/**
 * mds — A micro-display server
 * Copyright © 2014, 2015, 2016  Mattias Andrée (maandree@member.fsf.org)
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include "mds.h"

#include <libmdsserver/config.h>
#include <libmdsserver/macros.h>
#include <libmdsserver/util.h>

#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <time.h>
#include <dirent.h>


/**
 * Number of elements in `argv`
 */
static int argc;

/**
 * Command line arguments
 */
static char **argv;

/**
 * The master server
 */
static const char *master_server = LIBEXECDIR "/mds-server";

/**
 * The umask the server start with
 */
static mode_t saved_umask;


/**
 * Entry point of the program
 * 
 * @param   argc_  Number of elements in `argv_`
 * @param   argv_  Command line arguments
 * @return         Non-zero on error
 */
int main(int argc_, char **argv_)
{
	int fd = -1;
	int got_master_server = 0;
	struct sockaddr_un address;
	char pathname[PATH_MAX];
	char piddata[64];
	unsigned int display = DISPLAY_MAX;
	FILE *f;
	int rc;
	int j, r;
	char *arg;

	argc = argc_;
	argv = argv_;

	/* Sanity check the number of command line arguments. */
	exit_if (argc > ARGC_LIMIT,
	         eprint("that number of arguments is ridiculous, I will not allow it."););

	/* Parse command line arguments. */
	for (j = 1; j < argc; j++) {
		arg = argv[j];
		if (startswith(arg, "--master-server=")) { /* Master server. */
			exit_if (got_master_server,
			         eprintf("duplicate declaration of %s.", "--master-server"););
			got_master_server = 1;
			master_server = arg + strlen("--master-server=");
		}
	}

	/* Stymied if the effective user is not root. */
	exit_if (geteuid() != ROOT_USER_UID,
	         eprint("the effective user is not root, cannot continue."););

	/* Set up to ignore SIGUPDATE, used in mds for re-exec, but we cannot re-exec. */
	if (xsigaction(SIGUPDATE, SIG_IGN) < 0)
		xperror(*argv);

	/* Set up to ignore SIGDANGER. */
	if (xsigaction(SIGDANGER, SIG_IGN) < 0)
		xperror(*argv);

	/* Set up to ignore SIGINFO. */
	if (xsigaction(SIGINFO, SIG_IGN) < 0)
		xperror(*argv);

	/* Remove umask. */
	saved_umask = umask(0);

	/* Create directory for socket files, PID files and such. */
	fail_if (create_directory_root(MDS_RUNTIME_ROOT_DIRECTORY));

	/* Determine display index. */
	for (display = 0; display < DISPLAY_MAX; display++) {
		xsnprintf(pathname, "%s/%u.pid", MDS_RUNTIME_ROOT_DIRECTORY, display);

		fd = open(pathname, O_CREAT | O_EXCL, 0644);
		if (fd < 0) {
			/* Reuse display index if no longer used. */
			f = fopen(pathname, "r");
			if (!f) { /* Race, or error? */
				xperror(*argv);
				continue;
			}
			r = is_pid_file_reusable(f);
			xfclose(f);
			if (!r)
				continue;
		}
		xclose(fd);
		break;
	}
	exit_if (display == DISPLAY_MAX,
	         eprint("sorry, too many displays on the system."););
	/* Yes, the directory could have been removed, but it probably was not. */

	/* Create PID file. */
	fail_if (!(f = fopen(pathname, "w")));
	xsnprintf(piddata, "%u\n", getpid());
	if (fwrite(piddata, 1, strlen(piddata), f) < strlen(piddata)) {
		xfclose(f);
		fail_if (1);
	}
	fflush(f);
	xfclose(f);
	if (chmod(pathname, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) < 0)
		xperror(*argv);

	/* Create data storage directory. */
	xsnprintf(pathname, "%s/%u.data", MDS_STORAGE_ROOT_DIRECTORY, display);
	fail_if (create_directory_root(MDS_STORAGE_ROOT_DIRECTORY));
	fail_if (unlink_recursive(pathname));
	fail_if (create_directory_user(pathname));

	/* Save MDS_DISPLAY environment variable. */
	xsnprintf(pathname, /* Excuse the reuse without renaming. */
	          ":%u", display);
	fail_if (setenv(DISPLAY_ENV, pathname, 1) < 0);

	/* Create a new process group and export MDS_PGROUP */
	fail_if (setpgid(0, 0) < 0);
	xsnprintf(pathname, /* Excuse the reuse without renaming. */
	          "%ji", (intmax_t)getpgrp());
	fail_if (setenv(PGROUP_ENV, pathname, 1) < 0);

	/* Create display socket. */
	xsnprintf(pathname, "%s/%u.socket", MDS_RUNTIME_ROOT_DIRECTORY, display);
	address.sun_family = AF_UNIX;
	if (strlen(pathname) >= sizeof(address.sun_path))
		fail_if ((errno = ENAMETOOLONG));
	strcpy(address.sun_path, pathname);
	unlink(pathname);
	fail_if ((fd = socket(PF_UNIX, SOCK_STREAM, 0)) < 0);
	fail_if (fchmod(fd, S_IRWXU) < 0);
	fail_if (fchown(fd, getuid(), NOBODY_GROUP_GID) < 0);
	fail_if (bind(fd, (struct sockaddr*)(&address), sizeof(address)) < 0);

