Bug 1013: Don't assume errno is between 0 and 100000

[elinks.git] / src / protocol / bittorrent / peerwire.c

/* BitTorrent peer-wire protocol implementation */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <errno.h>
#include <sys/types.h>
#ifdef HAVE_SYS_SOCKET_H
#include <sys/socket.h> /* OS/2 needs this after sys/types.h */
#endif
#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#ifdef HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif

#include "elinks.h"

#include "config/options.h"
#include "main/select.h"
#include "main/timer.h"
#include "network/connection.h"
#include "network/socket.h"
#include "osdep/osdep.h"
#include "protocol/bittorrent/bittorrent.h"
#include "protocol/bittorrent/common.h"
#include "protocol/bittorrent/connection.h"
#include "protocol/bittorrent/peerconnect.h"
#include "protocol/bittorrent/peerwire.h"
#include "protocol/bittorrent/piececache.h"
#include "protocol/protocol.h"
#include "protocol/uri.h"
#include "util/bitfield.h"
#include "util/memory.h"
#include "util/string.h"


/* I give to you my sweet surrender. */

enum bittorrent_handshake_state {
        BITTORRENT_PEER_HANDSHAKE_OK,           /* Completely read and valid. */
        BITTORRENT_PEER_HANDSHAKE_ERROR,        /* The handshake was invalid. */
        BITTORRENT_PEER_HANDSHAKE_INFO_HASH,    /* All up to the hash was ok. */
        BITTORRENT_PEER_HANDSHAKE_INCOMPLETE,   /* All up to now was correct. */
};

/* The size of the handshake for version 1.0 is:
 * <1:protocolstrlen> <19:protocolstr> <8:reserved> <20:info-hash> <20:peer-id>
 * In total 68 bytes.*/
#define BITTORRENT_PEER_HANDSHAKE_SIZE  (1 + 19 + 8 + 20 + 20)

/* Storing the version identification part of the handshake as one entity
 * (length prefix and string) makes it much easier to verify and write. */
static const bittorrent_id_T BITTORRENT_ID = "\023BitTorrent protocol";

/* Has the last message written to the peer socket been sent or not? */
#define bittorrent_peer_is_sending(peer) ((peer)->socket->write_buffer)


static enum bittorrent_state
do_send_bittorrent_peer_message(struct bittorrent_peer_connection *peer,
                                struct bittorrent_peer_request *message);

static enum bittorrent_handshake_state
do_read_bittorrent_peer_handshake(struct socket *socket, struct read_buffer *buffer);


/* ************************************************************************** */
/* Peer state managing: */
/* ************************************************************************** */

/* Queue requests up to the configured limit. */
static void
queue_bittorrent_peer_connection_requests(struct bittorrent_peer_connection *peer)
{
        int size = get_opt_int("protocol.bittorrent.request_queue_size");
        int queue_size = list_size(&peer->local.requests);

        for ( ; queue_size < size; queue_size++) {
                struct bittorrent_peer_request *request;

                request = find_bittorrent_peer_request(peer);
                if (!request) break;

                /* TODO: Insert rarest first? --jonas */
                add_to_list_end(peer->local.requests, request);
        }

        /* Interest in the peer was lost if no request could be queued. */
        if (list_empty(peer->local.requests))
                set_bittorrent_peer_not_interested(peer);
}

/* Called both from the choke period handler of the main BitTorrent connection
 * and upon completion of message sending this handles allocation of peer
 * requests and sending of anything pending. */
/* XXX: Calling this function can cause the peer struct to disappear. */
void
update_bittorrent_peer_connection_state(struct bittorrent_peer_connection *peer)
{
        struct bittorrent_connection *bittorrent = peer->bittorrent;
        struct bittorrent_piece_cache *cache = bittorrent->cache;
        struct bittorrent_peer_request *request, *next_request;
        enum bittorrent_state state;

        /* Drop connections to other seeders or when a partial download has been
         * completed. */
        if ((peer->remote.seeder && bittorrent->mode == BITTORRENT_MODE_SEEDER)
            || (cache->partial && cache->partial_pieces == cache->completed_pieces)) {
                done_bittorrent_peer_connection(peer);
                return;
        }

        if (peer->local.interested && !peer->local.choked)
                queue_bittorrent_peer_connection_requests(peer);

