flexnbd-c/src/common/mirror.c

/*  FlexNBD server (C) Bytemark Hosting 2012

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/


#include "mirror.h"
#include "serve.h"
#include "util.h"
#include "ioutil.h"
#include "sockutil.h"
#include "parse.h"
#include "readwrite.h"
#include "bitset.h"
#include "self_pipe.h"
#include "status.h"

#include <stdlib.h>
#include <string.h>
#include <sys/un.h>
#include <unistd.h>
#include <sys/mman.h>
#include <ev.h>

/* compat with older libev */
#ifndef EVBREAK_ONE

#define ev_run( loop, flags ) ev_loop( loop, flags )

#define ev_break(loop, how) ev_unloop( loop, how )

#define EVBREAK_ONE EVUNLOOP_ONE
#define EVBREAK_ALL EVUNLOOP_ALL

#endif

/* We use this to keep track of the socket request data we need to send */
struct xfer {
	/* Store the bytes we need to send before the data, or receive back */
	union {
		struct nbd_request_raw req_raw;
		struct nbd_reply_raw rsp_raw;
	} hdr;

	/* what in mirror->mapped we should write, and how much of it we've done */
	uint64_t from;
	uint64_t len;
	uint64_t written;

	/* number of bytes of response read */
	uint64_t read;


};

struct mirror_ctrl {
	struct server *serve;
	struct mirror *mirror;

	/* libev stuff */
	struct ev_loop *ev_loop;
	ev_timer begin_watcher;
	ev_io read_watcher;
	ev_io write_watcher;
	ev_timer timeout_watcher;
	ev_timer limit_watcher;
	ev_io abandon_watcher;

	/* We set this if the bitset stream is getting uncomfortably full, and unset
	 * once it's emptier */
	int clear_events;

	/* This is set once all clients have been closed, to let the mirror know
	 * it's safe to finish once the queue is empty */
	int clients_closed;

	/* Use this to keep track of what we're copying at any moment */
	struct xfer xfer;

};

struct mirror * mirror_alloc(
		union mysockaddr * connect_to,
		union mysockaddr * connect_from,
		uint64_t max_Bps,
		enum mirror_finish_action action_at_finish,
		struct mbox * commit_signal)
{
	struct mirror * mirror;

	mirror = xmalloc(sizeof(struct mirror));
	mirror->connect_to = connect_to;
	mirror->connect_from = connect_from;
	mirror->max_bytes_per_second = max_Bps;
	mirror->action_at_finish = action_at_finish;
	mirror->commit_signal = commit_signal;
	mirror->commit_state = MS_UNKNOWN;
	mirror->abandon_signal = self_pipe_create();

	if ( mirror->abandon_signal == NULL ) {
		warn( "Couldn't create mirror abandon signal" );
		return NULL;
	}

	return mirror;
}

void mirror_set_state_f( struct mirror * mirror, enum mirror_state state )
{
	NULLCHECK( mirror );
	mirror->commit_state = state;
}

#define mirror_set_state( mirror, state ) do{\
	debug( "Mirror state => " #state );\
	mirror_set_state_f( mirror, state );\
} while(0)

enum mirror_state mirror_get_state( struct mirror * mirror )
{
	NULLCHECK( mirror );
	return mirror->commit_state;
}

#define mirror_state_is( mirror, state ) mirror_get_state( mirror ) == state


void mirror_init( struct mirror * mirror, const char * filename )
{
	int map_fd;
	uint64_t size;

	NULLCHECK( mirror );
	NULLCHECK( filename );

	FATAL_IF_NEGATIVE(
		open_and_mmap(
			filename,
			&map_fd,
			&size,
			(void**) &mirror->mapped
		),
		"Failed to open and mmap %s",
		filename
	);

	FATAL_IF_NEGATIVE(
		madvise( mirror->mapped, size, MADV_SEQUENTIAL ),
		SHOW_ERRNO( "Failed to madvise() %s", filename )
	);
}