	/* Start listening on socket. */
	fail_if (listen(fd, SOMAXCONN) < 0);

	/* Start master server and respawn it if it crashes. */
	rc = spawn_and_respawn_server(fd);

done:
	/* Shutdown, close and remove the socket. */
	if (fd >= 0) {
		shutdown(fd, SHUT_RDWR);
		xclose(fd);
		unlink(pathname);
	}

	if (display == DISPLAY_MAX)
		return rc;

	/* Remove PID file. */
	xsnprintf(pathname, "%s/%u.pid", MDS_RUNTIME_ROOT_DIRECTORY, display);
	unlink(pathname);

	/* Remove directories. */
	rmdir(MDS_RUNTIME_ROOT_DIRECTORY); /* Do not care if it fails, it is probably used by another display. */
	rmdir(MDS_STORAGE_ROOT_DIRECTORY); /* Do not care if it fails, it is probably used by another display. */
	xsnprintf(pathname, "%s/%u.data", MDS_STORAGE_ROOT_DIRECTORY, display);
	unlink_recursive(pathname); /* An error will be printed on error, do nothing more. */

	return rc;

fail:
	xperror(*argv);
	rc = 1;
	goto done;
}


/**
 * Read a PID-file and determine whether it refers to an non-existing process
 * 
 * @param   f  The PID-file
 * @return     Whether the PID-file is not longer used
 */
int
is_pid_file_reusable(FILE *f)
{
	char piddata[64];
	size_t read_len;
	pid_t pid;

	read_len = fread(piddata, 1, sizeof(piddata) / sizeof(char), f);
	fail_if (ferror(f)); /* Failed to read. */
	if (!feof(f)) { /* Did not read everything. */
		eprint("the content of a PID file is larger than expected.");
	} else {
		pid = parse_pid_t(piddata, read_len - 1);
		if (pid == (pid_t)-1)
			eprint("the content of a PID file is invalid.");
		else if (kill(pid, 0) < 0) /* Check if the PID is still allocated to any process. */
			return errno == ESRCH; /* PID is not used. */
	}

	return 0;
fail:
	xperror(*argv);
	return 0;
}


/**
 * Parse an LF-terminated string as a non-negative `pid_t`
 * 
 * @param   str  The string
 * @param   n    The length of the string, excluding LF-termination
 * @return       The pid, `(pid_t)-1` if malformated
 */
pid_t
parse_pid_t(const char *str, size_t n)
{
	pid_t pid = 0;
	size_t i;
	char c;

	for (i = 0; i < n; i++) {
		c = str[i];
		if ('0' <= c && c <= '9')
			pid = pid * 10 + (c & 15);
		else
			return (pid_t)-1;
	}

	if (str[n] != '\n')
		return (pid_t)-1;

	return pid;
}



/**
 * Drop privileges and change execution image into the master server's image.
 * This function will only return on error.
 * 
 * @param  child_args  Command line arguments for the new image
 */
static void __attribute__((nonnull))
exec_master_server(char **child_args)
{
	/* Drop privileges. They most not be propagated non-authorised components. */
	/* setgid should not be set, but just to be safe we are restoring both user and group. */
	fail_if (drop_privileges());

	/* Start master server. */
	execv(master_server, child_args);
	fail_if (1);
fail:
	return;
}


/**
 * Start master server and respawn it if it crashes
 * 
 * @param   fd  The file descriptor of the socket
 * @return      Non-zero on error
 */
int
spawn_and_respawn_server(int fd)
{
	int time_error = 0;
	int first_spawn = 1;
	int rc = 0;
	struct timespec time_start;
	struct timespec time_end;
	char *child_args[ARGC_LIMIT + LIBEXEC_ARGC_EXTRA_LIMIT + 1];
	char fdstr[(sizeof("--socket-fd=") / sizeof(char) - 1) + 64];
	int i;
	pid_t pid;
	int status;

	fail_if (xstrdup(child_args[0], master_server));
	for (i = 1; i < argc; i++)
		child_args[i] = argv[i];
	fail_if (xstrdup(child_args[argc + 0], "--initial-spawn"));
	xsnprintf(fdstr, "--socket-fd=%i", fd);
	child_args[argc + 1] = fdstr;
	child_args[argc + 2] = NULL;

#if (LIBEXEC_ARGC_EXTRA_LIMIT < 2)
# error LIBEXEC_ARGC_EXTRA_LIMIT is too small, need at least 2.
#endif

respawn:
	pid = fork();
	fail_if (pid == (pid_t)-1);

	if (!pid) { /* Child. */
		/* If this image exits, it should do so with failure status. */
		rc++;
		/* Reinstate original umask. */
		umask(saved_umask);
		/* Change image into the master server. */
		exec_master_server(child_args);
		fail_if (1);
	}

