xK/src/kike.c

/*
 * kike.c: the experimental IRC daemon
 *
 * Copyright (c) 2014, Přemysl Janouch <p.janouch@gmail.com>
 * All rights reserved.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 */

#define PROGRAM_NAME "kike"
#define PROGRAM_VERSION "alpha"

#include "common.c"
#include <nl_types.h>

// --- Configuration (application-specific) ------------------------------------

static struct config_item g_config_table[] =
{
	{ "server_name",     NULL,              "Server name"                    },
	{ "motd",            NULL,              "MOTD filename"                  },
	{ "catalog",         NULL,              "catgets localization catalog"   },

	{ "bind_host",       NULL,              "Address of the IRC server"      },
	{ "bind_port",       "6667",            "Port of the IRC server"         },
	{ "ssl_cert",        NULL,              "Server SSL certificate (PEM)"   },
	{ "ssl_key",         NULL,              "Server SSL private key (PEM)"   },

	{ "max_connections", NULL,              "Maximum client connections"     },
	{ NULL,              NULL,              NULL                             }
};

// --- Signals -----------------------------------------------------------------

static int g_signal_pipe[2];            ///< A pipe used to signal... signals

/// Program termination has been requested by a signal
static volatile sig_atomic_t g_termination_requested;

static void
sigterm_handler (int signum)
{
	(void) signum;

	g_termination_requested = true;

	int original_errno = errno;
	if (write (g_signal_pipe[1], "t", 1) == -1)
		soft_assert (errno == EAGAIN);
	errno = original_errno;
}

static void
setup_signal_handlers (void)
{
	if (pipe (g_signal_pipe) == -1)
		exit_fatal ("%s: %s", "pipe", strerror (errno));

	set_cloexec (g_signal_pipe[0]);
	set_cloexec (g_signal_pipe[1]);

	// So that the pipe cannot overflow; it would make write() block within
	// the signal handler, which is something we really don't want to happen.
	// The same holds true for read().
	set_blocking (g_signal_pipe[0], false);
	set_blocking (g_signal_pipe[1], false);

	signal (SIGPIPE, SIG_IGN);

	struct sigaction sa;
	sa.sa_flags = SA_RESTART;
	sigemptyset (&sa.sa_mask);
	sa.sa_handler = sigterm_handler;
	if (sigaction (SIGINT, &sa, NULL) == -1
	 || sigaction (SIGTERM, &sa, NULL) == -1)
		exit_fatal ("%s: %s", "sigaction", strerror (errno));
}

// --- IRC token validation ----------------------------------------------------

// Use the enum only if applicable and a simple boolean isn't sufficient.

enum validation_result
{
	VALIDATION_OK,
	VALIDATION_ERROR_EMPTY,
	VALIDATION_ERROR_TOO_LONG,
	VALIDATION_ERROR_INVALID
};

// Everything as per RFC 2812
#define IRC_MAX_NICKNAME          9
#define IRC_MAX_HOSTNAME         63
#define IRC_MAX_MESSAGE_LENGTH  510

static bool
irc_regex_match (const char *regex, const char *s)
{
	static struct str_map cache;
	static bool initialized;

	if (!initialized)
	{
		regex_cache_init (&cache);
		initialized = true;
	}

	struct error *e = NULL;
	bool result = regex_cache_match (&cache, regex,
		REG_EXTENDED | REG_NOSUB, s, &e);
	hard_assert (!e);
	return result;
}

static const char *
irc_validate_to_str (enum validation_result result)
{
	switch (result)
	{
	case VALIDATION_OK:              return "success";
	case VALIDATION_ERROR_EMPTY:     return "the value is empty";
	case VALIDATION_ERROR_INVALID:   return "invalid format";
	case VALIDATION_ERROR_TOO_LONG:  return "the value is too long";
	default:                         abort ();
	}
}

// Anything to keep it as short as possible
#define SN "[0-9A-Za-z][-0-9A-Za-z]*[0-9A-Za-z]*"
#define N4 "[0-9]{1,3}"
#define N6 "[0-9ABCDEFabcdef]{1,}"

#define LE "A-Za-z"
#define SP "][\\\\`_^{|}"

static enum validation_result
irc_validate_hostname (const char *hostname)
{
	if (!*hostname)
		return VALIDATION_ERROR_EMPTY;
	if (!irc_regex_match ("^" SN "(\\." SN ")*$", hostname))
		return VALIDATION_ERROR_INVALID;
	if (strlen (hostname) > IRC_MAX_HOSTNAME)
		return VALIDATION_ERROR_TOO_LONG;
	return VALIDATION_OK;
}

static bool
irc_is_valid_hostaddr (const char *hostaddr)
{
	if (irc_regex_match ("^" N4 "\\." N4 "\\." N4 "\\." N4 "$", hostaddr)
	 || irc_regex_match ("^" N6 ":" N6 ":" N6 ":" N6 ":"
		N6 ":" N6 ":" N6 ":" N6 "$", hostaddr)
	 || irc_regex_match ("^0:0:0:0:0:(0|[Ff]{4}):"
		N4 "\\." N4 "\\." N4 "\\." N4 "$", hostaddr))
		return true;
	return false;
}

static bool
irc_is_valid_host (const char *host)
{
	return irc_validate_hostname (host) == VALIDATION_OK
		|| irc_is_valid_hostaddr (host);
}

static bool
irc_is_valid_user (const char *user)
{
	return irc_regex_match ("^[^\r\n @]+$", user);
}

static bool
irc_validate_nickname (const char *nickname)
{
	if (!*nickname)
		return VALIDATION_ERROR_EMPTY;
	if (!irc_regex_match ("^[" SP LE "][" SP LE "0-9-]*$", nickname))
		return VALIDATION_ERROR_INVALID;
	if (strlen (nickname) > IRC_MAX_NICKNAME)
		return VALIDATION_ERROR_TOO_LONG;
	return VALIDATION_OK;
}

