git/daemon.c

#include "cache.h"
#include "pkt-line.h"
#include <signal.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

static const char daemon_usage[] = "git-daemon [--inetd | --port=n]";

static int upload(char *dir, int dirlen)
{
	if (chdir(dir) < 0)
		return -1;
	chdir(".git");

	/*
	 * Security on the cheap.
	 *
	 * We want a readable HEAD, usable "objects" directory, and 
	 * a "git-daemon-export-ok" flag that says that the other side
	 * is ok with us doing this.
	 */
	if (access("git-daemon-export-ok", F_OK) ||
	    access("objects/00", X_OK) ||
	    access("HEAD", R_OK))
		return -1;

	/*
	 * We'll ignore SIGTERM from now on, we have a
	 * good client.
	 */
	signal(SIGTERM, SIG_IGN);

	/* git-upload-pack only ever reads stuff, so this is safe */
	execlp("git-upload-pack", "git-upload-pack", ".", NULL);
	return -1;
}

static int execute(void)
{
	static char line[1000];
	int len;

	len = packet_read_line(0, line, sizeof(line));

	if (len && line[len-1] == '\n')
		line[--len] = 0;

	if (!strncmp("git-upload-pack /", line, 17))
		return upload(line + 16, len - 16);

	fprintf(stderr, "got bad connection '%s'\n", line);
	return -1;
}


/*
 * We count spawned/reaped separately, just to avoid any
 * races when updating them from signals. The SIGCHLD handler
 * will only update children_reaped, and the fork logic will
 * only update children_spawned.
 *
 * MAX_CHILDREN should be a power-of-two to make the modulus
 * operation cheap. It should also be at least twice
 * the maximum number of connections we will ever allow.
 */
#define MAX_CHILDREN 128

static int max_connections = 25;

/* These are updated by the signal handler */
static volatile unsigned int children_reaped = 0;
pid_t dead_child[MAX_CHILDREN];

/* These are updated by the main loop */
static unsigned int children_spawned = 0;
static unsigned int children_deleted = 0;

struct child {
	pid_t pid;
	int addrlen;
	struct sockaddr_in address;
} live_child[MAX_CHILDREN];

static void add_child(int idx, pid_t pid, struct sockaddr_in *addr, int addrlen)
{
	live_child[idx].pid = pid;
	live_child[idx].addrlen = addrlen;
	live_child[idx].address = *addr;
}

/*
 * Walk from "deleted" to "spawned", and remove child "pid".
 *
 * We move everything up by one, since the new "deleted" will
 * be one higher.
 */
static void remove_child(pid_t pid, unsigned deleted, unsigned spawned)
{
	struct child n;

	deleted %= MAX_CHILDREN;
	spawned %= MAX_CHILDREN;
	if (live_child[deleted].pid == pid) {
		live_child[deleted].pid = -1;
		return;
	}
	n = live_child[deleted];
	for (;;) {
		struct child m;
		deleted = (deleted + 1) % MAX_CHILDREN;
		if (deleted == spawned)
			die("could not find dead child %d\n", pid);
		m = live_child[deleted];
		live_child[deleted] = n;
		if (m.pid == pid)
			return;
		n = m;
	}
}

/*
 * This gets called if the number of connections grows
 * past "max_connections".
 *
 * We _should_ start off by searching for connections
 * from the same IP, and if there is some address wth
 * multiple connections, we should kill that first.
 *
 * As it is, we just "randomly" kill 25% of the connections,
 * and our pseudo-random generator sucks too. I have no
 * shame.
 *
 * Really, this is just a place-holder for a _real_ algorithm.
 */
static void kill_some_children(int signo, unsigned start, unsigned stop)
{
	start %= MAX_CHILDREN;
	stop %= MAX_CHILDREN;
	while (start != stop) {
		if (!(start & 3))
			kill(live_child[start].pid, signo);
		start = (start + 1) % MAX_CHILDREN;
	}
}

static void check_max_connections(void)
{
	for (;;) {
		int active;
		unsigned spawned, reaped, deleted;

		spawned = children_spawned;
		reaped = children_reaped;
		deleted = children_deleted;

		while (deleted < reaped) {
			pid_t pid = dead_child[deleted % MAX_CHILDREN];
			remove_child(pid, deleted, spawned);
			deleted++;
		}
		children_deleted = deleted;

		active = spawned - deleted;
		if (active <= max_connections)
			break;

		/* Kill some unstarted connections with SIGTERM */
		kill_some_children(SIGTERM, deleted, spawned);
		if (active <= max_connections << 1)
			break;

		/* If the SIGTERM thing isn't helping use SIGKILL */
		kill_some_children(SIGKILL, deleted, spawned);
		sleep(1);
	}
}

static void handle(int incoming, struct sockaddr_in *addr, int addrlen)
{
	pid_t pid = fork();

	if (pid) {
		unsigned idx;

		close(incoming);
		if (pid < 0)
			return;

		idx = children_spawned % MAX_CHILDREN;
		children_spawned++;
		add_child(idx, pid, addr, addrlen);

		check_max_connections();
		return;
	}

	dup2(incoming, 0);
	dup2(incoming, 1);
	close(incoming);
	exit(execute());
}

static void child_handler(int signo)
{
	for (;;) {
		pid_t pid = waitpid(-1, NULL, WNOHANG);

		if (pid > 0) {
			unsigned reaped = children_reaped;
			dead_child[reaped % MAX_CHILDREN] = pid;
			children_reaped = reaped + 1;
			continue;
		}
		break;
	}
}

static int serve(int port)
{
	int sockfd;
	struct sockaddr_in addr;

	signal(SIGCHLD, child_handler);
	sockfd = socket(PF_INET, SOCK_STREAM, IPPROTO_IP);
	if (sockfd < 0)
		die("unable to open socket (%s)", strerror(errno));
	memset(&addr, 0, sizeof(addr));
	addr.sin_port = htons(port);
	addr.sin_family = AF_INET;
	if (bind(sockfd, (void *)&addr, sizeof(addr)) < 0)
		die("unable to bind to port %d (%s)", port, strerror(errno));
	if (listen(sockfd, 5) < 0)
		die("unable to listen to port %d (%s)", port, strerror(errno));

	for (;;) {
		struct sockaddr_in in;
		socklen_t addrlen = sizeof(in);
		int incoming = accept(sockfd, (void *)&in, &addrlen);

		if (incoming < 0) {
			switch (errno) {
			case EAGAIN:
			case EINTR:
			case ECONNABORTED:
				continue;
			default:
				die("accept returned %s", strerror(errno));
			}
		}
		handle(incoming, &in, addrlen);
	}
}

int main(int argc, char **argv)
{
	int port = DEFAULT_GIT_PORT;
	int inetd_mode = 0;
	int i;

	for (i = 1; i < argc; i++) {
		char *arg = argv[i];

		if (!strncmp(arg, "--port=", 7)) {
			char *end;
			unsigned long n;
			n = strtoul(arg+7, &end, 0);
			if (arg[7] && !*end) {
				port = n;
				continue;
			}
		}

		if (!strcmp(arg, "--inetd")) {
			inetd_mode = 1;
			continue;
		}

		usage(daemon_usage);
	}

	if (inetd_mode)
		return execute();

	return serve(port);
}