Distributed engine: remove support for temp replies from slaves.

[pachi.git] / distributed / distributed.c

/* This is a master for the "distributed" engine. It receives connections
 * from slave machines, sends them gtp commands, then aggregates the
 * results. It can also act as a proxy for the logs of all slave machines.
 * The slave machines must run with engine "uct" (not "distributed").
 * The master sends pachi-genmoves gtp commands regularly to each slave,
 * gets as replies a list of candidate moves, their number of playouts
 * and their value. The master then picks the most popular move. */

/* With time control, the master waits for all slaves, except
 * when the allowed time is already passed. In this case the
 * master picks among the available replies, or waits for just
 * one reply if there is none yet.
 * Without time control, the master waits until the desired
 * number of games have been simulated. In this case the -t
 * parameter for the master should be the sum of the parameters
 * for all slaves. */

/* The master sends updated statistics for the best moves
 * in each genmoves command. In this version only the
 * children of the root node are updated. The slaves
 * reply with just their own stats; they remember what was
 * previously received from or sent to the master, to
 * distinguish their own contribution from that of other slaves. */

/* The master-slave protocol has has fault tolerance. If a slave is
 * out of sync, the master sends it the appropriate command history. */

/* Pass me arguments like a=b,c=d,...
 * Supported arguments:
 * slave_port=SLAVE_PORT  slaves connect to this port; this parameter is mandatory.
 * max_slaves=MAX_SLAVES  default 100
 * slaves_quit=0|1        quit gtp command also sent to slaves, default false.
 * proxy_port=PROXY_PORT  slaves optionally send their logs to this port.
 *    Warning: with proxy_port, the master stderr mixes the logs of all
 *    machines but you can separate them again:
 *      slave logs:  sed -n '/< .*:/s/.*< /< /p' logfile
 *      master logs: perl -0777 -pe 's/<[ <].*:.*\n//g' logfile
 */

/* A configuration without proxy would have one master run on masterhost as:
 *    zzgo -e distributed slave_port=1234
 * and N slaves running as:
 *    zzgo -e uct -g masterhost:1234 slave
 * With log proxy:
 *    zzgo -e distributed slave_port=1234,proxy_port=1235
 *    zzgo -e uct -g masterhost:1234 -l masterhost:1235 slave
 * If the master itself runs on a machine other than that running gogui,
 * gogui-twogtp, kgsGtp or cgosGtp, it can redirect its gtp port:
 *    zzgo -e distributed -g 10000 slave_port=1234,proxy_port=1235
 */

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <limits.h>
#include <ctype.h>
#include <time.h>
#include <alloca.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define DEBUG

#include "board.h"
#include "engine.h"
#include "move.h"
#include "timeinfo.h"
#include "network.h"
#include "playout.h"
#include "random.h"
#include "stats.h"
#include "mq.h"
#include "debug.h"
#include "distributed/distributed.h"

/* Internal engine state. */
struct distributed {
        char *slave_port;
        char *proxy_port;
        int max_slaves;
        bool slaves_quit;
        struct move my_last_move;
        struct move_stats my_last_stats;
};

static coord_t select_best_move(struct board *b, struct move_stats *best_stats,
                                int *total_playouts, int *total_threads,
                                char *all_stats, char *end, bool *keep_looking);

/* Default number of simulations to perform per move.
 * Note that this is in total over all slaves! */
#define DIST_GAMES      80000
static const struct time_info default_ti = {
        .period = TT_MOVE,
        .dim = TD_GAMES,
        .len = { .games = DIST_GAMES },
};

#define get_value(value, color) \
        ((color) == S_BLACK ? (value) : 1 - (value))

/* Max size for one line of reply or slave log. */
#define BSIZE 4096

/* Max size of all gtp commands for one game.
 * 60 chars for the first line of genmoves plus 100 lines
 * of 30 chars each for the stats at last move. */
#define CMDS_SIZE (60*MAX_GAMELEN + 30*100)

/* All gtp commands for current game separated by \n */
static char gtp_cmds[CMDS_SIZE];

/* Latest gtp command sent to slaves. */
static char *gtp_cmd = NULL;