#undef SN
#undef N4
#undef N6

#undef LE
#undef SP

// --- Application data --------------------------------------------------------

#define IRC_SUPPORTED_USER_MODES "aiwros"

enum
{
	IRC_USER_MODE_INVISIBLE          = (1 << 0),
	IRC_USER_MODE_RX_WALLOPS         = (1 << 1),
	IRC_USER_MODE_RESTRICTED         = (1 << 2),
	IRC_USER_MODE_OPERATOR           = (1 << 3),
	IRC_USER_MODE_RX_SERVER_NOTICES  = (1 << 4)
};

struct client
{
	struct client *next;                ///< The next link in a chain
	struct client *prev;                ///< The previous link in a chain

	struct server_context *ctx;         ///< Server context

	int socket_fd;                      ///< The TCP socket
	struct str read_buffer;             ///< Unprocessed input
	struct str write_buffer;            ///< Output yet to be sent out

	unsigned initialized    : 1;        ///< Has any data been received yet?
	unsigned registered     : 1;        ///< The user has registered

	unsigned ssl_rx_want_tx : 1;        ///< SSL_read() wants to write
	unsigned ssl_tx_want_rx : 1;        ///< SSL_write() wants to read
	SSL *ssl;                           ///< SSL connection

	char *nickname;                     ///< IRC nickname (main identifier)
	char *username;                     ///< IRC username
	char *realname;                     ///< IRC realname (e-mail)

	char *hostname;                     ///< Hostname shown to the network

	unsigned mode;                      ///< User's mode
	char *away_message;                 ///< Away message
};

static void
client_init (struct client *self)
{
	memset (self, 0, sizeof *self);

	self->socket_fd = -1;
	str_init (&self->read_buffer);
	str_init (&self->write_buffer);
}

static void
client_free (struct client *self)
{
	if (!soft_assert (self->socket_fd == -1))
		xclose (self->socket_fd);
	if (self->ssl)
		SSL_free (self->ssl);

	str_free (&self->read_buffer);
	str_free (&self->write_buffer);

	free (self->nickname);
	free (self->username);
	free (self->realname);

	free (self->hostname);
	free (self->away_message);
}

static char *
client_get_mode (struct client *self)
{
	struct str mode;
	str_init (&mode);

	if (self->away_message)                    str_append_c (&mode, 'a');

	unsigned m = self->mode;
	if (m & IRC_USER_MODE_INVISIBLE)           str_append_c (&mode, 'i');
	if (m & IRC_USER_MODE_RX_WALLOPS)          str_append_c (&mode, 'w');
	if (m & IRC_USER_MODE_RESTRICTED)          str_append_c (&mode, 'r');
	if (m & IRC_USER_MODE_OPERATOR)            str_append_c (&mode, 'o');
	if (m & IRC_USER_MODE_RX_SERVER_NOTICES)   str_append_c (&mode, 's');

	if (mode.len)
		return str_steal (&mode);

	str_free (&mode);
	return NULL;
}

#define IRC_SUPPORTED_CHAN_MODES "ov" "imnqpst" "kl"

enum
{
	IRC_CHAN_MODE_INVITE_ONLY      = (1 << 0),
	IRC_CHAN_MODE_MODERATED        = (1 << 1),
	IRC_CHAN_MODE_NO_OUTSIDE_MSGS  = (1 << 2),
	IRC_CHAN_MODE_QUIET            = (1 << 3),
	IRC_CHAN_MODE_PRIVATE          = (1 << 4),
	IRC_CHAN_MODE_SECRET           = (1 << 5),
	IRC_CHAN_MODE_PROTECTED_TOPIC  = (1 << 6),

	IRC_CHAN_MODE_OPERATOR         = (1 << 7),
	IRC_CHAN_MODE_VOICE            = (1 << 8)
};

struct channel_user
{
	struct channel_user *prev;
	struct channel_user *next;

	unsigned modes;
	char nickname[];
};

struct channel
{
	struct server_context *ctx;         ///< Server context

	char *name;                         ///< Channel name
	unsigned modes;                     ///< Channel modes
	char *key;                          ///< Channel key
	long user_limit;                    ///< User limit or -1

	struct channel_user *users;         ///< Channel users

	struct str_vector ban_list;         ///< Ban list
	struct str_vector exception_list;   ///< Exceptions from bans
	struct str_vector invite_list;      ///< Exceptions from +I
};

static void
channel_init (struct channel *self)
{
	memset (self, 0, sizeof *self);

	str_vector_init (&self->ban_list);
	str_vector_init (&self->exception_list);
	str_vector_init (&self->invite_list);
}

static void
channel_free (struct channel *self)
{
	free (self->name);
	free (self->key);

	struct channel_user *link, *tmp;
	for (link = self->users; link; link = tmp)
	{
		tmp = link->next;
		free (link);
	}

	str_vector_free (&self->ban_list);
	str_vector_free (&self->exception_list);
	str_vector_free (&self->invite_list);
}

struct server_context
{
	struct str_map config;              ///< Server configuration

	int listen_fd;                      ///< Listening socket FD
	struct client *clients;             ///< Clients
	SSL_CTX *ssl_ctx;                   ///< SSL context

	char *server_name;                  ///< Our server name
	struct str_map users;               ///< Maps nicknames to clients
	struct str_map channels;            ///< Maps channel names to data
	struct str_map handlers;            ///< Message handlers