/* Call this before a mirror attempt. */
void mirror_reset( struct mirror * mirror )
{
	NULLCHECK( mirror );
	mirror_set_state( mirror, MS_INIT );

	mirror->all_dirty = 0;
	mirror->migration_started = 0;
	mirror->offset = 0;

	return;
}


struct mirror * mirror_create(
		const char * filename,
		union mysockaddr * connect_to,
		union mysockaddr * connect_from,
		uint64_t max_Bps,
		int action_at_finish,
		struct mbox * commit_signal)
{
	/* FIXME: shouldn't map_fd get closed? */
	struct mirror * mirror;

	mirror = mirror_alloc( connect_to,
			connect_from,
			max_Bps,
			action_at_finish,
			commit_signal);

	mirror_init( mirror, filename );
	mirror_reset( mirror );


	return mirror;
}


void mirror_destroy( struct mirror *mirror )
{
	NULLCHECK( mirror );
	self_pipe_destroy( mirror->abandon_signal );
	free(mirror->connect_to);
	free(mirror->connect_from);
	free(mirror);
}


/** The mirror code will split NBD writes, making them this long as a maximum */
static const int mirror_longest_write = 8<<20;

/* This must not be called if there's any chance of further I/O. Methods to
 * ensure this include:
 *   - Ensure image size is 0
 *   - call server_forbid_new_clients() followed by a successful
 *     server_close_clients() ; server_join_clients()
 */
void mirror_on_exit( struct server * serve )
{
	/* If we're still here, we can shut the server down.
	 *
	 *
	 */
	debug("serve_signal_close");
	serve_signal_close( serve );

	/* We have to wait until the server is closed before unlocking
	 * IO.  This is because the client threads check to see if the
	 * server is still open before reading or writing inside their
	 * own locks. If we don't wait for the close, there's no way to
	 * guarantee the server thread will win the race and we risk the
	 * clients seeing a "successful" write to a dead disc image.
	 */
	debug("serve_wait_for_close");
	serve_wait_for_close( serve );

	if ( ACTION_UNLINK == serve->mirror->action_at_finish ) {
		debug("Unlinking %s", serve->filename );
		server_unlink( serve );
	}

	debug("Sending disconnect");
	socket_nbd_disconnect( serve->mirror->client );
	info("Mirror sent.");
}


void mirror_cleanup( struct server * serve,
		int fatal __attribute__((unused)))
{
	NULLCHECK( serve );
	struct mirror * mirror = serve->mirror;
	NULLCHECK( mirror );
	info( "Cleaning up mirror thread");

	if ( mirror->mapped ) {
		munmap( mirror->mapped, serve->size );
	}
	mirror->mapped = NULL;

	if( mirror->client && mirror->client > 0 ){
		close( mirror->client );
	}
	mirror->client = -1;
}


int mirror_connect( struct mirror * mirror, uint64_t local_size )
{
	struct sockaddr * connect_from = NULL;
	int connected =  0;

	if ( mirror->connect_from ) {
		connect_from = &mirror->connect_from->generic;
	}

	NULLCHECK( mirror->connect_to );

	mirror->client = socket_connect(&mirror->connect_to->generic, connect_from);
	if ( 0 < mirror->client ) {
		fd_set fds;
		struct timeval tv = { MS_HELLO_TIME_SECS, 0};
		FD_ZERO( &fds );
		FD_SET( mirror->client, &fds );

		FATAL_UNLESS( 0 <= select( FD_SETSIZE, &fds, NULL, NULL, &tv ),
				"Select failed." );

		if( FD_ISSET( mirror->client, &fds ) ){
			uint64_t remote_size;
			if ( socket_nbd_read_hello( mirror->client, &remote_size ) ) {
				if( remote_size == local_size ){
					connected = 1;
					mirror_set_state( mirror, MS_GO );
				}
				else {
					warn("Remote size (%d) doesn't match local (%d)",
							remote_size, local_size );
					mirror_set_state( mirror, MS_FAIL_SIZE_MISMATCH );
				}
			}
			else {
				warn( "Mirror attempt rejected." );
				mirror_set_state( mirror, MS_FAIL_REJECTED );
			}
		}
		else {
			warn( "No NBD Hello received." );
			mirror_set_state( mirror, MS_FAIL_NO_HELLO );
		}

		if ( !connected ) { close( mirror->client ); }
	}
	else {
		warn( "Mirror failed to connect.");
		mirror_set_state( mirror, MS_FAIL_CONNECT );
	}

	return connected;
}


int mirror_should_quit( struct mirror * mirror )
{
	switch( mirror->action_at_finish ) {
		case ACTION_EXIT:
		case ACTION_UNLINK:
			return 1;
		default:
			return 0;
	}
}

