Merge remote branch 'fml/master'

[paraslash.git] / afs.c

/*
 * Copyright (C) 2007-2010 Andre Noll <maan@systemlinux.org>
 *
 * Licensed under the GPL v2. For licencing details see COPYING.
 */

/** \file afs.c Paraslash's audio file selector. */

#include <regex.h>
#include <signal.h>
#include <fnmatch.h>
#include <openssl/rc4.h>
#include <osl.h>

#include "server.cmdline.h"
#include "para.h"
#include "error.h"
#include "crypt.h"
#include "string.h"
#include "afh.h"
#include "afs.h"
#include "server.h"
#include <dirent.h> /* readdir() */
#include <sys/mman.h>
#include <sys/time.h>
#include "net.h"
#include "ipc.h"
#include "list.h"
#include "sched.h"
#include "signal.h"
#include "fd.h"
#include "mood.h"

/** The osl tables used by afs. \sa blob.c. */
enum afs_table_num {
        /** Contains audio file information. See aft.c. */
        TBLNUM_AUDIO_FILES,
        /** The table for the paraslash attributes. See attribute.c. */
        TBLNUM_ATTRIBUTES,
        /**
         * Paraslash's scoring system is based on Gaussian normal
         * distributions, and the relevant data is stored in the rbtrees of an
         * osl table containing only volatile columns.  See score.c for
         * details.
         */
        TBLNUM_SCORES,
        /**
         * A standard blob table containing the mood definitions. For details
         * see mood.c.
         */
        TBLNUM_MOODS,
        /** A blob table containing lyrics on a per-song basis. */
        TBLNUM_LYRICS,
        /** Another blob table for images (for example album cover art). */
        TBLNUM_IMAGES,
        /** Yet another blob table for storing standard playlists. */
        TBLNUM_PLAYLIST,
        /** How many tables are in use? */
        NUM_AFS_TABLES
};

static struct afs_table afs_tables[NUM_AFS_TABLES] = {
        [TBLNUM_AUDIO_FILES] = {.init = aft_init, .name = "audio_files"},
        [TBLNUM_ATTRIBUTES] = {.init = attribute_init, .name = "attributes"},
        [TBLNUM_SCORES] = {.init = score_init, .name = "scores"},
        [TBLNUM_MOODS] = {.init = moods_init, .name = "moods"},
        [TBLNUM_LYRICS] = {.init = lyrics_init, .name = "lyrics"},
        [TBLNUM_IMAGES] = {.init = images_init, .name = "images"},
        [TBLNUM_PLAYLIST] = {.init = playlists_init, .name = "playlists"},
};

struct command_task {
        /** The file descriptor for the local socket. */
        int fd;
        /**
         * Value sent by the command handlers to identify themselves as
         * children of the running para_server.
         */
        uint32_t cookie;
        /** The associated task structure. */
        struct task task;
};

extern int mmd_mutex;
extern struct misc_meta_data *mmd;

static int server_socket;
static struct command_task command_task_struct;
static struct signal_task signal_task_struct;

static enum play_mode current_play_mode;
static char *current_mop; /* mode or playlist specifier. NULL means dummy mooe */

/**
 * A random number used to "authenticate" the connection.
 *
 * para_server picks this number by random before forking the afs process.  The
 * command handlers write this number together with the id of the shared memory
 * area containing the query. This way, a malicious local user has to know this
 * number to be able to cause the afs process to crash by sending fake queries.
 */
extern uint32_t afs_socket_cookie;

/**
 * Struct to let command handlers execute a callback in afs context.
 *
 * Commands that need to change the state of afs can't change the relevant data
 * structures directly because commands are executed in a child process, i.e.
 * they get their own virtual address space.
 *
 * This structure is used by \p send_callback_request() (executed from handler
 * context) in order to let the afs process call the specified function. An
 * instance of that structure is written to a shared memory area together with
 * the arguments to the callback function. The identifier of the shared memory
 * area is written to the command socket.
 *
 * The afs process accepts connections on the command socket and reads the
 * shared memory id, attaches the corresponing area, calls the given handler to
 * perform the desired action and to optionally compute a result.
 *
 * The result and a \p callback_result structure is then written to another
 * shared memory area. The identifier for that area is written to the handler's
 * command socket, so that the handler process can read the id, attach the
 * shared memory area and use the result.
 *
 * \sa struct callback_result.
 */
struct callback_query {
        /** The function to be called. */
        callback_function *handler;
        /** The number of bytes of the query */
        size_t query_size;
};