	struct poller poller;               ///< Manages polled description
	bool quitting;                      ///< User requested quitting
	bool polling;                       ///< The event loop is running

	struct str_vector motd;             ///< MOTD (none if empty)
	nl_catd catalog;                    ///< Message catalog for server msgs
};

static void
server_context_init (struct server_context *self)
{
	str_map_init (&self->config);
	self->config.free = free;
	load_config_defaults (&self->config, g_config_table);

	self->listen_fd = -1;
	self->clients = NULL;

	self->server_name = NULL;
	str_map_init (&self->users);
	self->users.key_xfrm = irc_strxfrm;
	// TODO: set channel_free() as the free function?
	str_map_init (&self->channels);
	self->channels.key_xfrm = irc_strxfrm;
	str_map_init (&self->handlers);
	self->handlers.key_xfrm = irc_strxfrm;

	poller_init (&self->poller);
	self->quitting = false;
	self->polling = false;

	str_vector_init (&self->motd);
	self->catalog = (nl_catd) -1;
}

static void
server_context_free (struct server_context *self)
{
	str_map_free (&self->config);

	if (self->listen_fd != -1)
		xclose (self->listen_fd);
	if (self->ssl_ctx)
		SSL_CTX_free (self->ssl_ctx);

	// TODO: terminate the connections properly before this is called
	struct client *link, *tmp;
	for (link = self->clients; link; link = tmp)
	{
		tmp = link->next;
		client_free (link);
		free (link);
	}

	free (self->server_name);
	str_map_free (&self->users);
	str_map_free (&self->channels);
	str_map_free (&self->handlers);
	poller_free (&self->poller);

	str_vector_free (&self->motd);
	if (self->catalog != (nl_catd) -1)
		catclose (self->catalog);
}

// --- Main program ------------------------------------------------------------

static void
client_kill (struct client *c, const char *reason)
{
	// TODO: multicast a QUIT message with `reason' || "Client exited"
	(void) reason;

	// TODO: do further cleanup if the client has successfully registered etc.

	struct server_context *ctx = c->ctx;
	ssize_t i = poller_find_by_fd (&ctx->poller, c->socket_fd);
	if (i != -1)
		poller_remove_at_index (&ctx->poller, i);

	xclose (c->socket_fd);
	c->socket_fd = -1;
	client_free (c);
	LIST_UNLINK (ctx->clients, c);
	free (c);
}

static void
irc_send_str (struct client *c, const struct str *s)
{
	// TODO: kill the connection above some "SendQ" threshold (careful!)

	str_append_data (&c->write_buffer, s->str,
		s->len > IRC_MAX_MESSAGE_LENGTH ? IRC_MAX_MESSAGE_LENGTH : s->len);
	str_append (&c->write_buffer, "\r\n");
}

static void irc_send (struct client *c,
	const char *format, ...) ATTRIBUTE_PRINTF (2, 3);

static void
irc_send (struct client *c, const char *format, ...)
{
	struct str tmp;
	str_init (&tmp);

	va_list ap;
	va_start (ap, format);
	str_append_vprintf (&tmp, format, ap);
	va_end (ap);

	irc_send_str (c, &tmp);
	str_free (&tmp);
}

static const char *
irc_get_text (struct server_context *ctx, int id, const char *def)
{
	if (!soft_assert (def != NULL))
		def = "";
	if (ctx->catalog == (nl_catd) -1)
		return def;
	return catgets (ctx->catalog, 1, id, def);
}

// --- IRC command handling ----------------------------------------------------

enum
{
	IRC_RPL_WELCOME               =   1,
	IRC_RPL_YOURHOST              =   2,
	IRC_RPL_CREATED               =   3,
	IRC_RPL_MYINFO                =   4,

	IRC_RPL_MOTD                  = 372,
	IRC_RPL_MOTDSTART             = 375,
	IRC_RPL_ENDOFMOTD             = 376,

	IRC_ERR_NOORIGIN              = 409,
	IRC_ERR_UNKNOWNCOMMAND        = 421,
	IRC_ERR_NOMOTD                = 422,
	IRC_ERR_NONICKNAMEGIVEN       = 431,
	IRC_ERR_ERRONEOUSNICKNAME     = 432,
	IRC_ERR_NICKNAMEINUSE         = 433,
	IRC_ERR_NOTREGISTERED         = 451,
	IRC_ERR_NEEDMOREPARAMS        = 461,
	IRC_ERR_ALREADYREGISTERED     = 462
};

static const char *g_default_replies[] =
{
	[IRC_RPL_WELCOME] = ":Welcome to the Internet Relay Network %s!%s@%s",
	[IRC_RPL_YOURHOST] = ":Your host is %s, running version %s",
	[IRC_RPL_CREATED] = ":This server was created %s",
	[IRC_RPL_MYINFO] = "%s %s %s %s",

	[IRC_RPL_MOTD] = ":- %s",
	[IRC_RPL_MOTDSTART] = ":- %s Message of the day - ",
	[IRC_RPL_ENDOFMOTD] = ":End of MOTD command",

	[IRC_ERR_NOORIGIN] = ":No origin specified",
	[IRC_ERR_UNKNOWNCOMMAND] = "%s: Unknown command",
	[IRC_ERR_NOMOTD] = ":MOTD File is missing",
	[IRC_ERR_NONICKNAMEGIVEN] = ":No nickname given",
	[IRC_ERR_ERRONEOUSNICKNAME] = "%s :Erroneous nickname",
	[IRC_ERR_NICKNAMEINUSE] = "%s :Nickname is already in use",
	[IRC_ERR_NOTREGISTERED] = "%s :You have not registered",
	[IRC_ERR_NEEDMOREPARAMS] = "%s :Not enough parameters",
	[IRC_ERR_ALREADYREGISTERED] = ":Unauthorized command (already registered)",
};