/* Bandwidth limiting - we hang around if bps is too high, unless we need to
 * empty out the bitset stream a bit */
int mirror_should_wait( struct mirror_ctrl *ctrl )
{
	int bps_over = server_mirror_bps( ctrl->serve ) >
		ctrl->serve->mirror->max_bytes_per_second;

	int stream_full = bitset_stream_size( ctrl->serve->allocation_map ) >
		( BITSET_STREAM_SIZE / 2 );

	return bps_over && !stream_full;
}

/*
 * If there's an event in the bitset stream of the serve allocation map, we
 * use it to construct the next transfer request, covering precisely the area
 * that has changed. If there are no events, we take the next
 * TODO: should we detect short events and lengthen them to reduce overhead?
 *
 * iterates through the bitmap, finding a dirty run to form the basis of the
 * next transfer, then puts it together. */
int mirror_setup_next_xfer( struct mirror_ctrl *ctrl )
{
	struct mirror* mirror = ctrl->mirror;
	struct server* serve = ctrl->serve;
	struct bitset_stream_entry e = { .event = BITSET_STREAM_UNSET };
	uint64_t current = mirror->offset, run = 0, size = serve->size;

	/* Technically, we'd be interested in UNSET events too, but they are never
	 * generated. TODO if that changes.
	 *
	 * We use ctrl->clear_events to start emptying the stream when it's half
	 * full, and stop when it's a quarter full. This stops a busy client from
	 * stalling a migration forever. FIXME: made-up numbers.
	 */
	if ( mirror->offset < serve->size && bitset_stream_size( serve->allocation_map ) > BITSET_STREAM_SIZE / 2 ) {
		ctrl->clear_events = 1;
	}


	while ( ( mirror->offset == serve->size || ctrl->clear_events ) && e.event != BITSET_STREAM_SET ) {
		uint64_t events =  bitset_stream_size( serve->allocation_map );

		if ( events == 0 ) {
			break;
		}

		debug("Dequeueing event");
		bitset_stream_dequeue( ctrl->serve->allocation_map, &e );
		debug("Dequeued event %i, %zu, %zu", e.event, e.from, e.len);

		if ( events < ( BITSET_STREAM_SIZE / 4 ) ) {
			ctrl->clear_events = 0;
		}
	}

	if ( e.event == BITSET_STREAM_SET ) {
		current = e.from;
		run = e.len;
	} else if ( current < serve->size ) {
		current = mirror->offset;
		run = mirror_longest_write;

		/* Adjust final block if necessary */
		if ( current + run > serve->size ) {
			run = size - current;
		}
		mirror->offset += run;
	} else {
		return 0;
	}

	debug( "Next transfer: current=%"PRIu64", run=%"PRIu64, current, run );
	struct nbd_request req = {
		.magic = REQUEST_MAGIC,
		.type = REQUEST_WRITE,
		.handle = ".MIRROR.",
		.from = current,
		.len = run
	};
	nbd_h2r_request( &req, &ctrl->xfer.hdr.req_raw );

	ctrl->xfer.from = current;
	ctrl->xfer.len  = run;

	ctrl->xfer.written = 0;
	ctrl->xfer.read = 0;

	return 1;
}

// ONLY CALL THIS AFTER CLOSING CLIENTS
void mirror_complete( struct server *serve )
{
	/* FIXME: Pretty sure this is broken, if action != !QUIT. Just moving code
	 * around for now, can fix it later. Action is always quit in production */
	if ( mirror_should_quit( serve->mirror ) ) {
		debug("exit!");
		/* FIXME: This depends on blocking I/O right now, so make sure we are */
		sock_set_nonblock( serve->mirror->client, 0 );
		mirror_on_exit( serve );
		info("Server closed, quitting after successful migration");
	}

	mirror_set_state( serve->mirror, MS_DONE );
	return;
}

static void mirror_write_cb( struct ev_loop *loop, ev_io *w, int revents )
{
	struct mirror_ctrl* ctrl = (struct mirror_ctrl*) w->data;
	NULLCHECK( ctrl );

	struct xfer *xfer = &ctrl->xfer;