/**
 * Structure embedded in the result of a callback.
 *
 * If the callback produced a result, an instance of that structure is embeeded
 * into the shared memory area holding the result, mainly to let the command
 * handler know the size of the result.
 *
 * \sa struct callback_query.
 */
struct callback_result {
        /** The number of bytes of the result. */
        size_t result_size;
};

static int dispatch_result(int result_shmid, callback_result_handler *handler,
                void *private_result_data)
{
        struct osl_object result;
        void *result_shm;
        int ret2, ret = shm_attach(result_shmid, ATTACH_RO, &result_shm);
        struct callback_result *cr = result_shm;

        if (ret < 0) {
                PARA_ERROR_LOG("attach failed: %s\n", para_strerror(-ret));
                return ret;
        }
        result.size = cr->result_size;
        result.data = result_shm + sizeof(*cr);
        if (result.size) {
                assert(handler);
                ret = handler(&result, private_result_data);
                if (ret < 0)
                        PARA_NOTICE_LOG("result handler error: %s\n",
                                para_strerror(-ret));
        }
        ret2 = shm_detach(result_shm);
        if (ret2 < 0) {
                PARA_ERROR_LOG("detach failed: %s\n", para_strerror(-ret2));
                if (ret >= 0)
                        ret = ret2;
        }
        return ret;
}

/**
 * Ask the afs process to call a given function.
 *
 * \param f The function to be called.
 * \param query Pointer to arbitrary data for the callback.
 * \param result_handler Called for each shm area sent by the callback.
 * \param private_result_data Passed verbatim to \a result_handler.
 *
 * This function creates a socket for communication with the afs process and a
 * shared memory area (sma) to which the buffer pointed to by \a query is
 * copied.  It then notifies the afs process that the callback function \a f
 * should be executed by sending the shared memory identifier (shmid) to the
 * socket.

 * If the callback produces a result, it sends any number of shared memory
 * identifiers back via the socket. For each such identifier received, \a
 * result_handler is called. The contents of the sma identified by the received
 * shmid are passed to that function as an osl object. The private_result_data
 * pointer is passed as the second argument to \a result_handler.
 *
 * \return Standard.
 *
 * \sa send_option_arg_callback_request(), send_standard_callback_request().
 */
int send_callback_request(callback_function *f, struct osl_object *query,
                callback_result_handler *result_handler,
                void *private_result_data)
{
        struct callback_query *cq;
        int ret, fd = -1, query_shmid, result_shmid;
        void *query_shm;
        char buf[sizeof(afs_socket_cookie) + sizeof(int)];
        size_t query_shm_size = sizeof(*cq);
        int dispatch_error = 0;

        if (query)
                query_shm_size += query->size;
        ret = shm_new(query_shm_size);
        if (ret < 0)
                return ret;
        query_shmid = ret;
        ret = shm_attach(query_shmid, ATTACH_RW, &query_shm);
        if (ret < 0)
                goto out;
        cq = query_shm;
        cq->handler = f;
        cq->query_size = query_shm_size - sizeof(*cq);

        if (query)
                memcpy(query_shm + sizeof(*cq), query->data, query->size);
        ret = shm_detach(query_shm);
        if (ret < 0)
                goto out;

        *(uint32_t *) buf = afs_socket_cookie;
        *(int *) (buf + sizeof(afs_socket_cookie)) = query_shmid;

        ret = create_remote_socket(conf.afs_socket_arg);
        if (ret < 0)
                goto out;
        fd = ret;
        ret = send_bin_buffer(fd, buf, sizeof(buf));
        if (ret < 0)
                goto out;
        /*
         * Read all shmids from afs.
         *
         * Even if the dispatcher returns an error we _must_ continue to read
         * shmids from fd so that we can destroy all shared memory areas that
         * have been created for us by the afs process.
         */
        for (;;) {
                ret = recv_bin_buffer(fd, buf, sizeof(int));
                if (ret <= 0)
                        goto out;
                assert(ret == sizeof(int));
                ret = *(int *) buf;
                assert(ret > 0);
                result_shmid = ret;
                if (!dispatch_error) {
                        ret = dispatch_result(result_shmid, result_handler,
                                private_result_data);
                        if (ret < 0)
                                dispatch_error = 1;
                }
                ret = shm_destroy(result_shmid);
                if (ret < 0)
                        PARA_CRIT_LOG("destroy result failed: %s\n",
                                para_strerror(-ret));
        }
out:
        if (shm_destroy(query_shmid) < 0)
                PARA_CRIT_LOG("shm destroy error\n");
        if (fd >= 0)
                close(fd);
//      PARA_DEBUG_LOG("callback_ret: %d\n", ret);
        return ret;
}