	/* Parent. */

	/* Get the current time. (Start of child process.) */
	if ((time_error = (monotone(&time_start) < 0)))
		xperror(*argv);

	/* Wait for master server to die. */
	fail_if (uninterruptable_waitpid(pid, &status, 0) == (pid_t)-1);

	/* If the server exited normally or SIGTERM, do not respawn. */
	if (WIFEXITED(status) ? !WEXITSTATUS(status) :
	    (WTERMSIG(status) == SIGTERM || WTERMSIG(status) == SIGINT)) {
		/* Child exited normally, stop. */
		rc--;
		goto done;
	}

	/* Get the current time. (End of child process.) */
	time_error |= (monotone(&time_end) < 0);

	if (WIFEXITED(status))
		eprintf("`%s' exited with code %i.", master_server, WEXITSTATUS(status));
	else
		eprintf("`%s' died by signal %i.", master_server, WTERMSIG(status));

	/* Do not respawn if we could not read the time. */
	if (time_error) {
		xperror(*argv);
		eprintf("`%s' died abnormally, not respawning because we could not read the time.", master_server);
		goto done;
	}

	/* Respawn if the server did not die too fast. */
	if (time_end.tv_sec - time_start.tv_sec >= RESPAWN_TIME_LIMIT_SECONDS) {
		eprintf("`%s' died abnormally, respawning.", master_server);
	} else {
		eprintf("`%s' died abnormally, died too fast, not respawning.", master_server);
		goto done;
	}

	if (first_spawn) {
		first_spawn = 0;
		free(child_args[argc + 0]);
		fail_if (xstrdup(child_args[argc + 0], "--respawn"));
	}

	goto respawn;

done:
	rc++;
	free(child_args[0]);
	free(child_args[argc + 0]);
	if (rc == 2)
		_exit(1);
	return rc;

fail:
	xperror(*argv);
	goto done;
}


/**
 * Create a directory owned by the root user and root group
 * 
 * @param   pathname  The pathname of the directory to create
 * @return            Non-zero on error
 */
int
create_directory_root(const char *pathname)
{
	struct stat attr;

	if (!stat(pathname, &attr)) {
		/* Cannot create the directory, its pathname refers to an existing. */
		if (!S_ISDIR(attr.st_mode)) {
			/* But it is not a directory so we cannot continue. */
			eprintf("%s already exists but is not a directory.", pathname);
			return 1;
		}
	} else {
		/* Directory is missing, create it. */
		if (mkdir(pathname, S_IRWXU | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH) < 0)
			/* Unlikely race condition. */
			fail_if (errno != EEXIST);
		else
			/* Set ownership. */
			fail_if (chown(pathname, ROOT_USER_UID, ROOT_GROUP_GID) < 0);
	}

	return 0;
fail:
	xperror(*argv);
	return 1;
}


/**
 * Create a directory owned by the real user and nobody group
 * 
 * @param   pathname  The pathname of the directory to create
 * @return            Non-zero on error
 */
int
create_directory_user(const char *pathname)
{
	struct stat attr;

	if (!stat(pathname, &attr)) {
		/* Cannot create the directory, its pathname refers to an existing. */
		if (!S_ISDIR(attr.st_mode)) {
			/* But it is not a directory so we cannot continue. */
			eprintf("%s already exists but is not a directory.", pathname);
			return 1;
		}
	} else {
		/* Directory is missing, create it. */
		if (mkdir(pathname, S_IRWXU) < 0)
			/* Unlikely race condition. */
			fail_if (errno != EEXIST);
		else
			/* Set ownership. */
			fail_if (chown(pathname, getuid(), NOBODY_GROUP_GID) < 0);
	}

	return 0;
fail:
	xperror(*argv);
	return 1;
}


/**
 * Recursively remove a directory
 * 
 * @param   pathname  The pathname of the directory to remove
 * @return            Non-zero on error, but zero if the directory does not exist
 */
int
unlink_recursive(const char *pathname)
{
	DIR *dir = opendir(pathname);
	int rc = 0;
	struct dirent *file;
	int saved_errno;
	struct stat _attr;

	/* Check that we could examine the directory. */
	if (!dir) {
		saved_errno = errno;
		if (stat(pathname, &_attr) < 0)
			return 0; /* Directory does not exist. */
		errno = saved_errno;
		fail_if (1);
	}

	/* Remove the content of the directory. */
	while ((file = readdir(dir))) {
		if (strcmp(file->d_name, ".") &&
		    strcmp(file->d_name, "..") &&
		    (unlink(file->d_name) < 0)) {
			fail_if (errno != EISDIR);
			unlink_recursive(file->d_name);
		}
	}

	/* Remove the drectory. */
	fail_if (rmdir(pathname) < 0);

done:
	if (dir)
		closedir(dir);
	return rc;

fail:
	xperror(*argv);
	rc = -1;
	goto done;
}