// XXX: this way we cannot typecheck the arguments, so we must be careful
static void
irc_send_reply (struct client *c, int id, ...)
{
	struct str tmp;
	str_init (&tmp);

	va_list ap;
	va_start (ap, id);
	str_append_printf (&tmp, ":%s %03d %s ",
		c->ctx->server_name, id, c->nickname ? c->nickname : "");
	str_append_vprintf (&tmp,
		irc_get_text (c->ctx, id, g_default_replies[id]), ap);
	va_end (ap);

	irc_send_str (c, &tmp);
	str_free (&tmp);
}

static void
irc_send_motd (struct client *c)
{
	struct server_context *ctx = c->ctx;
	if (!ctx->motd.len)
	{
		irc_send_reply (c, IRC_ERR_NOMOTD);
		return;
	}

	irc_send_reply (c, IRC_RPL_MOTDSTART, ctx->server_name);
	for (size_t i = 0; i < ctx->motd.len; i++)
		irc_send_reply (c, IRC_RPL_MOTD, ctx->motd.vector[i]);
	irc_send_reply (c, IRC_RPL_ENDOFMOTD);
}

static void
irc_try_finish_registration (struct client *c)
{
	struct server_context *ctx = c->ctx;
	if (!c->nickname || !c->username || !c->realname)
		return;

	c->registered = true;
	irc_send_reply (c, IRC_RPL_WELCOME, c->nickname, c->username, c->hostname);

	irc_send_reply (c, IRC_RPL_YOURHOST, ctx->server_name, PROGRAM_VERSION);
	// The purpose of this message eludes me
	irc_send_reply (c, IRC_RPL_CREATED, __DATE__);
	irc_send_reply (c, IRC_RPL_MYINFO, ctx->server_name, PROGRAM_VERSION,
		IRC_SUPPORTED_USER_MODES, IRC_SUPPORTED_CHAN_MODES);

	// Although not strictly required, bots often need this to work
	irc_send_motd (c);

	char *mode = client_get_mode (c);
	if (mode)
		irc_send (c, ":%s MODE %s :+%s", c->nickname, c->nickname, mode);
	free (mode);
}

static void
irc_handle_pass (const struct irc_message *msg, struct client *c)
{
	if (c->registered)
		irc_send_reply (c, IRC_ERR_ALREADYREGISTERED);
	else if (msg->params.len < 1)
		irc_send_reply (c, IRC_ERR_NEEDMOREPARAMS, msg->command);

	// We have SSL client certificates for this purpose; ignoring
}

static void
irc_handle_nick (const struct irc_message *msg, struct client *c)
{
	struct server_context *ctx = c->ctx;

	if (c->registered)
	{
		irc_send_reply (c, IRC_ERR_ALREADYREGISTERED);
		return;
	}
	if (msg->params.len < 1)
	{
		irc_send_reply (c, IRC_ERR_NONICKNAMEGIVEN);
		return;
	}

	const char *nickname = msg->params.vector[0];
	if (irc_validate_nickname (nickname) != VALIDATION_OK)
	{
		irc_send_reply (c, IRC_ERR_ERRONEOUSNICKNAME, nickname);
		return;
	}
	if (str_map_find (&ctx->users, nickname))
	{
		irc_send_reply (c, IRC_ERR_NICKNAMEINUSE, nickname);
		return;
	}
	if (c->nickname)
	{
		str_map_set (&ctx->users, c->nickname, NULL);
		free (c->nickname);
	}

	// Allocate the nickname
	c->nickname = xstrdup (nickname);
	str_map_set (&ctx->users, nickname, c);

	irc_try_finish_registration (c);
}

static void
irc_handle_user (const struct irc_message *msg, struct client *c)
{
	if (c->registered)
	{
		irc_send_reply (c, IRC_ERR_ALREADYREGISTERED);
		return;
	}
	if (msg->params.len < 4)
	{
		irc_send_reply (c, IRC_ERR_NEEDMOREPARAMS, msg->command);
		return;
	}

	const char *username = msg->params.vector[0];
	const char *mode     = msg->params.vector[1];
	const char *realname = msg->params.vector[3];

	// Unfortunately the protocol doesn't give us any means of rejecting it
	if (!irc_is_valid_user (username))
		username = "xxx";

	free (c->username);
	c->username = xstrdup (username);
	free (c->realname);
	c->realname = xstrdup (realname);

	unsigned long m;
	if (xstrtoul (&m, mode, 10))
	{
		if (m & 4)  c->mode |= IRC_USER_MODE_RX_WALLOPS;
		if (m & 8)  c->mode |= IRC_USER_MODE_INVISIBLE;
	}

	irc_try_finish_registration (c);
}

static void
irc_handle_ping (const struct irc_message *msg, struct client *c)
{
	// XXX: the RFC is pretty incomprehensible about the exact usage
	if (msg->params.len < 1)
		irc_send_reply (c, IRC_ERR_NOORIGIN);
	else
		irc_send (c, ":%s PONG :%s",
			c->ctx->server_name, msg->params.vector[0]);
}

// -----------------------------------------------------------------------------

struct irc_command
{
	const char *name;
	bool requires_registration;
	void (*handler) (const struct irc_message *, struct client *);
};

static void
irc_register_handlers (struct server_context *ctx)
{
	static const struct irc_command message_handlers[] =
	{
		{ "PASS", false, irc_handle_pass },
		{ "NICK", false, irc_handle_nick },
		{ "USER", false, irc_handle_user },