/**
 * Send a callback request passing an options structure and an argument vector.
 *
 * \param options pointer to an arbitrary data structure.
 * \param argc Argument count.
 * \param argv Standard argument vector.
 * \param f The callback function.
 * \param result_handler See \ref send_callback_request.
 * \param private_result_data See \ref send_callback_request.
 *
 * Some commands have a couple of options that are parsed in child context for
 * syntactic correctness and are stored in a special options structure for that
 * command. This function allows to pass such a structure together with a list
 * of further arguments (often a list of audio files) to the parent process.
 *
 * \sa send_standard_callback_request(), send_callback_request().
 */
int send_option_arg_callback_request(struct osl_object *options,
                int argc,  char * const * const argv, callback_function *f,
                callback_result_handler *result_handler,
                void *private_result_data)
{
        char *p;
        int i, ret;
        struct osl_object query = {.size = options? options->size : 0};

        for (i = 0; i < argc; i++)
                query.size += strlen(argv[i]) + 1;
        query.data = para_malloc(query.size);
        p = query.data;
        if (options) {
                memcpy(query.data, options->data, options->size);
                p += options->size;
        }
        for (i = 0; i < argc; i++) {
                strcpy(p, argv[i]); /* OK */
                p += strlen(argv[i]) + 1;
        }
        ret = send_callback_request(f, &query, result_handler,
                private_result_data);
        free(query.data);
        return ret;
}

/**
 * Send a callback request with an argument vector only.
 *
 * \param argc The same meaning as in send_option_arg_callback_request().
 * \param argv The same meaning as in send_option_arg_callback_request().
 * \param f The same meaning as in send_option_arg_callback_request().
 * \param result_handler See \ref send_callback_request.
 * \param private_result_data See \ref send_callback_request.
 *
 * This is similar to send_option_arg_callback_request(), but no options buffer
 * is passed to the parent process.
 *
 * \return The return value of the underlying call to
 * send_option_arg_callback_request().
 */
int send_standard_callback_request(int argc,  char * const * const argv,
                callback_function *f, callback_result_handler *result_handler,
                void *private_result_data)
{
        return send_option_arg_callback_request(NULL, argc, argv, f, result_handler,
                private_result_data);
}

static int action_if_pattern_matches(struct osl_row *row, void *data)
{
        struct pattern_match_data *pmd = data;
        struct osl_object name_obj;
        const char *p, *name;
        int ret = osl(osl_get_object(pmd->table, row, pmd->match_col_num, &name_obj));
        const char *pattern_txt = (const char *)pmd->patterns.data;

        if (ret < 0)
                return ret;
        name = (char *)name_obj.data;
        if ((!name || !*name) && (pmd->pm_flags & PM_SKIP_EMPTY_NAME))
                return 1;
        if (!pmd->patterns.size && (pmd->pm_flags & PM_NO_PATTERN_MATCHES_EVERYTHING))
                return pmd->action(pmd->table, row, name, pmd->data);
        for (p = pattern_txt; p < pattern_txt + pmd->patterns.size;
                        p += strlen(p) + 1) {
                ret = fnmatch(p, name, pmd->fnmatch_flags);
                if (ret == FNM_NOMATCH)
                        continue;
                if (ret)
                        return -E_FNMATCH;
                return pmd->action(pmd->table, row, name, pmd->data);
        }
        return 1;
}

/**
 * Execute the given function for each matching row.
 *
 * \param pmd Describes what to match and how.
 *
 * \return Standard.
 */
int for_each_matching_row(struct pattern_match_data *pmd)
{
        if (pmd->pm_flags & PM_REVERSE_LOOP)
                return osl(osl_rbtree_loop_reverse(pmd->table, pmd->loop_col_num, pmd,
                        action_if_pattern_matches));
        return osl(osl_rbtree_loop(pmd->table, pmd->loop_col_num, pmd,
                        action_if_pattern_matches));
}

/**
 * Compare two osl objects of string type.
 *
 * \param obj1 Pointer to the first object.
 * \param obj2 Pointer to the second object.
 *
 * In any case, only \p MIN(obj1->size, obj2->size) characters of each string
 * are taken into account.
 *
 * \return It returns an integer less than, equal to, or greater than zero if
 * \a obj1 is found, respectively, to be less than, to match, or be greater than
 * obj2.
 *
 * \sa strcmp(3), strncmp(3), osl_compare_func.
 */
int string_compare(const struct osl_object *obj1, const struct osl_object *obj2)
{
        const char *str1 = (const char *)obj1->data;
        const char *str2 = (const char *)obj2->data;
        return strncmp(str1, str2, PARA_MIN(obj1->size, obj2->size));
}

static int pass_afd(int fd, char *buf, size_t size)
{
        struct msghdr msg = {.msg_iov = NULL};
        struct cmsghdr *cmsg;
        char control[255];
        int ret;
        struct iovec iov;

        iov.iov_base = buf;
        iov.iov_len  = size;