        /* Is there a write in progress? */
        if (bittorrent_peer_is_sending(peer)) {
                assert(get_handler(peer->socket->fd, SELECT_HANDLER_WRITE));
                return;
        }

        /* Send the shorter state messages first ... */

        /* Send any peer state oriented messages. */
        foreachsafe (request, next_request, peer->queue) {
                state = do_send_bittorrent_peer_message(peer, request);
                if (state == BITTORRENT_STATE_OK)
                        return;
        }

        /* Send local piece requests which has not already been requested. */
        foreachsafe (request, next_request, peer->local.requests) {
                if (request->requested)
                        continue;

                request->id = BITTORRENT_MESSAGE_REQUEST;

                state = do_send_bittorrent_peer_message(peer, request);
                if (state == BITTORRENT_STATE_OK)
                        return;
        }

        /* Ship the longer piece data messages. */
        foreachsafe (request, next_request, peer->remote.requests) {
                request->id = BITTORRENT_MESSAGE_PIECE;

                state = do_send_bittorrent_peer_message(peer, request);
                if (state == BITTORRENT_STATE_OK)
                        return;
        }
}

static inline double
get_bittorrent_rate(struct bittorrent_peer_stats *stats, time_t now,
                    double rate, uint32_t loaded)
{
        return (rate * (stats->last_time - stats->age) + loaded)
                / (now - stats->age);
}

void
update_bittorrent_peer_connection_stats(struct bittorrent_peer_connection *peer,
                                        uint32_t downloaded, uint32_t have_piece,
                                        uint32_t uploaded)
{
        struct bittorrent_peer_stats *stats = &peer->stats;
        time_t now = time(NULL);

        stats->download_rate = get_bittorrent_rate(stats, now, stats->download_rate, downloaded);
        stats->have_rate     = get_bittorrent_rate(stats, now, stats->have_rate, have_piece);

        stats->downloaded   += downloaded;
        stats->uploaded     += uploaded;
        stats->last_time     = now;

        /* Push the age along, so it will be no older than the requested number
         * of seconds. */
        if (stats->age < now - 20)
                stats->age = stats->age ? now - 20 : now - 1;
}


/* ************************************************************************** */
/* Peer message handling: */
/* ************************************************************************** */

/* The layout of the length prefixed peer messages.
 *
 * - All indexes and offsets are encoded as 4-bytes big-endian.
 * - Indexes are zero-based.
 * - Variable length fields depends on the message length.
 * - All messages begin with <4:message-length> <1:message-id>
 *   The only exception is the keep-alive message which has length set to zero
 *   and doesn't carry any message ID or payload.
 *
 * Message without payload:
 * ------------------------
 *
 * - choke
 * - unchoke
 * - interested
 * - not-interested
 *
 * Messages with payload:
 * ----------------------
 *
 * - have:               <4:piece-index>
 * - bitfield:           <x:bitfield-data>
 * - request and cancel: <4:piece-index> <4:piece-offset> <4:block-length>
 * - piece:              <4:piece-index> <4:piece-offset> <x:block-data>
 */


/* ************************************************************************** */
/* Peer message sending: */
/* ************************************************************************** */

/* Meesage write completion callback. */
static void
sent_bittorrent_peer_message(struct socket *socket)
{
        assert(!socket->write_buffer);
        /* Check if there are pending messages or requests. */
        update_bittorrent_peer_connection_state(socket->conn);
}

static inline void
add_bittorrent_peer_integer(struct string *string, uint32_t integer)
{
        uint32_t data = htonl(integer);

        add_bytes_to_string(string, (unsigned char *) &data, sizeof(data));
}

/* Common lowlevel backend for composing a peer message and writing it to the
 * socket. */
static enum bittorrent_state
do_send_bittorrent_peer_message(struct bittorrent_peer_connection *peer,
                                struct bittorrent_peer_request *message)
{
        struct bittorrent_connection *bittorrent = peer->bittorrent;
        struct string string;
        unsigned char msgid_str[1] = { (unsigned char) message->id };
        uint32_t msglen = 0;

        assert(!bittorrent_peer_is_sending(peer));
        assert(!get_handler(peer->socket->fd, SELECT_HANDLER_WRITE));

        if (!init_string(&string))
                return BITTORRENT_STATE_OUT_OF_MEM;

