ENV_VAR_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_VAR_LEN, ERL_NIF_LATIN1) <
	- 1)
	- return enif_make_badarg(env);
	- exec_env[i] = _env_temp[i];
	- list = tail;
	+ if (enif_get_int(env, argv[3], &stderr_to_console_int) != true) {
	+ error("failed to read stderr_to_console int");
	+ term = enif_make_badarg(env);
	+ goto exit;
	}
	- exec_env[env_len] = NULL;
	+ stderr_to_console = stderr_to_console_int == 1 ? true : false;

	- bool stderr_to_console = true;
	- int tmp_int;
	- 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, exec_env);
	- ExecContext *ctx = NULL;
	- ERL_NIF_TERM term;
	+ data = enif_priv_data(env);
	+ result = start_process(exec_args, stderr_to_console, dir, exec_env);

	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);
	+ term = make_ok(env, term);
	} else {
	- return make_error(env, enif_make_int(env, result.err));
	+ term = make_error(env, enif_make_int(env, result.err));
	}
	+
	+exit:
	+ free_array_of_cstr(&exec_args);
	+ free_array_of_cstr(&exec_env);
	+
	+ if (dir)
	+ free(dir);
	+
	+ return term;
	}

	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_errno = 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_errno == EAGAIN \|\| write_errno == EWOULDBLOCK) { // 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_errno));
	}
	}

	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 \|\| read_errno == EWOULDBLOCK) { // 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 status = 0;
	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", 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 5c6af87..bbc30a6 100644
	--- a/lib/exile/process.ex
	+++ b/lib/exile/process.ex
	@@ -1,395 +1,397 @@
	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".
	"""

	alias Exile.ProcessNif
	require Exile.ProcessNif
	require Logger
	use GenServer

	# delay between exit_check when io is busy (in milliseconds)
	@exit_check_timeout 5

	@default_opts [stderr_to_console: false, 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` - an enumerable of tuples containing environment key-value. These can be accessed in the external program
	* `stderr_to_console` - whether to print stderr output to console. Defaults to `false`
	"""
	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

	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) 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

	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 [
	:args,
	:errno,
	: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
	%{cmd_with_args: cmd_with_args, cd: cd, env: env, stderr_to_console: stderr_to_console} =
	state.args

	case ProcessNif.execute(cmd_with_args, env, cd, stderr_to_console) do
	{:ok, context} ->
	{:ok, _} = Exile.Watcher.watch(self(), context)
	{:noreply, %Process{state \| context: context, status: :start}}

	{:error, errno} ->
	raise "Failed to start command: #{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, ProcessNif.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}, @exit_check_timeout)
	{:noreply, put_timer(state, from, :check, tref)}

	- {:error, {-1, status}} ->
	- GenServer.reply(from, {:error, status})
	+ {:error, {-1, _}} ->
	+ GenServer.reply(from, :error)
	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, ProcessNif.to_process_fd(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])

	- defp normalize_cmd(cmd) do
	+ 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_args(args) do
	- if is_list(args) do
	+ 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
	user_env =
	Map.new(env, fn {key, value} ->
	{String.trim(key), String.trim(value)}
	end)

	# spawned process env will be beam env at that time + user env.
	# this is similar to erlang behavior
	env_list =
	Map.merge(System.get_env(), user_env)
	\|> Enum.map(fn {k, v} ->
	to_charlist(k <> "=" <> v)
	end)

	{:ok, env_list}
	end

	defp normalize_stderr_to_console(nil), do: {:ok, ProcessNif.nif_false()}

	defp normalize_stderr_to_console(term) do
	if term, do: {:ok, ProcessNif.nif_true()}, else: {:ok, ProcessNif.nif_false()}
	end

	defp validate_opts_fields(opts) do
	{_, additional_opts} = Keyword.split(opts, [:cd, :stderr_to_console, :env])

	if Enum.empty?(additional_opts) do
	:ok
	else
	{:error, "invalid opts: #{inspect(additional_opts)}"}
	end
	end

	- defp normalize_args([cmd \| args], opts) when is_list(opts) do
	- with {:ok, cmd} <- normalize_cmd(cmd),
	- {:ok, args} <- normalize_cmd_args(args),
	+ 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, stderr_to_console} <- normalize_stderr_to_console(opts[:stderr_to_console]),
	{:ok, env} <- normalize_env(opts[:env]) do
	{:ok,
	%{cmd_with_args: [cmd \| args], cd: cd, stderr_to_console: stderr_to_console, env: env}}
	end
	end
	-
	- defp normalize_args(_, _), do: {:error, "invalid arguments"}
	end

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