	size_t to_write, hdr_size = sizeof( struct nbd_request_raw );
	char *data_loc;
	ssize_t count;

	if ( !( revents & EV_WRITE ) ) {
		warn( "No write event signalled in mirror write callback" );
		return;
	}

	debug( "Mirror write callback invoked with events %d. fd: %i", revents, ctrl->mirror->client );

	if ( xfer->written < hdr_size ) {
		data_loc = ( (char*) &xfer->hdr.req_raw ) + ctrl->xfer.written;
		to_write = hdr_size - xfer->written;
	} else {
		data_loc = ctrl->mirror->mapped  + xfer->from + ( xfer->written - hdr_size );
		to_write = xfer->len - ( ctrl->xfer.written - hdr_size );
	}

	// Actually write some bytes
	if ( ( count = write( ctrl->mirror->client, data_loc, to_write ) ) < 0 ) {
		if ( errno != EAGAIN && errno != EWOULDBLOCK && errno != EINTR ) {
			warn( SHOW_ERRNO( "Couldn't write to listener" ) );
			ev_break( loop, EVBREAK_ONE );
		}
		return;
	}
	debug( "Wrote %"PRIu64" bytes", count );
	debug( "to_write was %"PRIu64", xfer->written was %"PRIu64, to_write, xfer->written );

	// We wrote some bytes, so reset the timer and keep track for the next pass
	if ( count > 0 ) {
		ctrl->xfer.written += count;
		ev_timer_again( ctrl->ev_loop, &ctrl->timeout_watcher );
	}

	// All bytes written, so now we need to read the NBD reply back.
	if ( ctrl->xfer.written == ctrl->xfer.len + hdr_size ) {
		ev_io_start( loop, &ctrl->read_watcher  );
		ev_io_stop(  loop, &ctrl->write_watcher );
	}

	return;
}

static void mirror_read_cb( struct ev_loop *loop, ev_io *w, int revents )
{
	struct mirror_ctrl* ctrl = (struct mirror_ctrl*) w->data;
	NULLCHECK( ctrl );

	struct mirror *m = ctrl->mirror;
	NULLCHECK( m );

	struct xfer *xfer = &ctrl->xfer;
	NULLCHECK( xfer );

	if ( !( revents & EV_READ ) ) {
		warn( "No read event signalled in mirror read callback" );
		return;
	}

	struct nbd_reply rsp;
	ssize_t count;
	uint64_t left = sizeof( struct nbd_reply_raw ) - xfer->read;

	debug( "Mirror read callback invoked with events %d. fd:%i", revents, m->client );

	/* Start / continue reading the NBD response from the mirror. */
	if ( ( count = read( m->client, ((void*) &xfer->hdr.rsp_raw) + xfer->read, left ) ) < 0 ) {
		if ( errno != EAGAIN && errno != EWOULDBLOCK && errno != EINTR ) {
			warn( SHOW_ERRNO( "Couldn't read from listener" ) );
			ev_break( loop, EVBREAK_ONE );
		}
		debug( SHOW_ERRNO( "Couldn't read from listener (non-scary)" ) );
		return;
	}

	if ( count == 0 ) {
		warn( "EOF reading response from server!" );
		ev_break( loop, EVBREAK_ONE );
		return;
	}

	// We read some bytes, so reset the timer
	ev_timer_again( ctrl->ev_loop, &ctrl->timeout_watcher );

	debug( "Read %i bytes", count );
	debug( "left was %"PRIu64", xfer->read was %"PRIu64, left, xfer->read );
	xfer->read += count;

	if ( xfer->read < sizeof( struct nbd_reply_raw ) ) {
		// Haven't read the whole response yet
		return;
	}

	nbd_r2h_reply( &xfer->hdr.rsp_raw, &rsp );

	// validate reply, break event loop if bad
	if ( rsp.magic != REPLY_MAGIC ) {
		warn( "Bad reply magic from listener" );
		ev_break( loop, EVBREAK_ONE );
		return;
	}

	if ( rsp.error != 0 ) {
		warn( "Error returned from listener: %i", rsp.error );
		ev_break( loop, EVBREAK_ONE );
		return;
	}

	if ( memcmp( ".MIRROR.", &rsp.handle[0], 8 ) != 0 ) {
		warn( "Bad handle returned from listener" );
		ev_break( loop, EVBREAK_ONE );
		return;
	}