        /* Reserve 4 bytes to the message length and add the message ID byte. */
        add_bytes_to_string(&string, (unsigned char *) &msglen, sizeof(msglen));

        /* XXX: Can't use add_char_to_string() here because the message ID
         * can be zero. */
        if (message->id != BITTORRENT_MESSAGE_KEEP_ALIVE)
                add_bytes_to_string(&string, msgid_str, 1);

        switch (message->id) {
        case BITTORRENT_MESSAGE_HAVE:
        {
                add_bittorrent_peer_integer(&string, message->piece);
                assert(string.length == 9);
                break;
        }
        case BITTORRENT_MESSAGE_BITFIELD:
        {
                struct bitfield *bitfield = bittorrent->cache->bitfield;
                size_t bytes = get_bitfield_byte_size(bitfield->bitsize);

                /* Are bitfield messages allowed at this point? */
                assert(!peer->local.bitfield);

                add_bytes_to_string(&string, bitfield->bits, bytes);

                assert(string.length == 5 + bytes);
                break;
        }
        case BITTORRENT_MESSAGE_REQUEST:
        case BITTORRENT_MESSAGE_CANCEL:
        {
                assert(!peer->local.choked);

                add_bittorrent_peer_integer(&string, message->piece);
                add_bittorrent_peer_integer(&string, message->offset);
                add_bittorrent_peer_integer(&string, message->length);

                message->requested = 1;

                assert(string.length == 17);
                break;
        }
        case BITTORRENT_MESSAGE_PIECE:
        {
                unsigned char *data;

                assert(!peer->remote.choked);
                assert(test_bitfield_bit(bittorrent->cache->bitfield, message->piece));

                data = get_bittorrent_piece_cache_data(bittorrent, message->piece);
                if (!data) {
                        done_string(&string);
                        return BITTORRENT_STATE_CACHE_FAILURE;
                }

                data += message->offset;

                add_bittorrent_peer_integer(&string, message->piece);
                add_bittorrent_peer_integer(&string, message->offset);
                add_bytes_to_string(&string, data, message->length);

                update_bittorrent_peer_connection_stats(peer, 0, 0,
                                                        message->length);
                update_bittorrent_connection_stats(peer->bittorrent,
                                                   0, message->length, 0);
                assert(string.length == 13 + message->length);
                break;
        }
        case BITTORRENT_MESSAGE_KEEP_ALIVE:
                assert(string.length == 4);
                break;

        case BITTORRENT_MESSAGE_CHOKE:
        case BITTORRENT_MESSAGE_UNCHOKE:
        case BITTORRENT_MESSAGE_INTERESTED:
        case BITTORRENT_MESSAGE_NOT_INTERESTED:
                assert(string.length == 5);
                break;

        default:
                INTERNAL("Bad message ID");
        }

        /* Insert the real message length. */
        msglen = string.length - sizeof(uint32_t);
        msglen = htonl(msglen);
        memcpy(string.source, (unsigned char *) &msglen, sizeof(msglen));

        /* Any message will cause bitfield messages to become invalid. */
        peer->local.bitfield = 1;

        if (message->id != BITTORRENT_MESSAGE_REQUEST) {
                del_from_list(message);
                mem_free(message);
        }

        write_to_socket(peer->socket, string.source, string.length,
                        connection_state(S_TRANS),
                        sent_bittorrent_peer_message);

        done_string(&string);

        return BITTORRENT_STATE_OK;
}

/* Highlevel backend for sending peer messages. It handles queuing of messages.
 * In order to make this function safe to call from any contexts, messages are
 * NEVER directly written to the peer socket, since that could cause the peer
 * connection struct to disappear from under us. */
void
send_bittorrent_peer_message(struct bittorrent_peer_connection *peer,
                             enum bittorrent_message_id message_id, ...)
{
        struct bittorrent_peer_request message_store, *message = &message_store;
        va_list args;

        memset(message, 0, sizeof(*message));
        message->id = message_id;

        va_start(args, message_id);