/* Remember at most 12 gtp ids per move: play pass,
 * 10 genmoves (1s), play pass.
 * For move 0 we always resend the whole history. */
#define MAX_CMDS_PER_MOVE 12

/* History of gtp commands sent for current game, indexed by move. */
static int id_history[MAX_GAMELEN][MAX_CMDS_PER_MOVE];
static char *cmd_history[MAX_GAMELEN][MAX_CMDS_PER_MOVE];

/* Number of active slave machines working for this master. */
static int active_slaves = 0;

/* Number of replies to last gtp command already received. */
static int reply_count = 0;

/* All replies to latest gtp command are in gtp_replies[0..reply_count-1]. */
static char **gtp_replies;

/* Mutex protecting gtp_cmds, gtp_cmd, id_history, cmd_history,
 * active_slaves, reply_count & gtp_replies */
static pthread_mutex_t slave_lock = PTHREAD_MUTEX_INITIALIZER;

/* Condition signaled when a new gtp command is available. */
static pthread_cond_t cmd_cond = PTHREAD_COND_INITIALIZER;

/* Condition signaled when reply_count increases. */
static pthread_cond_t reply_cond = PTHREAD_COND_INITIALIZER;

/* Mutex protecting stderr. Must not be held at same time as slave_lock. */
static pthread_mutex_t log_lock = PTHREAD_MUTEX_INITIALIZER;

/* Absolute time when this program was started.
 * For debugging only. */
static double start_time;

/* Write the time, client address, prefix, and string s to stderr atomically.
 * s should end with a \n */
static void
logline(struct in_addr *client, char *prefix, char *s)
{
        double now = time_now();
        char addr[INET_ADDRSTRLEN];
        if (client) {
                inet_ntop(AF_INET, client, addr, sizeof(addr));
        } else {
                addr[0] = '\0';
        }
        pthread_mutex_lock(&log_lock);
        fprintf(stderr, "%s%15s %9.3f: %s", prefix, addr, now - start_time, s);
        pthread_mutex_unlock(&log_lock);
}

/* Thread opening a connection on the given socket and copying input
 * from there to stderr. */
static void *
proxy_thread(void *arg)
{
        int proxy_sock = (long)arg;
        assert(proxy_sock >= 0);
        for (;;) {
                struct in_addr client;
                int conn = open_server_connection(proxy_sock, &client);
                FILE *f = fdopen(conn, "r");
                char buf[BSIZE];
                while (fgets(buf, BSIZE, f)) {
                        logline(&client, "< ", buf);
                }
                fclose(f);
        }
}

/* Get a reply to one gtp command. Return the gtp command id,
 * or -1 if error. reply must have at least CMDS_SIZE bytes.
 * slave_lock is not held on either entry or exit of this function. */
static int
get_reply(FILE *f, struct in_addr client, char *reply)
{
        int reply_id = -1;
        *reply = '\0';
        char *line = reply;
        while (fgets(line, reply + CMDS_SIZE - line, f) && *line != '\n') {
                if (DEBUGL(2))
                        logline(&client, "<<", line);
                if (reply_id < 0 && (*line == '=' || *line == '?') && isdigit(line[1]))
                        reply_id = atoi(line+1);
                line += strlen(line);
        }
        if (*line != '\n') return -1;
        return reply_id;
}

/* Main loop of a slave thread.
 * Send the current command to the slave machine and wait for a reply.
 * Resend command history if the slave machine is out of sync.
 * Returns when the connection with the slave machine is cut.
 * slave_lock is held on both entry and exit of this function. */
static void
slave_loop(FILE *f, struct in_addr client, char *buf, bool resend)
{
        char *to_send = gtp_cmd;
        int cmd_id = -1;
        int reply_id = -1;
        for (;;) {
                while (cmd_id == reply_id && !resend) {
                        // Wait for a new gtp command.
                        pthread_cond_wait(&cmd_cond, &slave_lock);
                        if (gtp_cmd)
                                cmd_id = atoi(gtp_cmd);
                        to_send = gtp_cmd;
                }