	/* transfer was completed, so now we need to either set up the next
	 * transfer of this pass, set up the first transfer of the next pass, or
	 * complete the migration */
	xfer->read = 0;
	xfer->written = 0;

	/* We don't account for bytes written in this mode, to stop high-throughput
	 * discs getting stuck in "drain the event queue!" mode forever
	 */
	if ( !ctrl->clear_events ) {
		m->all_dirty += xfer->len;
	}


	/* This next bit could take a little while, which is fine */
	ev_timer_stop( ctrl->ev_loop, &ctrl->timeout_watcher );

	/* Set up the next transfer, which may be offset + mirror_longest_write
	 * or an event from the bitset stream. When offset hits serve->size,
	 * xfers will be constructed solely from the event stream. Once our estimate
	 * of time left reaches a sensible number (or the event stream empties),
	 * we stop new clients from connecting, disconnect existing ones, then
	 * continue emptying the bitstream. Once it's empty again, we're finished.
	 */
	int next_xfer = mirror_setup_next_xfer( ctrl );
	debug( "next_xfer: %d", next_xfer );

	/* Regardless of time estimates, if there's no waiting transfer, we can start closing clients down. */
	if ( !ctrl->clients_closed && ( !next_xfer || server_mirror_eta( ctrl->serve ) < MS_CONVERGE_TIME_SECS ) ) {
		info( "Closing clients to allow mirroring to converge" );
		server_forbid_new_clients( ctrl->serve );
		server_close_clients( ctrl->serve );
		server_join_clients( ctrl->serve );
		ctrl->clients_closed = 1;

		/* One more try - a new event may have been pushed since our last check  */
		if ( !next_xfer ) {
			next_xfer = mirror_setup_next_xfer( ctrl );
		}
	}

	if ( ctrl->clients_closed && !next_xfer ) {
		mirror_complete( ctrl->serve );
		ev_break( loop, EVBREAK_ONE );
		return;
	}

	/* This is a guard Just In Case */
	ERROR_IF( !next_xfer, "Unknown problem - no next transfer to do!" );

	ev_io_stop( loop, &ctrl->read_watcher );

	/* FIXME: Should we ignore the bwlimit after server_close_clients has been called? */

	if ( mirror_should_wait( ctrl ) ) {
		/* We're over the bandwidth limit, so don't move onto the next transfer
		 * yet. Our limit_watcher will move us on once we're OK. timeout_watcher
		 * was disabled further up, so don't need to stop it here too */
		debug( "max_bps exceeded, waiting" );
		ev_timer_again( loop, &ctrl->limit_watcher );
	} else {
		/* We're waiting for the socket to become writable again, so re-enable */
		ev_timer_again( loop, &ctrl->timeout_watcher );
		ev_io_start( loop, &ctrl->write_watcher );
	}

	return;
}

static void mirror_timeout_cb( struct ev_loop *loop, ev_timer *w __attribute__((unused)), int revents )
{
	if ( !(revents & EV_TIMER ) ) {
		warn( "Mirror timeout called but no timer event signalled" );
		return;
	}

	info( "Mirror timeout signalled" );
	ev_break( loop, EVBREAK_ONE );
	return;
}

static void mirror_abandon_cb( struct ev_loop *loop, ev_io *w, int revents )
{
	struct mirror_ctrl* ctrl = (struct mirror_ctrl*) w->data;
	NULLCHECK( ctrl );

	if ( !(revents & EV_READ ) ) {
		warn( "Mirror abandon called but no abandon event signalled" );
		return;
	}

	debug( "Abandon message received" );
	mirror_set_state( ctrl->mirror, MS_ABANDONED );
	self_pipe_signal_clear( ctrl->mirror->abandon_signal );
	ev_break( loop, EVBREAK_ONE );
	return;
}


static void mirror_limit_cb( struct ev_loop *loop, ev_timer *w, int revents )
{
	struct mirror_ctrl* ctrl = (struct mirror_ctrl*) w->data;
	NULLCHECK( ctrl );

	if ( !(revents & EV_TIMER ) ) {
		warn( "Mirror limit callback executed but no timer event signalled" );
		return;
	}