        switch (message_id) {
        case BITTORRENT_MESSAGE_CANCEL:
                message->piece  = va_arg(args, uint32_t);
                message->offset = va_arg(args, uint32_t);
                message->length = va_arg(args, uint32_t);
                break;

        case BITTORRENT_MESSAGE_HAVE:
                message->piece = va_arg(args, uint32_t);
                break;

        case BITTORRENT_MESSAGE_BITFIELD:
        case BITTORRENT_MESSAGE_CHOKE:
        case BITTORRENT_MESSAGE_INTERESTED:
        case BITTORRENT_MESSAGE_KEEP_ALIVE:
        case BITTORRENT_MESSAGE_NOT_INTERESTED:
        case BITTORRENT_MESSAGE_UNCHOKE:
                break;

        case BITTORRENT_MESSAGE_REQUEST:
        case BITTORRENT_MESSAGE_PIECE:
                /* Piece and piece request messages are generated automaticalle
                 * from the request queue the local and remote peer status. */
        default:
                INTERNAL("Bad message ID");
        }

        va_end(args);

        message = mem_alloc(sizeof(*message));
        if (!message) return;

        memcpy(message, &message_store, sizeof(*message));

        /* Prioritize bitfield cancel messages by putting them in the start of
         * the queue so that bandwidth is not wasted. This way bitfield messages
         * will always be sent before anything else and cancel messages will
         * always arrive before have messages, which our client prefers. */
        if (message->id == BITTORRENT_MESSAGE_BITFIELD
            || message->id == BITTORRENT_MESSAGE_CANCEL)
                add_to_list(peer->queue, message);
        else
                add_to_list_end(peer->queue, message);
}


/* ************************************************************************** */
/* Peer message receiving: */
/* ************************************************************************** */

static inline uint32_t
get_bittorrent_peer_integer(struct read_buffer *buffer, int offset)
{
        assert(offset + sizeof(uint32_t) <= buffer->length);
        return ntohl(*((uint32_t *) (buffer->data + offset)));
}

static enum bittorrent_message_id
check_bittorrent_peer_message(struct bittorrent_peer_connection *peer,
                              struct read_buffer *buffer, uint32_t *length)
{
        uint32_t message_length;
        enum bittorrent_message_id message_id;

        *length = 0;

        assert(peer->remote.handshake);

        if (buffer->length < sizeof(message_length))
                return BITTORRENT_MESSAGE_INCOMPLETE;

        message_length = get_bittorrent_peer_integer(buffer, 0);

        if (message_length > get_bittorrent_peerwire_max_message_length())
                return BITTORRENT_MESSAGE_ERROR;

        if (buffer->length - sizeof(message_length) < message_length)
                return BITTORRENT_MESSAGE_INCOMPLETE;

        if (message_length == 0)
                return BITTORRENT_MESSAGE_KEEP_ALIVE;

        message_id = buffer->data[sizeof(message_length)];

        *length = message_length;

        return message_id;
}

static enum bittorrent_state
read_bittorrent_peer_message(struct bittorrent_peer_connection *peer,
                             enum bittorrent_message_id message_id,
                             struct read_buffer *buffer, uint32_t message_length,
                             int *write_errno)
{
        struct bittorrent_connection *bittorrent = peer->bittorrent;
        enum bittorrent_state state;
        uint32_t piece, offset, length;
        unsigned char *data;

        assert(message_id != BITTORRENT_MESSAGE_INCOMPLETE);

        *write_errno = 0;

        switch (message_id) {
        case BITTORRENT_MESSAGE_CHOKE:
                /* Return all pending requests to the free list. */
                peer->local.choked = 1;
                add_requests_to_bittorrent_piece_cache(peer, &peer->local);
                {
                        struct bittorrent_peer_request *message, *next;

                        foreachsafe (message, next, peer->queue) {
                                if (message->id == BITTORRENT_MESSAGE_CANCEL) {
                                        del_from_list(message);
                                        mem_free(message);
                                }
                        }
                }
                break;

        case BITTORRENT_MESSAGE_UNCHOKE:
                peer->local.choked = 0;
                break;

        case BITTORRENT_MESSAGE_INTERESTED:
                peer->remote.interested = 1;
                break;

        case BITTORRENT_MESSAGE_NOT_INTERESTED:
                peer->remote.interested = 0;
                break;

        case BITTORRENT_MESSAGE_HAVE:
                if (message_length < sizeof(uint32_t))
                        return BITTORRENT_STATE_ERROR;

                piece = get_bittorrent_peer_integer(buffer, 5);
                /* Is piece out of bound? */
                if (piece >= bittorrent->meta.pieces)
                        break;