                /* Command available, send it to slave machine.
                 * If slave was out of sync, send the history. */
                assert(to_send && gtp_cmd);
                strncpy(buf, to_send, CMDS_SIZE);
                cmd_id = atoi(gtp_cmd);

                pthread_mutex_unlock(&slave_lock);

                if (DEBUGL(1) && resend) {
                        if (to_send == gtp_cmds) {
                                logline(&client, "? ", "Slave out-of-sync, resending all history\n");
                        } else {
                                logline(&client, "? ", "Slave behind, partial resend\n");
                        }
                }
                if (DEBUGL(2))
                        logline(&client, ">>", buf);
                fputs(buf, f);
                fflush(f);

                /* Read the reply, which always ends with \n\n
                 * The slave machine sends "=id reply" or "?id reply"
                 * with id == cmd_id if it is in sync. */
                char reply[CMDS_SIZE];
                reply_id = get_reply(f, client, reply);

                pthread_mutex_lock(&slave_lock);
                if (reply_id == -1) return;

                // Make sure we are still in sync:
                cmd_id = atoi(gtp_cmd);
                if (reply_id == cmd_id && *reply == '=') {
                        resend = false;
                        strncpy(buf, reply, CMDS_SIZE);
                        gtp_replies[reply_count++] = buf;
                        pthread_cond_signal(&reply_cond);
                        continue;
                }
                resend = true;
                to_send = gtp_cmds;
                /* Resend everything if slave got latest command,
                 *  but doesn't have a correct board. */
                if (reply_id == cmd_id) continue;

                /* The slave is ouf-of-sync. Check whether the last command
                 * it received belongs to the current game. If so resend
                 * starting at the last move known by slave, otherwise
                 * resend the whole history. */
                int reply_move = move_number(reply_id);
                if (reply_move > move_number(cmd_id)) continue;

                for (int slot = 0; slot < MAX_CMDS_PER_MOVE; slot++) {
                        if (reply_id == id_history[reply_move][slot]) {
                                to_send = cmd_history[reply_move][slot];
                                break;
                        }
                }
        }
}

/* Thread sending gtp commands to one slave machine, and
 * reading replies. If a slave machine dies, this thread waits
 * for a connection from another slave. */
static void *
slave_thread(void *arg)
{
        int slave_sock = (long)arg;
        assert(slave_sock >= 0);
        char slave_buf[CMDS_SIZE];
        bool resend = false;

        for (;;) {
                /* Wait for a connection from any slave. */
                struct in_addr client;
                int conn = open_server_connection(slave_sock, &client);

                FILE *f = fdopen(conn, "r+");
                if (DEBUGL(2))
                        logline(&client, "= ", "new slave\n");

                /* Minimal check of the slave identity. */
                fputs("name\n", f);
                if (!fgets(slave_buf, sizeof(slave_buf), f)
                    || strncasecmp(slave_buf, "= Pachi", 7)
                    || !fgets(slave_buf, sizeof(slave_buf), f)
                    || strcmp(slave_buf, "\n")) {
                        logline(&client, "? ", "bad slave\n");
                        fclose(f);
                        continue;
                }

                pthread_mutex_lock(&slave_lock);
                active_slaves++;
                slave_loop(f, client, slave_buf, resend);

                assert(active_slaves > 0);
                active_slaves--;
                // Unblock main thread if it was waiting for this slave.
                pthread_cond_signal(&reply_cond);
                pthread_mutex_unlock(&slave_lock);

                resend = true;
                if (DEBUGL(2))
                        logline(&client, "= ", "lost slave\n");
                fclose(f);
        }
}

/* Create a new gtp command for all slaves. The slave lock is held
 * upon entry and upon return, so the command will actually be
 * sent when the lock is released. The last command is overwritten
 * if gtp_cmd points to a non-empty string. cmd is a single word;
 * args has all arguments and is empty or has a trailing \n */
static void
update_cmd(struct board *b, char *cmd, char *args)
{
        assert(gtp_cmd);
        /* To make sure the slaves are in sync, we ignore the original id
         * and use the board number plus some random bits as gtp id.
         * Make sure the new command has a new id otherwise slaves
         * won't send it. */
        static int gtp_id = -1;
        int id;
        int moves = is_reset(cmd) ? 0 : b->moves;
        do {
                /* fast_random() is 16-bit only so the multiplication can't overflow. */
                id = force_reply(moves + fast_random(65535) * DIST_GAMELEN);
        } while (id == gtp_id);
        gtp_id = id;
        snprintf(gtp_cmd, gtp_cmds + CMDS_SIZE - gtp_cmd, "%d %s %s",
                 id, cmd, *args ? args : "\n");
        reply_count = 0;