	if ( mirror_should_wait( ctrl ) ) {
		debug( "max_bps exceeded, waiting", ctrl->mirror->max_bytes_per_second );
		ev_timer_again( loop, w );
	} else {
		/* We're below the limit, so do the next request */
		debug("max_bps not exceeded, performing next transfer" );
		ev_io_start( loop, &ctrl->write_watcher );
		ev_timer_stop( loop, &ctrl->limit_watcher );
		ev_timer_again( loop, &ctrl->timeout_watcher );
	}

	return;
}

/* We use this to periodically check whether the allocation map has built, and
 * if it has, start migrating. If it's not finished, then enabling the bitset
 * stream does not go well for us.
 */
static void mirror_begin_cb( struct ev_loop *loop, ev_timer *w, int revents )
{
	struct mirror_ctrl* ctrl = (struct mirror_ctrl*) w->data;
	NULLCHECK( ctrl );

	if ( !(revents & EV_TIMER ) ) {
		warn( "Mirror limit callback executed but no timer event signalled" );
		return;
	}

	if ( ctrl->serve->allocation_map_built || ctrl->serve->allocation_map_not_built ) {
		info( "allocation map builder is finished, beginning migration" );
		ev_timer_stop( loop, w );
		/* Start by writing xfer 0 to the listener */
		ev_io_start( loop, &ctrl->write_watcher );
		/* We want to timeout during the first write as well as subsequent ones */
		ev_timer_again( loop, &ctrl->timeout_watcher );
		/* We're now interested in events */
		bitset_enable_stream( ctrl->serve->allocation_map );
	} else {
		/* not done yet, so wait another second */
		ev_timer_again( loop, w );
	}

	return;
}

void mirror_run( struct server *serve )
{
	NULLCHECK( serve );
	NULLCHECK( serve->mirror );

	struct mirror *m = serve->mirror;

	m->migration_started = monotonic_time_ms();
	info("Starting mirror" );

	/* mirror_setup_next_xfer won't be able to cope with this, so special-case
	 * it here. There can't be any writes going on, so don't bother locking
	 * anything.
	 *
	 */
	if ( serve->size == 0 ) {
		info( "0-byte image special case" );
		mirror_complete( serve );
		return;
	}

	struct mirror_ctrl ctrl;
	memset( &ctrl, 0, sizeof( struct mirror_ctrl ) );

	ctrl.serve  = serve;
	ctrl.mirror = m;

	ctrl.ev_loop = EV_DEFAULT;

	/* gcc warns with -Wstrict-aliasing on -O2. clang doesn't
	 * implement this warning. Seems to be the fault of ev.h */
	ev_init( &ctrl.begin_watcher, mirror_begin_cb );
	ctrl.begin_watcher.repeat = 1.0; // We check bps every second. seems sane.
	ctrl.begin_watcher.data = (void*) &ctrl;

	ev_io_init( &ctrl.read_watcher, mirror_read_cb, m->client, EV_READ  );
	ctrl.read_watcher.data = (void*) &ctrl;

	ev_io_init( &ctrl.write_watcher, mirror_write_cb, m->client, EV_WRITE );
	ctrl.write_watcher.data = (void*) &ctrl;

	ev_init( &ctrl.timeout_watcher, mirror_timeout_cb );

	char * env_request_limit = getenv( "FLEXNBD_MS_REQUEST_LIMIT_SECS" );
	double timeout_limit = MS_REQUEST_LIMIT_SECS_F;

	if ( NULL != env_request_limit ) {
		char *endptr = NULL;
		errno = 0;
		double limit = strtod( env_request_limit, &endptr );
		warn( SHOW_ERRNO( "Got %f from strtod", limit ) );

		if ( errno == 0 ) {
			timeout_limit = limit;
		}
	}

	ctrl.timeout_watcher.repeat = timeout_limit;

	ev_init( &ctrl.limit_watcher, mirror_limit_cb );
	ctrl.limit_watcher.repeat = 1.0; // We check bps every second. seems sane.
	ctrl.limit_watcher.data = (void*) &ctrl;

	ev_init( &ctrl.abandon_watcher, mirror_abandon_cb );
	ev_io_set( &ctrl.abandon_watcher, m->abandon_signal->read_fd, EV_READ );
	ctrl.abandon_watcher.data = (void*) &ctrl;
	ev_io_start( ctrl.ev_loop, &ctrl.abandon_watcher );