		{ "PING", true,  irc_handle_ping }
	};

	for (size_t i = 0; i < N_ELEMENTS (message_handlers); i++)
	{
		const struct irc_command *cmd = &message_handlers[i];
		str_map_set (&ctx->handlers, cmd->name, (void *) cmd);
	}
}

static void
irc_process_message (const struct irc_message *msg,
	const char *raw, void *user_data)
{
	(void) raw;

	// XXX: we may want to discard everything following a QUIT etc.
	//   We can set a flag within the client object.
	// TODO: see RFC 2812 :!

	struct client *c = user_data;
	struct irc_command *cmd = str_map_find (&c->ctx->handlers, msg->command);
	if (!cmd)
		irc_send_reply (c, IRC_ERR_UNKNOWNCOMMAND, msg->command);
	else if (cmd->requires_registration && !c->registered)
		irc_send_reply (c, IRC_ERR_NOTREGISTERED);
	else
		cmd->handler (msg, c);
}

// --- Network I/O -------------------------------------------------------------

static bool
irc_try_read (struct client *c)
{
	struct str *buf = &c->read_buffer;
	ssize_t n_read;

	while (true)
	{
		str_ensure_space (buf, 512);
		n_read = recv (c->socket_fd, buf->str + buf->len,
			buf->alloc - buf->len - 1 /* null byte */, 0);

		if (n_read > 0)
		{
			buf->str[buf->len += n_read] = '\0';
			// TODO: discard characters above the 512 character limit
			irc_process_buffer (buf, irc_process_message, c);
			continue;
		}
		if (n_read == 0)
		{
			client_kill (c, NULL);
			return false;
		}

		if (errno == EAGAIN)
			return true;
		if (errno == EINTR)
			continue;

		print_debug ("%s: %s: %s", __func__, "recv", strerror (errno));
		client_kill (c, strerror (errno));
		return false;
	}
}

static bool
irc_try_read_ssl (struct client *c)
{
	if (c->ssl_tx_want_rx)
		return true;

	struct str *buf = &c->read_buffer;
	c->ssl_rx_want_tx = false;
	while (true)
	{
		str_ensure_space (buf, 512);
		int n_read = SSL_read (c->ssl, buf->str + buf->len,
			buf->alloc - buf->len - 1 /* null byte */);

		const char *error_info = NULL;
		switch (xssl_get_error (c->ssl, n_read, &error_info))
		{
		case SSL_ERROR_NONE:
			buf->str[buf->len += n_read] = '\0';
			// TODO: discard characters above the 512 character limit
			irc_process_buffer (buf, irc_process_message, c);
			continue;
		case SSL_ERROR_ZERO_RETURN:
			client_kill (c, NULL);
			return false;
		case SSL_ERROR_WANT_READ:
			return true;
		case SSL_ERROR_WANT_WRITE:
			c->ssl_rx_want_tx = true;
			return true;
		case XSSL_ERROR_TRY_AGAIN:
			continue;
		default:
			print_debug ("%s: %s: %s", __func__, "SSL_read", error_info);
			client_kill (c, error_info);
			return false;
		}
	}
}

static bool
irc_try_write (struct client *c)
{
	struct str *buf = &c->write_buffer;
	ssize_t n_written;

	while (buf->len)
	{
		n_written = send (c->socket_fd, buf->str, buf->len, 0);
		if (n_written >= 0)
		{
			str_remove_slice (buf, 0, n_written);
			continue;
		}

		if (errno == EAGAIN)
			return true;
		if (errno == EINTR)
			continue;

		print_debug ("%s: %s: %s", __func__, "send", strerror (errno));
		client_kill (c, strerror (errno));
		return false;
	}
	return true;
}

static bool
irc_try_write_ssl (struct client *c)
{
	if (c->ssl_rx_want_tx)
		return true;

	struct str *buf = &c->write_buffer;
	c->ssl_tx_want_rx = false;
	while (buf->len)
	{
		int n_written = SSL_write (c->ssl, buf->str, buf->len);

		const char *error_info = NULL;
		switch (xssl_get_error (c->ssl, n_written, &error_info))
		{
		case SSL_ERROR_NONE:
			str_remove_slice (buf, 0, n_written);
			continue;
		case SSL_ERROR_ZERO_RETURN:
			client_kill (c, NULL);
			return false;
		case SSL_ERROR_WANT_WRITE:
			return true;
		case SSL_ERROR_WANT_READ:
			c->ssl_tx_want_rx = true;
			return true;
		case XSSL_ERROR_TRY_AGAIN:
			continue;
		default:
			print_debug ("%s: %s: %s", __func__, "SSL_write", error_info);
			client_kill (c, error_info);
			return false;
		}
	}
	return true;
}

static bool
irc_autodetect_ssl (struct client *c)
{
	// Trivial SSL/TLS autodetection.  The first block of data returned by
	// recv() must be at least three bytes long for this to work reliably,
	// but that should not pose a problem in practice.
	//
	// SSL2:      1xxx xxxx | xxxx xxxx |    <1>
	//               (message length)  (client hello)
	// SSL3/TLS:    <22>    |    <3>    | xxxx xxxx
	//           (handshake)|  (protocol version)
	//
	// Such byte sequences should never occur at the beginning of regular IRC
	// communication, which usually begins with USER/NICK/PASS/SERVICE.