        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;

        msg.msg_control = control;
        msg.msg_controllen = sizeof(control);

        cmsg = CMSG_FIRSTHDR(&msg);
        cmsg->cmsg_level = SOL_SOCKET;
        cmsg->cmsg_type = SCM_RIGHTS;
        cmsg->cmsg_len = CMSG_LEN(sizeof(int));
        *(int *)CMSG_DATA(cmsg) = fd;

        /* Sum of the length of all control messages in the buffer */
        msg.msg_controllen = cmsg->cmsg_len;
        PARA_DEBUG_LOG("passing %zu bytes and fd %d\n", size, fd);
        ret = sendmsg(server_socket, &msg, 0);
        if (ret < 0) {
                ret = -ERRNO_TO_PARA_ERROR(errno);
                return ret;
        }
        return 1;
}

/**
 * Open the audio file with highest score.
 *
 * This stores all information for streaming the "best" audio file in a shared
 * memory area. The id of that area and an open file descriptor for the next
 * audio file are passed to the server process.
 *
 * \return Standard.
 *
 * \sa open_and_update_audio_file().
 */
static int open_next_audio_file(void)
{
        struct osl_row *aft_row;
        struct audio_file_data afd;
        int ret, shmid;
        char buf[8];
        long score;
again:
        PARA_NOTICE_LOG("getting next audio file\n");
        ret = score_get_best(&aft_row, &score);
        if (ret < 0) {
                PARA_ERROR_LOG("%s\n", para_strerror(-ret));
                goto no_admissible_files;
        }
        ret = open_and_update_audio_file(aft_row, score, &afd);
        if (ret < 0) {
                PARA_ERROR_LOG("%s\n", para_strerror(-ret));
                ret = score_delete(aft_row);
                if (ret < 0) {
                        PARA_ERROR_LOG("%s\n", para_strerror(-ret));
                        goto no_admissible_files;
                }
                goto again;
        }
        shmid = ret;
        if (!write_ok(server_socket)) {
                ret = -E_AFS_SOCKET;
                goto destroy;
        }
        *(uint32_t *)buf = NEXT_AUDIO_FILE;
        *(uint32_t *)(buf + 4) = (uint32_t)shmid;
        ret = pass_afd(afd.fd, buf, 8);
        close(afd.fd);
        if (ret >= 0)
                return ret;
destroy:
        shm_destroy(shmid);
        return ret;
no_admissible_files:
        *(uint32_t *)buf = NO_ADMISSIBLE_FILES;
        *(uint32_t *)(buf + 4) = (uint32_t)0;
        return send_bin_buffer(server_socket, buf, 8);
}

/* Never fails if arg == NULL */
static int activate_mood_or_playlist(char *arg, int *num_admissible)
{
        enum play_mode mode;
        int ret;

        if (!arg) {
                ret = change_current_mood(NULL); /* always successful */
                mode = PLAY_MODE_MOOD;
        } else {
                if (!strncmp(arg, "p/", 2)) {
                        ret = playlist_open(arg + 2);
                        mode = PLAY_MODE_PLAYLIST;
                } else if (!strncmp(arg, "m/", 2)) {
                        ret = change_current_mood(arg + 2);
                        mode = PLAY_MODE_MOOD;
                } else
                        return -E_AFS_SYNTAX;
                if (ret < 0)
                        return ret;
        }
        if (num_admissible)
                *num_admissible = ret;
        current_play_mode = mode;
        if (arg != current_mop) {
                free(current_mop);
                if (arg) {
                        current_mop = para_strdup(arg);
                        mutex_lock(mmd_mutex);
                        strncpy(mmd->afs_mode_string, arg,
                                sizeof(mmd->afs_mode_string));
                        mmd->afs_mode_string[sizeof(mmd->afs_mode_string) - 1] = '\0';
                        mutex_unlock(mmd_mutex);
                } else {
                        mutex_lock(mmd_mutex);
                        strcpy(mmd->afs_mode_string, "dummy");
                        mutex_unlock(mmd_mutex);
                        current_mop = NULL;
                }
        }
        return 1;
}

static void com_select_callback(int fd, const struct osl_object *query)
{
        struct para_buffer pb = {
                .max_size = SHMMAX,
                .private_data = &fd,
                .max_size_handler = pass_buffer_as_shm
        };
        char *arg = query->data;
        int num_admissible, ret, ret2;