	ERROR_UNLESS(
		mirror_setup_next_xfer( &ctrl ),
		"Couldn't find first transfer for mirror!"
	);


	if ( serve->allocation_map_built ) {
		/* Start by writing xfer 0 to the listener */
		ev_io_start( ctrl.ev_loop, &ctrl.write_watcher );
		/* We want to timeout during the first write as well as subsequent ones */
		ev_timer_again( ctrl.ev_loop, &ctrl.timeout_watcher );
		bitset_enable_stream( serve->allocation_map );
	} else {
		debug( "Waiting for allocation map to be built" );
		ev_timer_again( ctrl.ev_loop, &ctrl.begin_watcher );
	}

	/* Everything up to here is blocking. We switch to non-blocking so we
	 * can handle rate-limiting and weird error conditions better. TODO: We
	 * should expand the event loop upwards so we can do the same there too */
	sock_set_nonblock( m->client, 1 );

	info( "Entering event loop" );
	ev_run( ctrl.ev_loop, 0 );
	info( "Exited event loop" );

	/* Parent code might expect a non-blocking socket */
	sock_set_nonblock( m->client, 0 );


	/* Errors in the event loop don't track I/O lock state or try to restore
	 * it to something sane - they just terminate the event loop with state !=
	 * MS_DONE. We re-allow new clients here if necessary.
	 */
	if ( m->action_at_finish == ACTION_NOTHING || m->commit_state != MS_DONE ) {
		server_allow_new_clients( serve );
	}

	/* Returning here says "mirroring complete" to the runner. The error
	 * call retries the migration from scratch. */

	if ( m->commit_state != MS_DONE ) {
		/* mirror_reset will be called before a retry, so keeping hold of events
		 * between now and our next mirroring attempt is not useful
		 */
		bitset_disable_stream( serve->allocation_map );
		error( "Event loop exited, but mirroring is not complete" );
	}

	return;
}


void mbox_post_mirror_state( struct mbox * mbox, enum mirror_state st )
{
	NULLCHECK( mbox );
	enum mirror_state * contents = xmalloc( sizeof( enum mirror_state ) );

	*contents = st;

	mbox_post( mbox, contents );
}


void mirror_signal_commit( struct mirror * mirror )
{
	NULLCHECK( mirror );

	mbox_post_mirror_state( mirror->commit_signal,
			mirror_get_state( mirror ) );
}

/** Thread launched to drive mirror process
 * This is needed for two reasons: firstly, it decouples the mirroring
 * from the control thread (although that's less valid with mboxes
 * passing state back and forth) and to provide an error context so that
 * retries can be cleanly handled without a bespoke error handling
 * mechanism.
 * */
void* mirror_runner(void* serve_params_uncast)
{
	/* The supervisor thread relies on there not being any ERROR
	 * calls until after the mirror_signal_commit() call in this
	 * function.
	 * However, *after* that, we should call ERROR_* instead of
	 * FATAL_* wherever possible.
	 */
	struct server *serve = (struct server*) serve_params_uncast;

	NULLCHECK( serve );
	NULLCHECK( serve->mirror );
	struct mirror * mirror = serve->mirror;

	error_set_handler( (cleanup_handler *) mirror_cleanup, serve );

	info( "Connecting to mirror" );

	time_t start_time = time(NULL);
	int connected = mirror_connect( mirror, serve->size );
	mirror_signal_commit( mirror );
	if ( !connected ) { goto abandon_mirror; }

	/* After this point, if we see a failure we need to disconnect
	 * and retry everything from mirror_set_state(_, MS_INIT), but
	 * *without* signaling the commit or abandoning the mirror.
	 * */

	if ( (time(NULL) - start_time) > MS_CONNECT_TIME_SECS ){
		/* If we get here, then we managed to connect but the
		 * control thread feeding status back to the user will
		 * have gone away, leaving the user without meaningful
		 * feedback. In this instance, they have to assume a
		 * failure, so we can't afford to let the mirror happen.
		 * We have to set the state to avoid a race.
		 */
		mirror_set_state( mirror, MS_FAIL_CONNECT );
		warn( "Mirror connected, but too slowly" );
		goto abandon_mirror;
	}

	mirror_run( serve );