                /* Is the piece already recorded? */
                if (test_bitfield_bit(peer->bitfield, piece))
                        break;

                length = get_bittorrent_piece_length(&bittorrent->meta, piece);
                update_bittorrent_peer_connection_stats(peer, 0, length, 0);

                set_bitfield_bit(peer->bitfield, piece);
                peer->remote.seeder = bitfield_is_set(peer->bitfield);

                update_bittorrent_piece_cache(peer, piece);

                if (peer->local.interested
                    || test_bitfield_bit(bittorrent->cache->bitfield, piece))
                        break;

                set_bittorrent_peer_interested(peer);
                break;

        case BITTORRENT_MESSAGE_BITFIELD:
                data   = buffer->data + 5;
                length = message_length - 1; /* Message ID byte. */

                if (length > get_bitfield_byte_size(peer->bitfield->bitsize))
                        break;

                /* Are bitfield messages allowed at this point? */
                if (peer->remote.bitfield)
                        break;

                /* XXX: The function tail will set the bitfield flag ... */
                copy_bitfield(peer->bitfield, data, length);
                peer->remote.seeder = bitfield_is_set(peer->bitfield);

                update_bittorrent_piece_cache_from_bitfield(peer);

                /* Force checking of the interested flag. */
                foreach_bitfield_set (piece, peer->bitfield) {
                        if (test_bitfield_bit(bittorrent->cache->bitfield, piece))
                                continue;

                        set_bittorrent_peer_interested(peer);
                        break;
                }
                break;

        case BITTORRENT_MESSAGE_REQUEST:
        case BITTORRENT_MESSAGE_CANCEL:
                if (message_length < sizeof(uint32_t) * 3)
                        return BITTORRENT_STATE_ERROR;

                piece  = get_bittorrent_peer_integer(buffer, 5);
                offset = get_bittorrent_peer_integer(buffer, 9);
                length = get_bittorrent_peer_integer(buffer, 13);

                /* FIXME: Should requests be allowed to overlap pieces? */
                if (peer->remote.choked
                    || piece >= bittorrent->meta.pieces
                    || offset + length > get_bittorrent_piece_length(&bittorrent->meta, piece)
                    || !test_bitfield_bit(bittorrent->cache->bitfield, piece))
                        break;

                if (length > get_bittorrent_peerwire_max_request_length())
                        return BITTORRENT_STATE_ERROR;

                if (message_id == BITTORRENT_MESSAGE_REQUEST) {
                        add_bittorrent_peer_request(&peer->remote, piece, offset, length);
                } else {
                        del_bittorrent_peer_request(&peer->remote, piece, offset, length);
                }
                break;

        case BITTORRENT_MESSAGE_PIECE:
                if (message_length < sizeof(uint32_t) * 2)
                        return BITTORRENT_STATE_ERROR;

                piece  = get_bittorrent_peer_integer(buffer, 5);
                offset = get_bittorrent_peer_integer(buffer, 9);
                length = message_length - 9; /* Msg ID byte + 2 ints */
                data   = buffer->data + 13; /* Offset includes msg len */

                if (peer->local.choked
                    || piece >= bittorrent->meta.pieces
                    || offset + length > get_bittorrent_piece_length(&bittorrent->meta, piece)
                    || length == 0)
                        break;

                update_bittorrent_peer_connection_stats(peer, length, 0, 0);
                state = add_to_bittorrent_piece_cache(peer, piece, offset, data, length, write_errno);
                if (state != BITTORRENT_STATE_OK)
                        return state;
                break;

        case BITTORRENT_MESSAGE_KEEP_ALIVE:
        default:
                /* Keep-alive messages doesn't require any special handling.
                 * Unknown peer messages are simply dropped. */
                break;
        }