        /* Remember history for out-of-sync slaves. */
        static int slot = 0;
        slot = (slot + 1) % MAX_CMDS_PER_MOVE;
        id_history[moves][slot] = id;
        cmd_history[moves][slot] = gtp_cmd;

        // Notify the slave threads about the new command.
        pthread_cond_broadcast(&cmd_cond);
}

/* Update the command history, then create a new gtp command
 * for all slaves. The slave lock is held upon entry and
 * upon return, so the command will actually be sent when the
 * lock is released. cmd is a single word; args has all
 * arguments and is empty or has a trailing \n */
static void
new_cmd(struct board *b, char *cmd, char *args)
{
        // Clear the history when a new game starts:
        if (!gtp_cmd || is_gamestart(cmd)) {
                gtp_cmd = gtp_cmds;
        } else {
                /* Preserve command history for new slaves.
                 * To indicate that the slave should only reply to
                 * the last command we force the id of previous
                 * commands to be just the move number. */
                int id = prevent_reply(atoi(gtp_cmd));
                int len = strspn(gtp_cmd, "0123456789");
                char buf[32];
                snprintf(buf, sizeof(buf), "%0*d", len, id);
                memcpy(gtp_cmd, buf, len);

                gtp_cmd += strlen(gtp_cmd);
        }

        // Let the slave threads send the new gtp command:
        update_cmd(b, cmd, args);
}

/* If time_limit > 0, wait until all slaves have replied, or if the
 * given absolute time is passed, wait for at least one reply.
 * If time_limit == 0, wait until we get at least min_playouts games
 * simulated in total by all the slaves, or until all slaves have replied.
 * The replies are returned in gtp_replies[0..reply_count-1]
 * slave_lock is held on entry and on return. */
static void
get_replies(double time_limit, int min_playouts, struct board *b)
{
        while (reply_count == 0 || reply_count < active_slaves) {
                if (time_limit && reply_count > 0) {
                        struct timespec ts;
                        double sec;
                        ts.tv_nsec = (int)(modf(time_limit, &sec)*1000000000.0);
                        ts.tv_sec = (int)sec;
                        pthread_cond_timedwait(&reply_cond, &slave_lock, &ts);
                } else {
                        pthread_cond_wait(&reply_cond, &slave_lock);
                }
                if (reply_count == 0) continue;
                if (reply_count >= active_slaves) return;
                if (time_limit) {
                        if (time_now() >= time_limit) break;
                } else {
                        int playouts;
                        select_best_move(b, NULL, &playouts, NULL, NULL, NULL, NULL);
                        if (playouts >= min_playouts) return;
                }
        }
        if (DEBUGL(1)) {
                char buf[1024];
                snprintf(buf, sizeof(buf),
                         "get_replies timeout %.3f >= %.3f, replies %d < active %d\n",
                         time_now() - start_time, time_limit - start_time,
                         reply_count, active_slaves);
                logline(NULL, "? ", buf);
        }
        assert(reply_count > 0);
}

/* Maximum time (seconds) to wait for answers to fast gtp commands
 * (all commands except pachi-genmoves and final_status_list). */
#define MAX_FAST_CMD_WAIT 1.0

/* How often to send a stats update to slaves (seconds) */
#define STATS_UPDATE_INTERVAL 0.1 /* 100ms */

/* Maximum time (seconds) to wait between genmoves
 * (all commands except pachi-genmoves and final_status_list). */
#define MAX_FAST_CMD_WAIT 1.0

/* Dispatch a new gtp command to all slaves.
 * The slave lock must not be held upon entry and is released upon return.
 * args is empty or ends with '\n' */
static enum parse_code
distributed_notify(struct engine *e, struct board *b, int id, char *cmd, char *args, char **reply)
{
        struct distributed *dist = e->data;