	char buf[3];
start:
	switch (recv (c->socket_fd, buf, sizeof buf, MSG_PEEK))
	{
	case 3:
		if ((buf[0] & 0x80) && buf[2] == 1)
			return true;
	case 2:
		if (buf[0] == 22 && buf[1] == 3)
			return true;
		break;
	case 1:
		if (buf[0] == 22)
			return true;
		break;
	case 0:
		break;
	default:
		if (errno == EINTR)
			goto start;
	}
	return false;
}

static bool
client_initialize_ssl (struct client *c)
{
	// SSL support not enabled
	if (!c->ctx->ssl_ctx)
		return false;

	c->ssl = SSL_new (c->ctx->ssl_ctx);
	if (!c->ssl)
		goto error_ssl_1;

	if (!SSL_set_fd (c->ssl, c->socket_fd))
		goto error_ssl_2;
	SSL_set_accept_state (c->ssl);
	return true;

error_ssl_2:
	SSL_free (c->ssl);
	c->ssl = NULL;
error_ssl_1:
	// XXX: these error strings are really nasty; also there could be
	//   multiple errors on the OpenSSL stack.
	print_debug ("%s: %s: %s", "could not initialize SSL",
		c->hostname, ERR_error_string (ERR_get_error (), NULL));
	return false;
}

static void
on_irc_client_ready (const struct pollfd *pfd, void *user_data)
{
	struct client *c = user_data;
	if (!c->initialized)
	{
		hard_assert (pfd->events == POLLIN);
		if (irc_autodetect_ssl (c) && !client_initialize_ssl (c))
		{
			client_kill (c, NULL);
			return;
		}
		c->initialized = true;
	}

	int new_events = 0;
	if (c->ssl)
	{
		// Reads may want to write, writes may want to read, poll() may
		// return unexpected things in `revents'... let's try both
		if (!irc_try_read_ssl (c) || !irc_try_write_ssl (c))
			return;

		new_events |= POLLIN;
		if (c->write_buffer.len || c->ssl_rx_want_tx)
			new_events |= POLLOUT;

		// While we're waiting for an opposite event, we ignore the original
		if (c->ssl_rx_want_tx)  new_events &= ~POLLIN;
		if (c->ssl_tx_want_rx)  new_events &= ~POLLOUT;
	}
	else
	{
		if (!irc_try_read (c) || !irc_try_write (c))
			return;

		new_events |= POLLIN;
		if (c->write_buffer.len)
			new_events |= POLLOUT;
	}

	hard_assert (new_events != 0);
	if (pfd->events != new_events)
		poller_set (&c->ctx->poller, c->socket_fd, new_events,
			(poller_dispatcher_func) on_irc_client_ready, c);
}

static void
on_irc_client_available (const struct pollfd *pfd, void *user_data)
{
	(void) pfd;
	struct server_context *ctx = user_data;

	// TODO: stop accepting new connections when `max_connections' is reached

	while (true)
	{
		// XXX: `struct sockaddr_storage' is not the most portable thing
		struct sockaddr_storage peer;
		socklen_t peer_len = sizeof peer;

		int fd = accept (ctx->listen_fd, (struct sockaddr *) &peer, &peer_len);
		if (fd == -1)
		{
			if (errno == EAGAIN)
				break;
			if (errno == EINTR)
				continue;
			if (errno == ECONNABORTED)
				continue;

			// TODO: handle resource exhaustion (EMFILE, ENFILE) specially
			//   (stop accepting new connections and wait until we close some).
			// FIXME: handle this better, bring the server down cleanly.
			exit_fatal ("%s: %s", "accept", strerror (errno));
		}

		char host[NI_MAXHOST] = "unknown", port[NI_MAXSERV] = "unknown";
		int err = getnameinfo ((struct sockaddr *) &peer, peer_len,
			host, sizeof host, port, sizeof port, NI_NUMERICSERV);
		if (err)
			print_debug ("%s: %s", "getnameinfo", gai_strerror (err));
		print_debug ("accepted connection from %s:%s", host, port);

		struct client *c = xmalloc (sizeof *c);
		client_init (c);
		c->ctx = ctx;
		c->socket_fd = fd;
		c->hostname = xstrdup (host);
		LIST_PREPEND (ctx->clients, c);

		// TODO: set a timeout on the socket, something like 3 minutes, then we
		//   should terminate the connection.
		set_blocking (fd, false);
		poller_set (&ctx->poller, fd, POLLIN,
			(poller_dispatcher_func) on_irc_client_ready, c);
	}
}

// -----------------------------------------------------------------------------

static int
irc_ssl_verify_callback (int verify_ok, X509_STORE_CTX *ctx)
{
	(void) verify_ok;
	(void) ctx;

	// We only want to provide additional privileges based on the client's
	// certificate, so let's not terminate the connection because of a failure.
	return 1;
}

static bool
irc_initialize_ssl (struct server_context *ctx, struct error **e)
{
	const char *ssl_cert = str_map_find (&ctx->config, "ssl_cert");
	const char *ssl_key = str_map_find (&ctx->config, "ssl_key");

	// Only try to enable SSL support if the user configures it; it is not
	// a failure if no one has requested it.
	if (!ssl_cert && !ssl_key)
		return true;

	if (!ssl_cert)
		error_set (e, "no SSL certificate set");
	else if (!ssl_key)
		error_set (e, "no SSL private key set");
	if (!ssl_cert || !ssl_key)
		return false;

	char *cert_path = resolve_config_filename (ssl_cert);
	char *key_path = resolve_config_filename (ssl_key);
	if (!cert_path)
		error_set (e, "%s: %s", "cannot open file", ssl_cert);
	else if (!key_path)
		error_set (e, "%s: %s", "cannot open file", ssl_key);
	if (!cert_path || !key_path)
		return false;

	ctx->ssl_ctx = SSL_CTX_new (SSLv23_server_method ());
	if (!ctx->ssl_ctx)
	{
		// XXX: these error strings are really nasty; also there could be
		//   multiple errors on the OpenSSL stack.
		error_set (e, "%s: %s", "could not initialize SSL",
			ERR_error_string (ERR_get_error (), NULL));
		goto error_ssl_1;
	}
	SSL_CTX_set_verify (ctx->ssl_ctx,
		SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE, irc_ssl_verify_callback);
	// XXX: maybe we should call SSL_CTX_set_options() for some workarounds

