[Spice-devel] [PATCH spice-gtk v2] spice-gtk: Use time comparisons that still work after wraparound

[Christophe Fergeau]

On Mon, Jul 17, 2017 at 02:00:01PM +0200, Christophe de Dinechin wrote:
> 
> > On 17 Jul 2017, at 13:49, Christophe Fergeau <cfergeau at redhat.com <mailto:cfergeau at redhat.com>> wrote:
> > 
> > On Mon, Jul 17, 2017 at 11:44:04AM +0200, Christophe de Dinechin wrote:
> >> From: Christophe de Dinechin <dinechin at redhat.com <mailto:dinechin at redhat.com>>
> >> 
> >> The mm timer is a millisecond timer that wraps around after ~49 days.
> >> All comparisons that look like a<b will fail once this happens.
> >> Instead, use signed ((int)(a-b)<0), which may fail if there is more than
> >> 25 days between a and b (should not be happening under normal conditions),
> >> but is robust to the timer wrapping around.
> >> 
> >> Signed-off-by: Christophe de Dinechin <dinechin at redhat.com <mailto:dinechin at redhat.com>>
> >> ---
> >> src/channel-display-gst.c   | 4 ++--
> >> src/channel-display-mjpeg.c | 4 ++--
> >> src/channel-display.c       | 2 +-
> >> src/channel-playback.c      | 2 +-
> >> src/spice-channel-priv.h    | 2 ++
> >> src/spice-session.c         | 4 ++--
> >> 6 files changed, 10 insertions(+), 8 deletions(-)
> >> 
> >> diff --git a/src/channel-display-gst.c b/src/channel-display-gst.c
> >> index adcd6ef..3f51361 100644
> >> --- a/src/channel-display-gst.c
> >> +++ b/src/channel-display-gst.c
> >> @@ -166,7 +166,7 @@ static void schedule_frame(SpiceGstDecoder *decoder)
> >>             break;
> >>         }
> >> 
> >> -        if (now < gstframe->frame->mm_time) {
> >> +        if (spice_mmtime_diff(now, gstframe->frame->mm_time) < 0) {
> >>             decoder->timer_id = g_timeout_add(gstframe->frame->mm_time - now,
> >>                                               display_frame, decoder);
> >>         } else if (g_queue_get_length(decoder->display_queue) == 1) {
> >> @@ -511,7 +511,7 @@ static gboolean spice_gst_decoder_queue_frame(VideoDecoder *video_decoder,
> >>         return TRUE;
> >>     }
> >> 
> >> -    if (frame->mm_time < decoder->last_mm_time) {
> >> +    if (spice_mmtime_diff(frame->mm_time, decoder->last_mm_time) < 0) {
> >>         SPICE_DEBUG("new-frame-time < last-frame-time (%u < %u):"
> >>                     " resetting stream",
> >>                     frame->mm_time, decoder->last_mm_time);
> >> diff --git a/src/channel-display-mjpeg.c b/src/channel-display-mjpeg.c
> >> index ee33b01..3ba098c 100644
> >> --- a/src/channel-display-mjpeg.c
> >> +++ b/src/channel-display-mjpeg.c
> >> @@ -201,7 +201,7 @@ static void mjpeg_decoder_schedule(MJpegDecoder *decoder)
> >>     decoder->cur_frame = NULL;
> >>     do {
> >>         if (frame) {
> >> -            if (time <= frame->mm_time) {
> >> +            if (spice_mmtime_diff(time, frame->mm_time) <= 0) {
> >>                 guint32 d = frame->mm_time - time;
> >>                 decoder->cur_frame = frame;
> >>                 decoder->timer_id = g_timeout_add(d, mjpeg_decoder_decode_frame, decoder);
> >> @@ -251,7 +251,7 @@ static gboolean mjpeg_decoder_queue_frame(VideoDecoder *video_decoder,
> >> 
> >>     last_frame = g_queue_peek_tail(decoder->msgq);
> >>     if (last_frame) {
> >> -        if (frame->mm_time < last_frame->mm_time) {
> >> +        if (spice_mmtime_diff(frame->mm_time, last_frame->mm_time) < 0) {
> >>             /* This should really not happen */
> >>             SPICE_DEBUG("new-frame-time < last-frame-time (%u < %u):"
> >>                         " resetting stream",
> >> diff --git a/src/channel-display.c b/src/channel-display.c
> >> index 3c98d79..06ed18a 100644
> >> --- a/src/channel-display.c
> >> +++ b/src/channel-display.c
> >> @@ -1366,7 +1366,7 @@ static void display_update_stream_report(SpiceDisplayChannel *channel, uint32_t
> >>     }
> >> 
> >>     if (st->report_num_frames >= st->report_max_window ||
> >> -        now - st->report_start_time >= st->report_timeout ||
> >> +        spice_mmtime_diff(now - st->report_start_time, st->report_timeout) >= 0 ||
> >>         st->report_drops_seq_len >= STREAM_REPORT_DROP_SEQ_LEN_LIMIT) {
> >>         SpiceMsgcDisplayStreamReport report;
> >>         SpiceSession *session = spice_channel_get_session(SPICE_CHANNEL(channel));
> >> diff --git a/src/channel-playback.c b/src/channel-playback.c
> >> index ca14b96..afc9059 100644
> >> --- a/src/channel-playback.c
> >> +++ b/src/channel-playback.c
> >> @@ -313,7 +313,7 @@ static void playback_handle_data(SpiceChannel *channel, SpiceMsgIn *in)
> >>                   packet->time, packet->data, packet->data_size);
> >> #endif
> >> 
> >> -    if (c->last_time > packet->time)
> >> +    if (spice_mmtime_diff(c->last_time, packet->time) > 0)
> >>         g_warn_if_reached();
> >> 
> >>     c->last_time = packet->time;
> >> diff --git a/src/spice-channel-priv.h b/src/spice-channel-priv.h
> >> index 7288920..20d3495 100644
> >> --- a/src/spice-channel-priv.h
> >> +++ b/src/spice-channel-priv.h
> >> @@ -43,6 +43,8 @@ G_BEGIN_DECLS
> >> #define CHANNEL_DEBUG(channel, fmt, ...) \
> >>     SPICE_DEBUG("%s: " fmt, SPICE_CHANNEL(channel)->priv->name, ## __VA_ARGS__)
> >> 
> >> +#define spice_mmtime_diff(now, time)            ((int32_t) ((now)-(time)))
> > 
> > Nit: I think the 'now' and 'time' naming here could be misleading, what
> > about just "mmtime0" and "mmtime1" for example?
> 
> Well, that was to match the typical usage where ‘now’ would be the
> current time, and ‘time’ would be some other reference time. To me,
> that’s more readable that way. But YMMV…

Dunno, looking at spice_mmtime_diff(c->last_time, packet->time), none of
the args strike me as the obvious "current" time. And the function is
generic, spice_mmtime_diff(old_time, current_time) would work equally
well as spice_mmtime_diff(current_time, old_time). So the "now" name
seems very artificial to me.

Christophe
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