        /* Commands that should not be sent to slaves.
         * time_left will be part of next pachi-genmoves,
         * we reduce latency by not forwarding it here. */
        if ((!strcasecmp(cmd, "quit") && !dist->slaves_quit)
            || !strcasecmp(cmd, "uct_genbook")
            || !strcasecmp(cmd, "uct_dumpbook")
            || !strcasecmp(cmd, "kgs-chat")
            || !strcasecmp(cmd, "time_left")

            /* and commands that will be sent to slaves later */
            || !strcasecmp(cmd, "genmove")
            || !strcasecmp(cmd, "kgs-genmove_cleanup")
            || !strcasecmp(cmd, "final_score")
            || !strcasecmp(cmd, "final_status_list"))
                return P_OK;

        pthread_mutex_lock(&slave_lock);

        // Create a new command to be sent by the slave threads.
        new_cmd(b, cmd, args);

        /* Wait for replies here. If we don't wait, we run the
         * risk of getting out of sync with most slaves and
         * sending command history too frequently. */
        get_replies(time_now() + MAX_FAST_CMD_WAIT, 0, b);

        pthread_mutex_unlock(&slave_lock);
        return P_OK;
}

/* genmoves returns a line "=id total_playouts threads keep_looking[ reserved]"
 * then a list of lines "coord playouts value".
 * Return the move with most playouts, and optional additional info.
 * If set, all_stats gathers the stats from all slaves except for
 * pass and resign; it must have room up to end and upon return
 * ends with an empty line.
 * Keep this code in sync with uct_getstats().
 * slave_lock is held on entry and on return. */
static coord_t
select_best_move(struct board *b, struct move_stats *best_stats,
                 int *total_playouts, int *total_threads,
                 char *all_stats, char *end, bool *keep_looking)
{
        assert(reply_count > 0);

        /* +2 for pass and resign. */
        struct move_stats *stats = alloca((board_size2(b)+2) * sizeof(struct move_stats));
        memset(stats, 0, (board_size2(b)+2) * sizeof(*stats));
        stats += 2;

        coord_t best_move = pass;
        int best_playouts = -1;
        int playouts = 0;
        int threads = 0;
        int keep = 0;

        for (int reply = 0; reply < reply_count; reply++) {
                char *r = gtp_replies[reply];
                int id, p, t, k;
                if (sscanf(r, "=%d %d %d %d", &id, &p, &t, &k) != 4) continue;
                playouts += p;
                threads += t;
                keep += k;
                // Skip the rest of the firt line if any (allow future extensions)
                r = strchr(r, '\n');

                char move[64];
                struct move_stats s;
                while (r && sscanf(++r, "%63s %d %f", move, &s.playouts, &s.value) == 3) {
                        coord_t *c = str2coord(move, board_size(b));
                        stats_add_result(&stats[*c], s.value, s.playouts);
                        if (stats[*c].playouts > best_playouts) {
                                best_playouts = stats[*c].playouts;
                                best_move = *c;
                        }
                        coord_done(c);
                        r = strchr(r, '\n');
                }
        }
        if (all_stats) {
                char *s = all_stats;
                int min_playouts = best_playouts /= 100;
                /* Send stats for all moves except pass and resign. */
                foreach_point(b) {
                        if (stats[c].playouts <= min_playouts) continue;
                        s += snprintf(s, end - s, "%s %d %.7f\n",
                                      coord2sstr(c, b),
                                      stats[c].playouts, stats[c].value);
                } foreach_point_end;
                s += snprintf(s, end - s, "\n");
        }
        if (best_stats) *best_stats = stats[best_move];
        if (total_playouts) *total_playouts = playouts;
        if (total_threads) *total_threads = threads;
        if (keep_looking) *keep_looking = keep > reply_count / 2;
        return best_move;
}

/* Time control is mostly done by the slaves, so we use default values here. */
#define FUSEKI_END 20
#define YOSE_START 40

static coord_t *
distributed_genmove(struct engine *e, struct board *b, struct time_info *ti, enum stone color, bool pass_all_alive)
{
        struct distributed *dist = e->data;
        double now = time_now();
        double first = now;

        char *cmd = pass_all_alive ? "pachi-genmoves_cleanup" : "pachi-genmoves";
        char args[CMDS_SIZE];
        char *end = args + sizeof(args);

        coord_t best;
        int playouts, threads;
        struct move_stats best_stats;

        if (ti->period == TT_NULL) *ti = default_ti;
        struct time_stop stop;
        time_stop_conditions(ti, b, FUSEKI_END, YOSE_START, &stop);
        struct time_info saved_ti = *ti;