        /* Any message will cause bitfield messages to become invalid. */
        peer->remote.bitfield = 1;

        return BITTORRENT_STATE_OK;
}

static void
read_bittorrent_peer_data(struct socket *socket, struct read_buffer *buffer)
{
        struct bittorrent_peer_connection *peer = socket->conn;

        if (!peer->remote.handshake) {
                enum bittorrent_handshake_state state;

                /* We still needs to read the peer ID from the handshake. */
                state = do_read_bittorrent_peer_handshake(socket, buffer);
                if (state != BITTORRENT_PEER_HANDSHAKE_OK)
                        return;

                /* If the handshake was ok'ed see if there are any messages to
                 * read. */
                assert(peer->remote.handshake);
        }

        /* All messages atleast contains an integer prefix. */
        while (buffer->length > sizeof(uint32_t)) {
                enum bittorrent_message_id message_id;
                uint32_t length;
                int write_errno = 0;

                message_id = check_bittorrent_peer_message(peer, buffer, &length);
                if (message_id == BITTORRENT_MESSAGE_INCOMPLETE)
                        break;

                if (message_id == BITTORRENT_MESSAGE_ERROR) {
                        done_bittorrent_peer_connection(peer);
                        return;
                }

                switch (read_bittorrent_peer_message(peer, message_id, buffer, length, &write_errno)) {
                case BITTORRENT_STATE_OK:
                        break;

                case BITTORRENT_STATE_OUT_OF_MEM:
                        abort_connection(peer->bittorrent->conn,
                                         connection_state(S_OUT_OF_MEM));
                        return;

                case BITTORRENT_STATE_ERROR:
                default:
                        if (!write_errno) {
                                done_bittorrent_peer_connection(peer);
                                return;
                        }

                        /* Shutdown on fatal errors! */
                        abort_connection(peer->bittorrent->conn,
                                         connection_state_for_errno(write_errno));
                        return;
                }

                /* Remove the processed data from the input buffer. */
                kill_buffer_data(buffer, length + sizeof(length));
                update_bittorrent_connection_stats(peer->bittorrent, 0, 0,
                                                   length + sizeof(length));
        }

        update_bittorrent_peer_connection_state(peer);
}


/* ************************************************************************** */
/* Peer handshake exchanging: */
/* ************************************************************************** */

/* XXX: Note, handshake messages are also handled above in the function
 * read_bittorrnt_peer_data() if it is incomplete when reading it below.
 * Often, peers will only send up to and including the info hash, so that the
 * peer ID arrives later. */

/* The handshake was sent, so notify the remote peer about completed piece in a
 * bitfield message and start reading any remaining bits of the handshake or any
 * other message.  */
static void
sent_bittorrent_peer_handshake(struct socket *socket)
{
        struct bittorrent_peer_connection *peer = socket->conn;
        struct read_buffer *buffer = peer->socket->read_buffer;

        assert(buffer);

        /* Only send the bitfield message if there is anything interesting to
         * report. */
        if (peer->bittorrent->cache->completed_pieces) {
                assert(list_empty(peer->queue));
                send_bittorrent_peer_message(peer, BITTORRENT_MESSAGE_BITFIELD);
        }

        read_from_socket(peer->socket, buffer, connection_state(S_TRANS),
                         read_bittorrent_peer_data);
}

/* This function is called when a handhake has been read from an incoming
 * connection and is used as a callback for when a new peer connection has
 * successfully been established. */
void
send_bittorrent_peer_handshake(struct socket *socket)
{
        struct bittorrent_peer_connection *peer = socket->conn;
        struct bittorrent_connection *bittorrent = peer->bittorrent;
        struct bittorrent_meta *meta = &bittorrent->meta;
        unsigned char reserved[8] = { 0, 0, 0, 0, 0, 0, 0, 0 };
        unsigned char handshake[BITTORRENT_PEER_HANDSHAKE_SIZE];
        int i = 0;

#define add_to_handshake(handshake, i, data) \
        do { \
                memcpy((handshake) + (i), data, sizeof(data)); \
                i += sizeof(data); \
        } while (0)

        add_to_handshake(handshake, i, BITTORRENT_ID);
        add_to_handshake(handshake, i, reserved);
        add_to_handshake(handshake, i, meta->info_hash);
        add_to_handshake(handshake, i, bittorrent->peer_id);