        ret = clear_score_table();
        if (ret < 0) {
                ret2 = para_printf(&pb, "%s\n", para_strerror(-ret));
                goto out;
        }
        if (current_play_mode == PLAY_MODE_MOOD)
                close_current_mood();
        else
                playlist_close();
        ret = activate_mood_or_playlist(arg, &num_admissible);
        if (ret < 0) {
                ret2 = para_printf(&pb, "%s\nswitching back to %s\n",
                        para_strerror(-ret), current_mop?
                        current_mop : "dummy");
                ret = activate_mood_or_playlist(current_mop, &num_admissible);
                if (ret < 0) {
                        if (ret2 >= 0)
                                ret2 = para_printf(&pb, "failed, switching to dummy\n");
                        activate_mood_or_playlist(NULL, &num_admissible);
                }
        } else
                ret2 = para_printf(&pb, "activated %s (%d admissible files)\n", current_mop?
                        current_mop : "dummy mood", num_admissible);
out:
        if (ret2 >= 0 && pb.offset)
                pass_buffer_as_shm(pb.buf, pb.offset, &fd);
        free(pb.buf);
}

/**
 * Result handler for sending data to the para_client process.
 *
 * \param result The data to be sent.
 * \param private Pointer to rc4 context.
 *
 * \return The return value of the underlying call to rc4_send_bin_buffer().
 *
 * \sa \ref callback_result_handler, \ref rc4_send_bin_buffer().
 */
int rc4_send_result(struct osl_object *result, void *private)
{
        struct rc4_context *rc4c = private;

        if (!result->size)
                return 1;
        return rc4_send_bin_buffer(rc4c, result->data, result->size);
}

int com_select(struct rc4_context *rc4c, int argc, char * const * const argv)
{
        struct osl_object query;

        if (argc != 2)
                return -E_AFS_SYNTAX;
        query.data = argv[1];
        query.size = strlen(argv[1]) + 1;
        return send_callback_request(com_select_callback, &query,
                &rc4_send_result, rc4c);
}

static void init_admissible_files(char *arg)
{
        if (activate_mood_or_playlist(arg, NULL) < 0)
                activate_mood_or_playlist(NULL, NULL); /* always successful */
}

static int setup_command_socket_or_die(void)
{
        int ret, socket_fd;
        char *socket_name = conf.afs_socket_arg;
        struct sockaddr_un unix_addr;

        unlink(socket_name);
        ret = create_local_socket(socket_name, &unix_addr,
                S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IWOTH);
        if (ret < 0) {
                PARA_EMERG_LOG("%s: %s\n", para_strerror(-ret), socket_name);
                exit(EXIT_FAILURE);
        }
        socket_fd = ret;
        if (listen(socket_fd , 5) < 0) {
                PARA_EMERG_LOG("can not listen on socket\n");
                exit(EXIT_FAILURE);
        }
        ret = mark_fd_nonblocking(socket_fd);
        if (ret < 0) {
                close(socket_fd);
                return ret;
        }
        PARA_INFO_LOG("listening on socket %s (fd %d)\n", socket_name,
                socket_fd);
        return socket_fd;
}

static void close_afs_tables(void)
{
        int i;
        PARA_NOTICE_LOG("closing afs_tables\n");
        for (i = 0; i < NUM_AFS_TABLES; i++)
                afs_tables[i].close();
}

static char *database_dir;

static void get_database_dir(void)
{
        if (!database_dir) {
                if (conf.afs_database_dir_given)
                        database_dir = para_strdup(conf.afs_database_dir_arg);
                else {
                        char *home = para_homedir();
                        database_dir = make_message(
                                "%s/.paraslash/afs_database-0.4", home);
                        free(home);
                }
        }
        PARA_INFO_LOG("afs_database dir %s\n", database_dir);
}

static int make_database_dir(void)
{
        int ret;

        get_database_dir();
        ret = para_mkdir(database_dir, 0777);
        if (ret >= 0 || is_errno(-ret, EEXIST))
                return 1;
        return ret;
}

static int open_afs_tables(void)
{
        int i, ret;

        get_database_dir();
        PARA_NOTICE_LOG("opening %u osl tables in %s\n", NUM_AFS_TABLES,
                database_dir);
        for (i = 0; i < NUM_AFS_TABLES; i++) {
                ret = afs_tables[i].open(database_dir);
                if (ret >= 0)
                        continue;
                PARA_ERROR_LOG("%s init: %s\n", afs_tables[i].name,
                        para_strerror(-ret));
                break;
        }
        if (ret >= 0)
                return ret;
        while (i)
                afs_tables[--i].close();
        return ret;
}

static void signal_pre_select(struct sched *s, struct task *t)
{
        struct signal_task *st = container_of(t, struct signal_task, task);
        para_fd_set(st->fd, &s->rfds, &s->max_fileno);
}

static void afs_signal_post_select(struct sched *s, struct task *t)
{
        int signum;