        /* Send the first genmoves without stats. This is
         * a multi-line command ending with \n\n.
         * Keep this code in sync with uct_genmoves(). */
        char *col = args + snprintf(args, sizeof(args), "%s", stone2str(color));
        char *s = col;
        if (ti->dim == TD_WALLTIME) {
                s += snprintf(s, end - s, " %.3f %.3f %d %d",
                              ti->len.t.main_time, ti->len.t.byoyomi_time,
                              ti->len.t.byoyomi_periods, ti->len.t.byoyomi_stones);
        }
        s += snprintf(s, end - s, "\n\n");

        pthread_mutex_lock(&slave_lock);
        new_cmd(b, cmd, args);

        /* Loop until most slaves want to quit or time elapsed. */
        for (;;) {
                double start = now;
                get_replies(now + STATS_UPDATE_INTERVAL, 0, b);
                now = time_now();
                s = col;
                if (ti->dim == TD_WALLTIME) {
                        time_sub(ti, now - start);
                        s += snprintf(s, end - s, " %.3f %.3f %d %d",
                                      ti->len.t.main_time, ti->len.t.byoyomi_time,
                                      ti->len.t.byoyomi_periods, ti->len.t.byoyomi_stones);
                }
                s += snprintf(s, end - s, "\n");
                bool keep_looking;
                best = select_best_move(b, &best_stats, &playouts,
                                        &threads, s, end, &keep_looking);

                if (!keep_looking) break;
                if (ti->dim == TD_WALLTIME) {
                        if (now - ti->len.t.timer_start >= stop.worst.time) break;
                } else {
                        if (playouts >= stop.worst.playouts) break;
                }
                if (DEBUGL(2)) {
                        char buf[BSIZE];
                        char *coord = coord2sstr(best, b);
                        snprintf(buf, sizeof(buf),
                                 "temp winner is %s %s with score %1.4f (%d/%d games)"
                                 " %d slaves %d threads\n",
                                 stone2str(color), coord, get_value(best_stats.value, color),
                                 best_stats.playouts, playouts, reply_count, threads);
                        logline(NULL, "* ", buf);
                }
                update_cmd(b, cmd, args);
        }
        int replies = reply_count;

        /* Do not subtract time spent twice (see gtp_parse). */
        *ti = saved_ti;

        dist->my_last_move.color = color;
        dist->my_last_move.coord = best;
        dist->my_last_stats = best_stats;

        /* Tell the slaves to commit to the selected move, overwriting
         * the last "pachi-genmoves" in the command history. */
        char *coord = coord2str(best, b);
        snprintf(args, sizeof(args), "%s %s\n", stone2str(color), coord);
        update_cmd(b, "play", args);
        pthread_mutex_unlock(&slave_lock);

        if (DEBUGL(1)) {
                char buf[BSIZE];
                double time = now - first + 0.000001; /* avoid divide by zero */
                snprintf(buf, sizeof(buf),
                         "GLOBAL WINNER is %s %s with score %1.4f (%d/%d games)\n"
                         "genmove in %0.2fs %d slaves %d threads (%d games/s,"
                         " %d games/s/slave, %d games/s/thread)\n",
                         stone2str(color), coord, get_value(best_stats.value, color),
                         best_stats.playouts, playouts, time, replies, threads,
                         (int)(playouts/time), (int)(playouts/time/replies),
                         (int)(playouts/time/threads));
                logline(NULL, "* ", buf);
        }
        free(coord);
        return coord_copy(best);
}

static char *
distributed_chat(struct engine *e, struct board *b, char *cmd)
{
        struct distributed *dist = e->data;
        static char reply[BSIZE];

        cmd += strspn(cmd, " \n\t");
        if (!strncasecmp(cmd, "winrate", 7)) {
                enum stone color = dist->my_last_move.color;
                snprintf(reply, BSIZE, "In %d playouts at %d machines, %s %s can win with %.2f%% probability.",
                         dist->my_last_stats.playouts, active_slaves, stone2str(color),
                         coord2sstr(dist->my_last_move.coord, b),
                         100 * get_value(dist->my_last_stats.value, color));
                return reply;
        }
        return NULL;
}