	// XXX: perhaps we should read the files ourselves for better messages
	if (!SSL_CTX_use_certificate_chain_file (ctx->ssl_ctx, cert_path))
	{
		error_set (e, "%s: %s", "setting the SSL client certificate failed",
			ERR_error_string (ERR_get_error (), NULL));
		goto error_ssl_2;
	}
	if (!SSL_CTX_use_PrivateKey_file (ctx->ssl_ctx, key_path, SSL_FILETYPE_PEM))
	{
		error_set (e, "%s: %s", "setting the SSL private key failed",
			ERR_error_string (ERR_get_error (), NULL));
		goto error_ssl_2;
	}

	// TODO: SSL_CTX_check_private_key()?  It has probably already been checked
	//   by SSL_CTX_use_PrivateKey_file() above.

	// Gah, spare me your awkward semantics, I just want to push data!
	// XXX: do we want SSL_MODE_AUTO_RETRY as well?  I guess not.
	SSL_CTX_set_mode (ctx->ssl_ctx,
		SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER | SSL_MODE_ENABLE_PARTIAL_WRITE);
	return true;

error_ssl_2:
	SSL_CTX_free (ctx->ssl_ctx);
	ctx->ssl_ctx = NULL;
error_ssl_1:
	return false;
}

static bool
irc_initialize_catalog (struct server_context *ctx, struct error **e)
{
	hard_assert (ctx->catalog == (nl_catd) -1);
	const char *catalog = str_map_find (&ctx->config, "catalog");
	if (!catalog)
		return true;

	char *path = resolve_config_filename (catalog);
	if (!path)
	{
		error_set (e, "%s: %s", "cannot open file", catalog);
		return false;
	}
	ctx->catalog = catopen (path, NL_CAT_LOCALE);
	free (path);

	if (ctx->catalog == (nl_catd) -1)
	{
		error_set (e, "%s: %s",
			"failed reading the message catalog file", strerror (errno));
		return false;
	}
	return true;
}

static bool
irc_initialize_motd (struct server_context *ctx, struct error **e)
{
	hard_assert (ctx->motd.len == 0);
	const char *motd = str_map_find (&ctx->config, "motd");
	if (!motd)
		return true;

	char *path = resolve_config_filename (motd);
	if (!path)
	{
		error_set (e, "%s: %s", "cannot open file", motd);
		return false;
	}
	FILE *fp = fopen (path, "r");
	free (path);

	if (!fp)
	{
		error_set (e, "%s: %s",
			"failed reading the MOTD file", strerror (errno));
		return false;
	}

	struct str line;
	str_init (&line);
	while (read_line (fp, &line))
		str_vector_add_owned (&ctx->motd, str_steal (&line));
	str_free (&line);

	fclose (fp);
	return true;
}

static bool
irc_initialize_server_name (struct server_context *ctx, struct error **e)
{
	enum validation_result res;
	const char *server_name = str_map_find (&ctx->config, "server_name");
	if (server_name)
	{
		res = irc_validate_hostname (server_name);
		if (res != VALIDATION_OK)
		{
			error_set (e, "invalid configuration value for `%s': %s",
				"server_name", irc_validate_to_str (res));
			return false;
		}
		ctx->server_name = xstrdup (server_name);
	}
	else
	{
		char hostname[HOST_NAME_MAX];
		if (gethostname (hostname, sizeof hostname))
		{
			error_set (e, "%s: %s",
				"getting the hostname failed", strerror (errno));
			return false;
		}
		res = irc_validate_hostname (hostname);
		if (res != VALIDATION_OK)
		{
			error_set (e,
				"`%s' is not set and the hostname (`%s') cannot be used: %s",
				"server_name", hostname, irc_validate_to_str (res));
			return false;
		}
		ctx->server_name = xstrdup (hostname);
	}
	return true;
}

static bool
irc_listen (struct server_context *ctx, struct error **e)
{
	const char *bind_host = str_map_find (&ctx->config, "bind_host");
	const char *bind_port = str_map_find (&ctx->config, "bind_port");
	hard_assert (bind_port != NULL);  // We have a default value for this

	struct addrinfo gai_hints, *gai_result, *gai_iter;
	memset (&gai_hints, 0, sizeof gai_hints);

	gai_hints.ai_socktype = SOCK_STREAM;
	gai_hints.ai_flags = AI_PASSIVE;

	int err = getaddrinfo (bind_host, bind_port, &gai_hints, &gai_result);
	if (err)
	{
		error_set (e, "%s: %s: %s",
			"network setup failed", "getaddrinfo", gai_strerror (err));
		return false;
	}

	int sockfd;
	char real_host[NI_MAXHOST], real_port[NI_MAXSERV];

	for (gai_iter = gai_result; gai_iter; gai_iter = gai_iter->ai_next)
	{
		sockfd = socket (gai_iter->ai_family,
			gai_iter->ai_socktype, gai_iter->ai_protocol);
		if (sockfd == -1)
			continue;
		set_cloexec (sockfd);

		int yes = 1;
		soft_assert (setsockopt (sockfd, SOL_SOCKET, SO_KEEPALIVE,
			&yes, sizeof yes) != -1);
		soft_assert (setsockopt (sockfd, SOL_SOCKET, SO_REUSEADDR,
			&yes, sizeof yes) != -1);