        if (getppid() == 1) {
                PARA_EMERG_LOG("para_server died\n");
                goto shutdown;
        }
        signum = para_next_signal(&s->rfds);
        if (signum == 0)
                return;
        if (signum == SIGHUP) {
                close_afs_tables();
                parse_config_or_die(1);
                t->error = open_afs_tables();
                if (t->error < 0)
                        return;
                init_admissible_files(current_mop);
                return;
        }
        PARA_EMERG_LOG("terminating on signal %d\n", signum);
shutdown:
        sched_shutdown();
        t->error = -E_AFS_SIGNAL;
}

static void register_signal_task(void)
{
        struct signal_task *st = &signal_task_struct;

        para_sigaction(SIGPIPE, SIG_IGN);
        st->fd = para_signal_init();
        PARA_INFO_LOG("signal pipe: fd %d\n", st->fd);
        para_install_sighandler(SIGINT);
        para_install_sighandler(SIGTERM);
        para_install_sighandler(SIGHUP);

        st->task.pre_select = signal_pre_select;
        st->task.post_select = afs_signal_post_select;
        sprintf(st->task.status, "signal task");
        register_task(&st->task);
}

static struct list_head afs_client_list;

/** Describes on connected afs client. */
struct afs_client {
        /** Position in the afs client list. */
        struct list_head node;
        /** The socket file descriptor for this client. */
        int fd;
        /** The time the client connected. */
        struct timeval connect_time;
};

static void command_pre_select(struct sched *s, struct task *t)
{
        struct command_task *ct = container_of(t, struct command_task, task);
        struct afs_client *client;

        para_fd_set(server_socket, &s->rfds, &s->max_fileno);
        para_fd_set(ct->fd, &s->rfds, &s->max_fileno);
        list_for_each_entry(client, &afs_client_list, node)
                para_fd_set(client->fd, &s->rfds, &s->max_fileno);
}

/**
 * Send data as shared memory to a file descriptor.
 *
 * \param buf The buffer holding the data to be sent.
 * \param size The size of \a buf.
 * \param fd_ptr A pointer to the file descriptor.
 *
 * This function is used as the \a max_size handler in a \ref para_buffer
 * structure. If used this way, it is called by \ref para_printf() whenever
 * the buffer passed to para_printf() is about to exceed its maximal size.
 *
 * This function creates a shared memory area large enough to hold
 * the content given by \a buf and \a size and sends the identifier
 * of this area to the file descriptor given by \a fd_ptr.
 *
 * \return Zero if \a buf is \p NULL or \a size is zero. Negative on errors,
 * and positive on success.
 */
int pass_buffer_as_shm(char *buf, size_t size, void *fd_ptr)
{
        int ret, shmid, fd = *(int *)fd_ptr;
        void *shm;
        struct callback_result *cr;

        if (!buf || !size)
                return 0;
        ret = shm_new(size + sizeof(struct callback_result));
        if (ret < 0)
                return ret;
        shmid = ret;
        ret = shm_attach(shmid, ATTACH_RW, &shm);
        if (ret < 0)
                goto err;
        cr = shm;
        cr->result_size = size;
        memcpy(shm + sizeof(*cr), buf, size);
        ret = shm_detach(shm);
        if (ret < 0)
                goto err;
        ret = send_bin_buffer(fd, (char *)&shmid, sizeof(int));
        if (ret >= 0)
                return ret;
err:
        if (shm_destroy(shmid) < 0)
                PARA_ERROR_LOG("destroy result failed\n");
        return ret;
}

/*
 * On errors, negative value is written to fd.
 * On success: If query produced a result, the result_shmid is written to fd.
 * Otherwise, zero is written.
 */
static int call_callback(int fd, int query_shmid)
{
        void *query_shm;
        struct callback_query *cq;
        struct osl_object query;
        int ret;

        ret = shm_attach(query_shmid, ATTACH_RW, &query_shm);
        if (ret < 0)
                return ret;
        cq = query_shm;
        query.data = (char *)query_shm + sizeof(*cq);
        query.size = cq->query_size;
        cq->handler(fd, &query);
        return shm_detach(query_shm);
}

static int execute_server_command(fd_set *rfds)
{
        char buf[8];
        size_t n;
        int ret = read_nonblock(server_socket, buf, sizeof(buf) - 1, rfds, &n);

        if (ret < 0 || n == 0)
                return ret;
        buf[n] = '\0';
        if (strcmp(buf, "new"))
                return -E_BAD_CMD;
        return open_next_audio_file();
}