#undef  add_to_handshake

        assert(handshake[0] == 19);

        if (!peer->socket->read_buffer) {
                /* Just return. Failure is handled by alloc_read_buffer(). */
                peer->socket->read_buffer = alloc_read_buffer(peer->socket);
                if (!peer->socket->read_buffer) {
                        done_bittorrent_peer_connection(peer);
                        return;
                }
        }

        peer->local.handshake = 1;

        /* Temporarily stop processing of all incoming messages while we send
         * the handshake so that reading of especially the bitfield message
         * won't cause any interested message to be queued _before_ we have a
         * chance to send the bitfield message which MUST be first. */
        clear_handlers(peer->socket->fd);

        /* Can't use request_.. version because it will create a new read buffer
         * and we might want to hold on to the old buffer if the peer ID of the
         * handshake was not read. */
        write_to_socket(peer->socket, handshake, sizeof(handshake),
                        connection_state(S_TRANS),
                        sent_bittorrent_peer_handshake);
}

#if 0
/* DHT is not supported, so commented out. */
/* Checks for the DHT flags used by atleast Brams client to indicate it supports
 * trackerless BitTorrent. */
static inline int
bittorrent_peer_supports_dht(unsigned char flags[8])
{
        return !!(flags[7] & 1);
}
#endif

/* This function is called each time there is something from the handshake
 * message to read. */
static enum bittorrent_handshake_state
check_bittorrent_peer_handshake(struct bittorrent_peer_connection *peer,
                                struct read_buffer *buffer)
{
        bittorrent_id_T info_hash;
        struct bittorrent_peer *peer_info;
        enum bittorrent_handshake_state state;

        if (buffer->length < 20)
                return BITTORRENT_PEER_HANDSHAKE_INCOMPLETE;

        if (memcmp(buffer->data, BITTORRENT_ID, sizeof(BITTORRENT_ID)))
                return BITTORRENT_PEER_HANDSHAKE_ERROR;

        if (buffer->length < 28)
                return BITTORRENT_PEER_HANDSHAKE_INCOMPLETE;

#if 0
        /* DHT is not supported, so commented out. */
        /* Check the reserved flags */
        peer->remote.dht = bittorrent_peer_supports_dht(&buffer->data[20]);
#endif

        if (buffer->length < 48)
                return BITTORRENT_PEER_HANDSHAKE_INCOMPLETE;

        memcpy(info_hash, &buffer->data[28], sizeof(info_hash));

        if (peer->local.initiater) {
                struct bittorrent_meta *meta = &peer->bittorrent->meta;

                assert(peer->bittorrent);

                /* Check if the info_hash matches the one in the associated
                 * bittorrent connection. */
                if (memcmp(meta->info_hash, info_hash, sizeof(info_hash)))
                        return BITTORRENT_PEER_HANDSHAKE_ERROR;

                if (buffer->length < BITTORRENT_PEER_HANDSHAKE_SIZE)
                        return BITTORRENT_PEER_HANDSHAKE_INFO_HASH;

                /* If we got the peer info using the compact tracker flag there
                 * is no peer ID, so set it. Else check if the peer has sent the
                 * expected ID. */
                if (bittorrent_id_is_empty(peer->id))
                        memcpy(peer->id, &buffer->data[48], sizeof(peer->id));

                else if (memcmp(peer->id, &buffer->data[48], sizeof(peer->id)))
                                return BITTORRENT_PEER_HANDSHAKE_ERROR;

        } else {
                struct bittorrent_connection *bittorrent = peer->bittorrent;

                /* If the peer ID is empty we didn't establish the connection. */
                assert(bittorrent_id_is_empty(peer->id));

                /* Look-up the bittorrent connection and drop peer connections
                 * to unknown torrents. */
                if (!bittorrent) {
                        bittorrent = find_bittorrent_connection(info_hash);
                        if (!bittorrent)
                                return BITTORRENT_PEER_HANDSHAKE_ERROR;

                        peer->bittorrent = bittorrent;