		real_host[0] = real_port[0] = '\0';
		err = getnameinfo (gai_iter->ai_addr, gai_iter->ai_addrlen,
			real_host, sizeof real_host, real_port, sizeof real_port,
			NI_NUMERICHOST | NI_NUMERICSERV);
		if (err)
			print_debug ("%s: %s", "getnameinfo", gai_strerror (err));

		if (bind (sockfd, gai_iter->ai_addr, gai_iter->ai_addrlen))
			print_error ("bind to %s:%s failed: %s",
				real_host, real_port, strerror (errno));
		else if (listen (sockfd, 16 /* arbitrary number */))
			print_error ("listen at %s:%s failed: %s",
				real_host, real_port, strerror (errno));
		else
			break;

		xclose (sockfd);
	}

	freeaddrinfo (gai_result);

	if (!gai_iter)
	{
		error_set (e, "network setup failed");
		return false;
	}

	set_blocking (sockfd, false);
	ctx->listen_fd = sockfd;
	poller_set (&ctx->poller, ctx->listen_fd, POLLIN,
		(poller_dispatcher_func) on_irc_client_available, ctx);

	print_status ("listening at %s:%s", real_host, real_port);
	return true;
}

static void
on_signal_pipe_readable (const struct pollfd *fd, struct server_context *ctx)
{
	char *dummy;
	(void) read (fd->fd, &dummy, 1);

	// TODO: send ERROR messages to anyone, wait for the messages to get
	//   dispatched for a few seconds, RST the rest and quit.
	if (g_termination_requested && !ctx->quitting)
	{
#if 0
		initiate_quit (ctx);
#endif
	}
}

static void
daemonize (void)
{
	// TODO: create and lock a PID file?
	print_status ("daemonizing...");

	if (chdir ("/"))
		exit_fatal ("%s: %s", "chdir", strerror (errno));

	pid_t pid;
	if ((pid = fork ()) < 0)
		exit_fatal ("%s: %s", "fork", strerror (errno));
	else if (pid)
		exit (EXIT_SUCCESS);

	setsid ();
	signal (SIGHUP, SIG_IGN);

	if ((pid = fork ()) < 0)
		exit_fatal ("%s: %s", "fork", strerror (errno));
	else if (pid)
		exit (EXIT_SUCCESS);

	openlog (PROGRAM_NAME, LOG_NDELAY | LOG_NOWAIT | LOG_PID, 0);
	g_log_message_real = log_message_syslog;

	// XXX: we may close our own descriptors this way, crippling ourselves
	for (int i = 0; i < 3; i++)
		xclose (i);

	int tty = open ("/dev/null", O_RDWR);
	if (tty != 0 || dup (0) != 1 || dup (0) != 2)
		exit_fatal ("failed to reopen FD's: %s", strerror (errno));
}

static void
print_usage (const char *program_name)
{
	fprintf (stderr,
		"Usage: %s [OPTION]...\n"
		"Experimental IRC server.\n"
		"\n"
		"  -d, --debug     run in debug mode (do not daemonize)\n"
		"  -h, --help      display this help and exit\n"
		"  -V, --version   output version information and exit\n"
		"  --write-default-cfg [filename]\n"
		"                  write a default configuration file and exit\n",
		program_name);
}

int
main (int argc, char *argv[])
{
	const char *invocation_name = argv[0];

	static struct option opts[] =
	{
		{ "debug",             no_argument,       NULL, 'd' },
		{ "help",              no_argument,       NULL, 'h' },
		{ "version",           no_argument,       NULL, 'V' },
		{ "write-default-cfg", optional_argument, NULL, 'w' },
		{ NULL,                0,                 NULL,  0  }
	};

	while (1)
	{
		int c, opt_index;

		c = getopt_long (argc, argv, "dhV", opts, &opt_index);
		if (c == -1)
			break;

		switch (c)
		{
		case 'd':
			g_debug_mode = true;
			break;
		case 'h':
			print_usage (invocation_name);
			exit (EXIT_SUCCESS);
		case 'V':
			printf (PROGRAM_NAME " " PROGRAM_VERSION "\n");
			exit (EXIT_SUCCESS);
		case 'w':
			call_write_default_config (optarg, g_config_table);
			exit (EXIT_SUCCESS);
		default:
			print_error ("wrong options");
			exit (EXIT_FAILURE);
		}
	}

	print_status (PROGRAM_NAME " " PROGRAM_VERSION " starting");
	setup_signal_handlers ();

	SSL_library_init ();
	atexit (EVP_cleanup);
	SSL_load_error_strings ();
	// XXX: ERR_load_BIO_strings()?  Anything else?
	atexit (ERR_free_strings);

	struct server_context ctx;
	server_context_init (&ctx);
	irc_register_handlers (&ctx);

	struct error *e = NULL;
	if (!read_config_file (&ctx.config, &e))
	{
		print_error ("error loading configuration: %s", e->message);
		error_free (e);
		exit (EXIT_FAILURE);
	}

	poller_set (&ctx.poller, g_signal_pipe[0], POLLIN,
		(poller_dispatcher_func) on_signal_pipe_readable, &ctx);

	if (!irc_initialize_ssl (&ctx, &e)
	 || !irc_initialize_server_name (&ctx, &e)
	 || !irc_initialize_motd (&ctx, &e)
	 || !irc_initialize_catalog (&ctx, &e)
	 || !irc_listen (&ctx, &e))
	{
		print_error ("%s", e->message);
		error_free (e);
		exit (EXIT_FAILURE);
	}

	if (!g_debug_mode)
		daemonize ();

	ctx.polling = true;
	while (ctx.polling)
		poller_run (&ctx.poller);

	server_context_free (&ctx);
	return EXIT_SUCCESS;
}