/* returns 0 if no data available, 1 else */
static int execute_afs_command(int fd, fd_set *rfds, uint32_t expected_cookie)
{
        uint32_t cookie;
        int query_shmid;
        char buf[sizeof(cookie) + sizeof(query_shmid)];
        size_t n;
        int ret = read_nonblock(fd, buf, sizeof(buf), rfds, &n);

        if (ret < 0)
                goto err;
        if (n == 0)
                return 0;
        if (n != sizeof(buf)) {
                PARA_NOTICE_LOG("short read (%d bytes, expected %lu)\n",
                        ret, (long unsigned) sizeof(buf));
                return 1;
        }
        cookie = *(uint32_t *)buf;
        if (cookie != expected_cookie) {
                PARA_NOTICE_LOG("received invalid cookie (got %u, expected %u)\n",
                        (unsigned)cookie, (unsigned)expected_cookie);
                return 1;
        }
        query_shmid = *(int *)(buf + sizeof(cookie));
        if (query_shmid < 0) {
                PARA_WARNING_LOG("received invalid query shmid %d)\n",
                        query_shmid);
                return 1;
        }
        ret = call_callback(fd, query_shmid);
        if (ret >= 0)
                return 1;
err:
        PARA_NOTICE_LOG("%s\n", para_strerror(-ret));
        return 1;
}

/** Shutdown connection if query has not arrived until this many seconds. */
#define AFS_CLIENT_TIMEOUT 3

static void command_post_select(struct sched *s, struct task *t)
{
        struct command_task *ct = container_of(t, struct command_task, task);
        struct sockaddr_un unix_addr;
        struct afs_client *client, *tmp;
        int fd, ret;

        ret = execute_server_command(&s->rfds);
        if (ret < 0) {
                PARA_EMERG_LOG("%s\n", para_strerror(-ret));
                sched_shutdown();
                return;
        }
        /* Check the list of connected clients. */
        list_for_each_entry_safe(client, tmp, &afs_client_list, node) {
                ret = execute_afs_command(client->fd, &s->rfds, ct->cookie);
                if (ret == 0) { /* prevent bogus connection flooding */
                        struct timeval diff;
                        tv_diff(now, &client->connect_time, &diff);
                        if (diff.tv_sec < AFS_CLIENT_TIMEOUT)
                                continue;
                        PARA_WARNING_LOG("connection timeout\n");
                }
                close(client->fd);
                list_del(&client->node);
                free(client);
        }
        /* Accept connections on the local socket. */
        ret = para_accept(ct->fd, &s->rfds, &unix_addr, sizeof(unix_addr), &fd);
        if (ret < 0)
                PARA_NOTICE_LOG("%s\n", para_strerror(-ret));
        if (ret <= 0)
                return;
        ret = mark_fd_nonblocking(fd);
        if (ret < 0) {
                PARA_NOTICE_LOG("%s\n", para_strerror(-ret));
                close(fd);
                return;
        }
        client = para_malloc(sizeof(*client));
        client->fd = fd;
        client->connect_time = *now;
        para_list_add(&client->node, &afs_client_list);
}

static void register_command_task(uint32_t cookie)
{
        struct command_task *ct = &command_task_struct;
        ct->fd = setup_command_socket_or_die();
        ct->cookie = cookie;

        ct->task.pre_select = command_pre_select;
        ct->task.post_select = command_post_select;
        sprintf(ct->task.status, "command task");
        register_task(&ct->task);
}

/**
 * Initialize the audio file selector process.
 *
 * \param cookie The value used for "authentication".
 * \param socket_fd File descriptor used for communication with the server.
 */
__noreturn void afs_init(uint32_t cookie, int socket_fd)
{
        static struct sched s;
        int i, ret;

        register_signal_task();
        INIT_LIST_HEAD(&afs_client_list);
        for (i = 0; i < NUM_AFS_TABLES; i++)
                afs_tables[i].init(&afs_tables[i]);
        ret = open_afs_tables();
        if (ret < 0)
                goto out;
        server_socket = socket_fd;
        ret = mark_fd_nonblocking(server_socket);
        if (ret < 0)
                goto out_close;
        PARA_INFO_LOG("server_socket: %d, afs_socket_cookie: %u\n",
                server_socket, (unsigned) cookie);
        init_admissible_files(conf.afs_initial_mode_arg);
        register_command_task(cookie);
        s.default_timeout.tv_sec = 0;
        s.default_timeout.tv_usec = 999 * 1000;
        ret = schedule(&s);
out_close:
        close_afs_tables();
out:
        if (ret < 0)
                PARA_EMERG_LOG("%s\n", para_strerror(-ret));
        exit(EXIT_FAILURE);
}

static void create_tables_callback(int fd, const struct osl_object *query)
{
        uint32_t table_mask = *(uint32_t *)query->data;
        int i, ret;
        struct para_buffer pb = {.buf = NULL};

        close_afs_tables();
        for (i = 0; i < NUM_AFS_TABLES; i++) {
                struct afs_table *t = &afs_tables[i];