static int
scmp(const void *p1, const void *p2)
{
        return strcasecmp(*(char * const *)p1, *(char * const *)p2);
}

static void
distributed_dead_group_list(struct engine *e, struct board *b, struct move_queue *mq)
{
        pthread_mutex_lock(&slave_lock);

        new_cmd(b, "final_status_list", "dead\n");
        get_replies(time_now() + MAX_FAST_CMD_WAIT, 0, b);

        /* Find the most popular reply. */
        qsort(gtp_replies, reply_count, sizeof(char *), scmp);
        int best_reply = 0;
        int best_count = 1;
        int count = 1;
        for (int reply = 1; reply < reply_count; reply++) {
                if (!strcmp(gtp_replies[reply], gtp_replies[reply-1])) {
                        count++;
                } else {
                        count = 1;
                }
                if (count > best_count) {
                        best_count = count;
                        best_reply = reply;
                }
        }

        /* Pick the first move of each line as group. */
        char *dead = gtp_replies[best_reply];
        dead = strchr(dead, ' '); // skip "id "
        while (dead && *++dead != '\n') {
                coord_t *c = str2coord(dead, board_size(b));
                mq_add(mq, *c);
                coord_done(c);
                dead = strchr(dead, '\n');
        }
        pthread_mutex_unlock(&slave_lock);
}

static struct distributed *
distributed_state_init(char *arg, struct board *b)
{
        struct distributed *dist = calloc(1, sizeof(struct distributed));

        dist->max_slaves = 100;
        if (arg) {
                char *optspec, *next = arg;
                while (*next) {
                        optspec = next;
                        next += strcspn(next, ",");
                        if (*next) { *next++ = 0; } else { *next = 0; }

                        char *optname = optspec;
                        char *optval = strchr(optspec, '=');
                        if (optval) *optval++ = 0;

                        if (!strcasecmp(optname, "slave_port") && optval) {
                                dist->slave_port = strdup(optval);
                        } else if (!strcasecmp(optname, "proxy_port") && optval) {
                                dist->proxy_port = strdup(optval);
                        } else if (!strcasecmp(optname, "max_slaves") && optval) {
                                dist->max_slaves = atoi(optval);
                        } else if (!strcasecmp(optname, "slaves_quit")) {
                                dist->slaves_quit = !optval || atoi(optval);
                        } else {
                                fprintf(stderr, "distributed: Invalid engine argument %s or missing value\n", optname);
                        }
                }
        }

        gtp_replies = calloc(dist->max_slaves, sizeof(char *));

        if (!dist->slave_port) {
                fprintf(stderr, "distributed: missing slave_port\n");
                exit(1);
        }
        int slave_sock = port_listen(dist->slave_port, dist->max_slaves);
        pthread_t thread;
        for (int id = 0; id < dist->max_slaves; id++) {
                pthread_create(&thread, NULL, slave_thread, (void *)(long)slave_sock);
        }

        if (dist->proxy_port) {
                int proxy_sock = port_listen(dist->proxy_port, dist->max_slaves);
                for (int id = 0; id < dist->max_slaves; id++) {
                        pthread_create(&thread, NULL, proxy_thread, (void *)(long)proxy_sock);
                }
        }
        return dist;
}

struct engine *
engine_distributed_init(char *arg, struct board *b)
{
        start_time = time_now();
        struct distributed *dist = distributed_state_init(arg, b);
        struct engine *e = calloc(1, sizeof(struct engine));
        e->name = "Distributed Engine";
        e->comment = "I'm playing the distributed engine. When I'm losing, I will resign, "
                "if I think I win, I play until you pass. "
                "Anyone can send me 'winrate' in private chat to get my assessment of the position.";
        e->notify = distributed_notify;
        e->genmove = distributed_genmove;
        e->dead_group_list = distributed_dead_group_list;
        e->chat = distributed_chat;
        e->data = dist;
        // Keep the threads and the open socket connections:
        e->keep_on_clear = true;

        return e;
}