reexec.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>

#define COOLDOWN_START_MS    100
#define COOLDOWN_BACKOFF     1.15
#define COOLDOWN_MAX_MS      30 * 1000
#define COOLDOWN_RESET_AFTER 600 /* seconds */
int do_cooldown = 1;
void cooldown() {
	int rc;
	struct timespec now;
	static struct timespec last;
	static unsigned int ms = 0;

	if (!do_cooldown)
		goto no_cooldown;

	if (ms == 0) { /* initialize */
		ms = COOLDOWN_START_MS;
		rc = clock_gettime(CLOCK_MONOTONIC, &last);
		if (rc == -1)
			goto failed;
	}

	rc = clock_gettime(CLOCK_MONOTONIC, &now);
	if (rc == -1)
		goto failed;

	if (now.tv_sec - last.tv_sec - 1 > COOLDOWN_RESET_AFTER)
		ms = COOLDOWN_START_MS;

	last.tv_sec  = now.tv_sec;
	last.tv_nsec = now.tv_nsec;
	usleep(ms * 1000);

	ms = ms * COOLDOWN_BACKOFF;
	if (ms > COOLDOWN_MAX_MS)
		ms = COOLDOWN_MAX_MS;
	return;

failed:
	fprintf(stderr, "failed to retrieve monotonic time: %s (error %d)\n", strerror(errno), errno);
no_cooldown:
	do_cooldown = 0;
	usleep(COOLDOWN_MAX_MS * 1000);
}

pid_t pid = 0;
int keep_going = 1;
void handle_sigterm(int sig) {
	keep_going = 0;
	if (pid)
		kill(pid, SIGTERM);
}
int main(int argc, char **argv, char **environ) {
	int i, bail, n;
	char **cmd, **envp, **p;

	bail = 0;
	envp = argv+1;
	cmd  = NULL;
	for (i = 1; i < argc; i++) {
		if (strcmp(argv[i], "--") == 0) {
			argv[i] = NULL;
			cmd = argv+i+1;
			break;
		}
	}
	if (!cmd || !*cmd) {
		fprintf(stderr, "USAGE: %s ENV=VALUE -- command --to --run --with /arg/u/ments\n", argv[0]);
		return 1;
	}

	fprintf(stderr, "setting up execution environment:\n");
	for (i = 0; envp[i]; i++) {
		if (!strchr(envp[i], '=')) {
			n = strlen(envp[i]);
			for (p = environ; *p; p++) {
				if (strncmp(*p, envp[i], n) == 0 && (*p)[n] == '=') {
					envp[i] = *p;
					break;
				}
			}
			if (!*p) {
				fprintf(stderr, "  %s (UNDEFINED)\n", envp[i]);
				bail = 1;
				continue;
			}
		}
		fprintf(stderr, "  %s\n", envp[i]);
	}
	if (bail) {
		return 1;
	}

	fprintf(stderr, "running command %s with arguments:\n", cmd[0]);
	for (i = 1; cmd[i]; i++) {
		fprintf(stderr, "  [%d]: '%s'\n", i-1, cmd[i]);
	}

	signal(SIGINT,  handle_sigterm);
	signal(SIGTERM, handle_sigterm);

	while (keep_going) {
		int status;

		pid = fork();
		if (pid < 0) {
			fprintf(stderr, "fork() failed: %s (error %d)\n", strerror(errno), errno);
			cooldown();
			continue;
		}

		if (pid == 0) {
			execve(cmd[0], cmd, envp);
			fprintf(stderr, "(child %d) execve(%s) failed: %s (error %d)\n",
				getpid(), cmd[0], strerror(errno), errno);
			exit(77);
		}

		waitpid(pid, &status, 0);
		if (WIFSIGNALED(status) && WTERMSIG(status) == SIGINT)
			keep_going = 0;
		if (keep_going)
			cooldown();
	}
	return 0;
}