/* SPDX-License-Identifier: GPL-2.0 */

/** \file exec.c Helper functions for spawning new processes. */

#include "para.h"
#include "error.h"
#include "fd.h"
#include "string.h"

static void xdup2(int oldfd, int newfd)
{
	while (dup2(oldfd, newfd) == -1) {
		if (errno == EINTR)
			continue;
		PARA_EMERG_LOG("dup2 failed: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}
}

/* Spawn a new process, redirect fds, have the child call execvp(2). */
static int fork_and_exec(pid_t *pid, const char *file,
		char *const *const args, int *fds)
{
	int ret, in[2] = {-1, -1}, out[2] = {-1, -1}, err[2] = {-1, -1},
		null = -1, control[2];
	int exec_errno;

	*pid = 0;
	ret = -E_DUP_PIPE;
	if (fds[0] > 0 && pipe(in) < 0)
		goto err_out;
	if (fds[1] > 0 && pipe(out) < 0)
		goto err_out;
	if (fds[2] > 0 && pipe(err) < 0)
		goto err_out;
	if (pipe2(control, O_CLOEXEC) < 0)
		goto err_out;
	if (!fds[0] || !fds[1] || !fds[2]) {
		ret = para_open("/dev/null", O_RDWR, 42);
		if (ret < 0)
			goto err_out;
		null = ret;
	}
	ret = fork();
	if (ret < 0) {
		ret = -ERRNO_TO_PARA_ERROR(errno);
		goto err_out;
	}
	*pid = ret;
	if (!(*pid)) { /* child */
		close(control[0]);
		if (fds[0] == 0)
			xdup2(null, STDIN_FILENO);
		else if (fds[0] > 0) {
			close(in[1]);
			if (in[0] != STDIN_FILENO)
				xdup2(in[0], STDIN_FILENO);
		}
		if (fds[1] == 0)
			xdup2(null, STDOUT_FILENO);
		else if (fds[1] > 0) {
			close(out[0]);
			if (out[1] != STDOUT_FILENO)
				xdup2(out[1], STDOUT_FILENO);
		}
		if (fds[2] == 0)
			xdup2(null, STDERR_FILENO);
		else if (fds[2] > 0) {
			close(err[0]);
			if (err[1] != STDERR_FILENO)
				xdup2(err[1], STDERR_FILENO);
		}
		if (null >= 0)
			close(null);
		/*
		 * If stdin is redirected, the foreground process group can no
		 * longer be signalled with CTRL+C. Set the process group ID of
		 * the child to its PID so that the parent can send a signal to
		 * the process group to kill the child and all its subprocesses.
		 */
		if (fds[0] >= 0)
			setpgid(0, 0);
		execvp(file, args); /* closes control[1] */
		/* exec failed, notify parent */
		xwrite(control[1], &errno, sizeof(int));
		_exit(EXIT_FAILURE);
	}
	close(control[1]);
	do
		ret = read(control[0], &exec_errno, sizeof(int));
	while (ret < 0 && errno == EINTR);
	close(control[0]);
	if (ret > 0) { /* exec failed */
		assert(ret == sizeof(int));
		ret = -ERRNO_TO_PARA_ERROR(exec_errno);
		goto err_out;
	}
	if (fds[0] > 0) {
		close(in[0]);
		*fds = in[1];
	}
	if (fds[1] > 0) {
		close(out[1]);
		*(fds + 1) = out[0];
	}
	if (fds[2] > 0) {
		close(err[1]);
		*(fds + 2) = err[0];
	}
	if (null >= 0)
		close(null);
	return 1;
err_out:
	if (err[0] >= 0)
		close(err[0]);
	if (err[1] >= 0)
		close(err[1]);
	if (out[0] >= 0)
		close(out[0]);
	if (out[1] >= 0)
		close(out[1]);
	if (in[0] >= 0)
		close(in[0]);
	if (in[1] >= 0)
		close(in[1]);
	if (null >= 0)
		close(null);
	return ret;
}

/**
 * Execute a file as a background process, honoring $PATH.
 *
 * \param pid Contains the PID of the child process on return.
 * \param cmdline Path to the file to execute, followed by arguments.
 * \param fds A pointer to a value-result array.
 *
 * Create a new process and return without waiting for the child
 * to terminate. The child replaces itself with the process image that
 * corresponds to the first word of the given command line.
 *
 * The fd pointer must point to an array of three integers. Initially, the
 * integers specify how to deal with fd 0, 1, 2 in the child:
 *
 *	< 0: Leave fd alone.
 *	== 0: Dup fd to /dev/null.
 *	> 0: Create a pipe and dup to one end of that pipe.
 *
 * In the third case, the function sets the corresponding integer of the
 * fd array to one end of the newly created pipe.
 *
 * \return Standard. Regardless of the return value, if pid points to a
 * non-zero value, a child process was created. It is the responsibility of
 * the caller to reap the child.
 *
 * \sa \ref create_argv(), null(4), pipe(2), dup2(2), fork(2), exec(3).
 */
int xexec(pid_t *pid, const char *cmdline, int *fds)
{
	int ret;
	char **argv;

	ret = create_argv(cmdline, " \t", &argv);
	if (ret < 0)
		return ret;
	PARA_INFO_LOG("cmdline: %s\n", cmdline);
	ret = fork_and_exec(pid, argv[0], argv, fds);
	free_argv(argv);
	return ret;
}