                if (!(table_mask & (1 << i)))
                        continue;
                if (!t->create)
                        continue;
                ret = t->create(database_dir);
                if (ret < 0)
                        goto out;
                para_printf(&pb, "successfully created %s table\n", t->name);
        }
        ret = open_afs_tables();
out:
        if (ret < 0)
                para_printf(&pb, "%s\n", para_strerror(-ret));
        if (pb.buf)
                pass_buffer_as_shm(pb.buf, pb.offset, &fd);
        free(pb.buf);
}

int com_init(struct rc4_context *rc4c, int argc, char * const * const argv)
{
        int i, j, ret;
        uint32_t table_mask = (1 << (NUM_AFS_TABLES + 1)) - 1;
        struct osl_object query = {.data = &table_mask,
                .size = sizeof(table_mask)};

        ret = make_database_dir();
        if (ret < 0)
                return ret;
        if (argc != 1) {
                table_mask = 0;
                for (i = 1; i < argc; i++) {
                        for (j = 0; j < NUM_AFS_TABLES; j++) {
                                struct afs_table *t = &afs_tables[j];

                                if (strcmp(argv[i], t->name))
                                        continue;
                                table_mask |= (1 << j);
                                break;
                        }
                        if (j == NUM_AFS_TABLES)
                                return -E_BAD_TABLE_NAME;
                }
        }
        ret = send_callback_request(create_tables_callback, &query,
                rc4_send_result, rc4c);
        if (ret < 0)
                return rc4_send_va_buffer(rc4c, "%s\n", para_strerror(-ret));
        return ret;
}

/**
 * Flags for the check command.
 *
 * \sa com_check().
 */
enum com_check_flags {
        /** Check the audio file table. */
        CHECK_AFT = 1,
        /** Check the mood table. */
        CHECK_MOODS = 2,
        /** Check the playlist table. */
        CHECK_PLAYLISTS = 4
};

int com_check(struct rc4_context *rc4c, int argc, char * const * const argv)
{
        unsigned flags = 0;
        int i, ret;

        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];
                if (arg[0] != '-')
                        break;
                if (!strcmp(arg, "--")) {
                        i++;
                        break;
                }
                if (!strcmp(arg, "-a")) {
                        flags |= CHECK_AFT;
                        continue;
                }
                if (!strcmp(arg, "-p")) {
                        flags |= CHECK_PLAYLISTS;
                        continue;
                }
                if (!strcmp(arg, "-m")) {
                        flags |= CHECK_MOODS;
                        continue;
                }
                return -E_AFS_SYNTAX;
        }
        if (i < argc)
                return -E_AFS_SYNTAX;
        if (!flags)
                flags = ~0U;
        if (flags & CHECK_AFT) {
                ret = send_callback_request(aft_check_callback, NULL,
                        rc4_send_result, rc4c);
                if (ret < 0)
                        return ret;
        }
        if (flags & CHECK_PLAYLISTS) {
                ret = send_callback_request(playlist_check_callback,
                        NULL, rc4_send_result, rc4c);
                if (ret < 0)
                        return ret;
        }
        if (flags & CHECK_MOODS) {
                ret = send_callback_request(mood_check_callback, NULL,
                        rc4_send_result, rc4c);
                if (ret < 0)
                        return ret;
        }
        return 1;
}

/**
 * The afs event dispatcher.
 *
 * \param event Type of the event.
 * \param pb May be \p NULL.
 * \param data Type depends on \a event.
 *
 * This function calls the table handlers of all tables and passes \a pb and \a
 * data verbatim. It's up to the handlers to interpret the \a data pointer.
 */
void afs_event(enum afs_events event, struct para_buffer *pb,
                void *data)
{
        int i, ret;

        for (i = 0; i < NUM_AFS_TABLES; i++) {
                struct afs_table *t = &afs_tables[i];
                if (!t->event_handler)
                        continue;
                ret = t->event_handler(event, pb, data);
                if (ret < 0)
                        PARA_CRIT_LOG("table %s, event %d: %s\n", t->name,
                                event, para_strerror(-ret));
        }
}

/**
 * Dummy event handler for the images table.
 *
 * \param event Unused.
 * \param pb Unused.
 * \param data Unused.
 *
 * This table does not honor events.
 */
int images_event_handler(__a_unused enum afs_events event,
        __a_unused  struct para_buffer *pb, __a_unused void *data)
{
        return 1;
}

/**
 * Dummy event handler for the lyrics table.
 *
 * \param event Unused.
 * \param pb Unused.
 * \param data Unused.
 *
 * This table does not honor events.
 */
int lyrics_event_handler(__a_unused enum afs_events event,
        __a_unused struct para_buffer *pb, __a_unused void *data)
{
        return 1;
}