                        /* Don't know if this is the right place to do this
                         * initialization, but it is the first time we know how
                         * many bits there should be room for. Also, it feels
                         * safer to have it created before the peer connection
                         * is moved to a bittorrent connection so all other code
                         * can assume it is always there. */
                        peer->bitfield = init_bitfield(bittorrent->meta.pieces);
                        if (!peer->bitfield)
                                return BITTORRENT_PEER_HANDSHAKE_ERROR;
                }

                /* FIXME: It would be possible to already add the peer to the
                 * neighbor list here. Should we? --jonas */

                if (buffer->length < BITTORRENT_PEER_HANDSHAKE_SIZE)
                        return BITTORRENT_PEER_HANDSHAKE_INFO_HASH;

                memcpy(peer->id, &buffer->data[48], sizeof(peer->id));
        }

        assert(peer->bittorrent);

        /* Remove any recorded peer from the peer info list. */
        /* XXX: This has to be done before checking if the peer is known. Since
         * known in this case means whether the peer already has a connection
         * associated. */
        peer_info = get_peer_from_bittorrent_pool(peer->bittorrent, peer->id);
        if (peer_info) {
                del_from_list(peer_info);
                mem_free(peer_info);
        }

        /* Even if the peer is already associated with a connection we still
         * needs to check if the ID is known since we might have just gotten it.
         * Removing it first makes that possible. */
        del_from_list(peer);

        /* Check if the peer is already connected to us. */
        state = bittorrent_id_is_known(peer->bittorrent, peer->id)
              ? BITTORRENT_PEER_HANDSHAKE_ERROR : BITTORRENT_PEER_HANDSHAKE_OK;

        /* For unassociated connections; move the peer connection from the list
         * of unknown pending peer connections to a bittorrent connection's list
         * of active peers. */
        add_to_list(peer->bittorrent->peers, peer);

        return state;
}

/* Common backend for reading handshakes. */
static enum bittorrent_handshake_state
do_read_bittorrent_peer_handshake(struct socket *socket, struct read_buffer *buffer)
{
        struct bittorrent_peer_connection *peer = socket->conn;
        enum bittorrent_handshake_state state;

        state = check_bittorrent_peer_handshake(peer, buffer);

        switch (state) {
        case BITTORRENT_PEER_HANDSHAKE_OK:
                /* The whole handshake was successfully read. */
                peer->remote.handshake = 1;
                kill_buffer_data(buffer, BITTORRENT_PEER_HANDSHAKE_SIZE);
                update_bittorrent_connection_stats(peer->bittorrent, 0, 0,
                                                   BITTORRENT_PEER_HANDSHAKE_SIZE);

                switch (get_bittorrent_blacklist_flags(peer->id)) {
                case BITTORRENT_BLACKLIST_PEER_POOL:
                case BITTORRENT_BLACKLIST_NONE:
                        break;

                case BITTORRENT_BLACKLIST_MALICIOUS:
                case BITTORRENT_BLACKLIST_BEHAVIOUR:
                default:
                        done_bittorrent_peer_connection(peer);
                        return BITTORRENT_PEER_HANDSHAKE_ERROR;
                }

                if (!peer->local.handshake)
                        send_bittorrent_peer_handshake(socket);
                break;

        case BITTORRENT_PEER_HANDSHAKE_INFO_HASH:
                if (!peer->local.handshake) {
                        send_bittorrent_peer_handshake(socket);
                        /* XXX: The peer connection might have disappear from
                         * under us at this point so do not reregister the
                         * socket for reading. */
                        break;
                }

                read_from_socket(peer->socket, buffer,
                                 connection_state(S_TRANS),
                                 read_bittorrent_peer_handshake);
                break;

        case BITTORRENT_PEER_HANDSHAKE_ERROR:
                done_bittorrent_peer_connection(peer);
                break;

        case BITTORRENT_PEER_HANDSHAKE_INCOMPLETE:
                /* The whole handshake was not read so wait for more. */
                read_from_socket(peer->socket, buffer,
                                 connection_state(S_TRANS),
                                 read_bittorrent_peer_handshake);
                break;
        }

        return state;
}

void
read_bittorrent_peer_handshake(struct socket *socket, struct read_buffer *buffer)
{
        do_read_bittorrent_peer_handshake(socket, buffer);
}