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

#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <dirent.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <sys/select.h>


#include "gcc-compat.h"
#include "err.h"
#include "log.h"
#include "str.h"
#include "file.h"
#include "sig.h"

static int signal_pipe[2];

/*
 * Initialize the signal subsystem.
 *
 * This function creates a pipe, the signal pipe, to deliver pending signals to
 * the application (Bernstein's trick). It is called at startup, followed by
 * subsequent calls to install_sighandler() for each signal that should be
 * caught.
 *
 * signal_init() installs a generic signal handler which is used for all
 * signals simultaneously. When a signal arrives, this generic signal handler
 * writes the corresponding signal number to the signal pipe so that the
 * application can test for pending signals by checking the signal pipe for
 * reading, e.g. by using the select(2) system call.
 *
 * This function either succeeds or calls exit(2) to terminate the current
 * process. On success, the file descriptor of the signal pipe is returned.
 */
int signal_init(void)
{
	int ret;
	if (pipe(signal_pipe) < 0) {
		ret = -ERRNO_TO_DSS_ERROR(errno);
		goto err_out;
	}
	ret = mark_fd_nonblocking(signal_pipe[0]);
	if (ret < 0)
		goto err_out;
	ret = mark_fd_nonblocking(signal_pipe[1]);
	if (ret < 0)
		goto err_out;
	return signal_pipe[0];
err_out:
	DSS_EMERG_LOG("%s\n", dss_strerror(-ret));
	exit(EXIT_FAILURE);
}

/* Write the signal number to the signal pipe, abort on errors. */
static void generic_signal_handler(int s)
{
	/*
	 * Signal handlers that make system calls must save a copy of errno on
	 * entry to the handler and restore it on exit, to prevent the
	 * possibility of overwriting a errno value that had previously been
	 * set in the main program.
	 */
	int save_errno = errno;
	ssize_t ret = write(signal_pipe[1], &s, sizeof(int));

	if (ret == sizeof(int)) {
		errno = save_errno;
		return;
	}
	if (ret < 0)
		DSS_EMERG_LOG("%s\n", strerror(errno));
	else
		DSS_EMERG_LOG("short write to signal pipe\n");
	exit(EXIT_FAILURE);
}

/*
 * If there is a child to reap, return its pid and exit status via the pointer
 * arguments.
 *
 * Returns a negative error code on errors, zero, if there was no child to
 * reap, one if a child was reaped successfully. Only if the function returns
 * one, the contents of pid and status are meaningful.
 */
int reap_child(pid_t *pid, int *status)
{
	*pid = waitpid(-1, status, WNOHANG);

	if (!*pid)
		return 0;
	if (*pid < 0)
		return -ERRNO_TO_DSS_ERROR(errno);
	if (WIFEXITED(*status))
		DSS_DEBUG_LOG("child %i exited. Exit status: %i\n", (int)*pid,
			WEXITSTATUS(*status));
	else if (WIFSIGNALED(*status))
		DSS_DEBUG_LOG("child %i was killed by signal %i\n", (int)*pid,
			WTERMSIG(*status));
	else
		DSS_WARNING_LOG("child %i terminated abormally\n", (int)*pid);
	return 1;
}

/*
 * Install the generic signal handler for the given signal, return 1 on success
 * and -E_SIGNAL_SIG_ERR on errors.
 */
int install_sighandler(int sig)
{
	DSS_DEBUG_LOG("catching signal %d\n", sig);
	if (signal(sig, &generic_signal_handler) != SIG_ERR)
		return 1;
	return -E_SIGNAL_SIG_ERR;
}

/*
 * This should be called if the fd for the signal pipe is ready for reading.
 * It returns the number of the next pending signal. If the read was
 * interrupted by another signal, the function returns 0. On errors, a negative
 * error code is returned.
 */
int next_signal(void)
{
	int s, err;
	ssize_t r;

	r = read(signal_pipe[0], &s, sizeof(s));
	if (r == sizeof(s)) {
		DSS_DEBUG_LOG("next signal: %d\n", s);
		return s;
	}
	err = errno;
	assert(r < 0);
	if (err == EAGAIN)
		return 0;
	DSS_ERROR_LOG("failed to read from signal pipe\n");
	return -ERRNO_TO_DSS_ERROR(err);
}

/* Close the signal pipe. */
void signal_shutdown(void)
{
	close(signal_pipe[1]);
}