/* systemd socket activation
 *
 * compile with:
 * gcc -shared -Wl,-soname,sockethack.so -o sockethack.so sockethack.c -ldl `pkg-config --cflags --libs libsystemd` -fPIC
 * usage: env LD_PRELOAD=libdivert.so LD_LIBRARY_PATH=. /path/to/program
 */

#include <dlfcn.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netdb.h>
#include <string.h>
#include <stdlib.h>
#include <systemd/sd-daemon.h>
#include <pthread.h>

#define SD_LISTEN_FDS_START 3

pthread_mutex_t sockethack = PTHREAD_MUTEX_INITIALIZER;
int systemd_fds;
char** systemd_fds_names;
extern int h_errno;

typedef struct {
	int domain;
	int type;
	int protocol;
	int index;
} mapping_t;
mapping_t* mappings;

__attribute__((destructor)) void hackdown () {
}

__attribute__((constructor)) void hackup1 () {
	pthread_mutex_lock(&sockethack);
	systemd_fds = sd_listen_fds_with_names(0, &systemd_fds_names);

	if (0) {
		fprintf(stderr, "Have %d systemd descriptors and want %ld memory for it\n", systemd_fds, sizeof(mapping_t));
	}
	mappings = malloc(systemd_fds * sizeof(mapping_t));
	for (int i=0; i < systemd_fds; i++) {
		if (mappings && sscanf(systemd_fds_names[i],"socket(%d,%d,%d)=%d\n", &mappings[i].domain, &mappings[i].type, &mappings[i].protocol, &mappings[i].index) == 4) {
			if (0) {
				fprintf(stderr, "match and stored selector %d_%d_%d_%d\n", mappings[i].domain, mappings[i].type, mappings[i].protocol, mappings[i].index);
			}
		}
		free(systemd_fds_names[i]);
	}
	free(systemd_fds_names);
	pthread_mutex_unlock(&sockethack);
}
__attribute__((constructor)) void gcc_wants_this_to_have_a_name() {
}


int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen) {
	int (*real_bind)(int, const struct sockaddr*, socklen_t);
	int result;
	real_bind = dlsym(RTLD_NEXT, "bind");
	if (sockfd > 2 + systemd_fds) {
		result =  (*real_bind)(sockfd, addr, addrlen);
		return result;
	}
	fprintf(stderr, "throwing away bind() call for %d\n", sockfd);
	return 0;
}

/**
 * intercept calls to make sockets
 * if it is the one that service uses to listen,
 * pass in the already opened fd
 */
int socket(int domain, int type, int protocol) {
	int (*real_socket)(int, int, int);
	int sockfd, i;
	real_socket = dlsym(RTLD_NEXT, "socket");
	if (mappings) {
		for (i=0; i < systemd_fds; i++) {
			if (
				mappings[i].domain == domain &&
				mappings[i].type == type &&
				mappings[i].protocol == protocol
			) {
				if (mappings[i].index == 0) {
					sockfd = 3+i;
					real_socket = NULL;
				}
				mappings[i].index--;
			}
		}
		for (i=0; i < systemd_fds; i++) {
			if (mappings[i].index > -1) {
				break;
			}
		}
		if (i == systemd_fds) {
			fprintf(stderr, "Every fd has been allocated \360\237\230\270\n");
			free(mappings);
			mappings = NULL;
		}
	}

	if (real_socket) {
		sockfd = (*real_socket)(domain, type, protocol);
	}
	if (1) {
		fprintf(stderr, "App requested socket %d %d %d and got fd %d\n", domain, type, protocol, sockfd);
	}
	fflush(stdout);
	return sockfd;
}

/**
 * prevent the service shutting down the descriptors obtained off systemd,
 * for now presuming they are listening sockets.
 * Would exempt FDSTORE=1 descriptors from this
 */
int shutdown(int sockfd, int how) {
	int (*real_shutdown)(int, int);
	real_shutdown = dlsym(RTLD_NEXT, "shutdown");
	if (sockfd <= 2 + systemd_fds) {
		return 0;
	}
	return (*real_shutdown)(sockfd, how);
}

int unlink(const char *pathname) {
	int (*real_unlink)(const char *);
	real_unlink = dlsym(RTLD_NEXT, "unlink");
	if (!strcmp(pathname,"/var/run/slapd/ldapi")) {
		return 0;
	}
	return (*real_unlink)(pathname);
}