	/* On success, this is unnecessary, and harmless ( mirror_cleanup does it
	 * for us ). But if we've failed and are going to retry on the next run, we
	 * must close this socket here to have any chance of it succeeding.
	 */
	if ( !mirror->client < 0 ) {
		sock_try_close( mirror->client );
		mirror->client = -1;
	}

abandon_mirror:
	return NULL;
}


struct mirror_super * mirror_super_create(
		const char * filename,
		union mysockaddr * connect_to,
		union mysockaddr * connect_from,
		uint64_t max_Bps,
		enum mirror_finish_action action_at_finish,
		struct mbox * state_mbox)
{
	struct mirror_super * super = xmalloc( sizeof( struct mirror_super) );
	super->mirror = mirror_create(
			filename,
			connect_to,
			connect_from,
			max_Bps,
			action_at_finish,
			mbox_create() ) ;
	super->state_mbox = state_mbox;
	return super;
}


/* Post the current state of the mirror into super->state_mbox.*/
void mirror_super_signal_committed(
		struct mirror_super * super ,
		enum mirror_state commit_state )
{
	NULLCHECK( super );
	NULLCHECK( super->state_mbox );

	mbox_post_mirror_state(
			super->state_mbox,
			commit_state );
}


void mirror_super_destroy( struct mirror_super * super )
{
	NULLCHECK( super );

	mbox_destroy( super->mirror->commit_signal );
	mirror_destroy( super->mirror );
	free( super );
}


/* The mirror supervisor thread.  Responsible for kicking off retries if
 * the mirror thread fails.
 * The mirror and mirror_super objects are never freed, and the
 * mirror_super_runner thread is never joined.
 */
void * mirror_super_runner( void * serve_uncast )
{
	struct server * serve = (struct server *) serve_uncast;
	NULLCHECK( serve );
	NULLCHECK( serve->mirror );
	NULLCHECK( serve->mirror_super );

	int first_pass = 1;
	int should_retry = 0;
	int success = 0, abandoned = 0;

	struct mirror * mirror = serve->mirror;
	struct mirror_super *  super  = serve->mirror_super;

	do {
		FATAL_IF( 0 != pthread_create(
					&mirror->thread,
					NULL,
					mirror_runner,
					serve),
				"Failed to create mirror thread");

		debug("Supervisor waiting for commit signal");
		enum mirror_state * commit_state =
			mbox_receive( mirror->commit_signal );

		debug( "Supervisor got commit signal" );
		if ( first_pass ) {
			/* Only retry if the connection attempt was successful.  Otherwise
			 * the user will see an error reported while we're still trying to
			 * retry behind the scenes. This may race with migration completing
			 * but since we "shouldn't retry" in that case either, that's fine
			 */
			should_retry = *commit_state == MS_GO;

			/* Only send this signal the first time */
			mirror_super_signal_committed(
					super,
					*commit_state);
			debug("Mirror supervisor committed");
		}
		/* We only care about the value of the commit signal on
		 * the first pass, so this is ok
		 */
		free( commit_state );

		debug("Supervisor waiting for mirror thread" );
		pthread_join( mirror->thread, NULL );

		/* If we can't connect to the remote end, the watcher for the abandon
		 * signal never gets installed at the moment, which is why we also check
		 * it here. */
		abandoned =
		  mirror_get_state( mirror ) == MS_ABANDONED ||
		  self_pipe_signal_clear( mirror->abandon_signal );

		success = MS_DONE == mirror_get_state( mirror );


		if( success ){
			info( "Mirror supervisor success, exiting" );
		} else if ( abandoned ) {
			info( "Mirror abandoned" );
			should_retry = 0;
		} else if ( should_retry ) {
			info( "Mirror failed, retrying" );
		} else {
			info( "Mirror failed before commit, giving up" );
		}

		first_pass = 0;

		if ( should_retry ) {
			/* We don't want to hammer the destination too
			 * hard, so if this is a retry, insert a delay. */
			sleep( MS_RETRY_DELAY_SECS );

			/* We also have to reset the bitmap to be sure
			 * we transfer everything */
			mirror_reset( mirror );
		}

	}
	while ( should_retry && !success );

	return NULL;
}