[vlc-devel] [PATCH] video_output: explicit vlc_testcancel

[Thomas Guillem]

On Wed, Feb 26, 2020, at 15:34, Quentin Chateau wrote:
> Thanks for the hindsight, I dug further:

> vlc_cond_timedwait is called, and so is pthread_cond_timedwait. The issue happens when the deadline parameter is in the past: when this happens, pthread_cond_timedwait does not act as a cancellation point anymore...

> In my case deadline happens to be in the past because ThreadDisplayPicture keeps failing (due to one of my video filters failing on the last frame)

> I could not find this behavior in the pthread documentation, but it seems to be the case anyway. Do you think that we need to:

>  1. Fix vlc_cond_timedwait so it will check for cancellation if the deadline is in the past
>  2. Acknowledge the fact that vlc_cond_timedwait is not always a cancellation point and fix the issue higher in the stack, either in vout_control_Pop, or similarly to the patch I sent, directly in Thread

Or 3. Investigate why deadline is in the past.

Rémi, any preference ?

> 

> On 26/02/2020 11:11, Thomas Guillem wrote:
>> Why is it not canceled from vlc_cond_timedwait() called from vout_control_Pop() ?

Is ctrl->can_sleep false in your case ?

Anyway, I would prefer a fix in vout_control_Pop() like this:

    if (ctrl->cmd.i_size <= 0 && deadline != VLC_TICK_INVALID && ctrl->can_sleep) {
        /* Spurious wakeups are perfectly fine */
        ctrl->is_waiting = true;
        vlc_cond_signal(&ctrl->wait_available);
        vlc_cond_timedwait(&ctrl->wait_request, &ctrl->lock, deadline);
        ctrl->is_waiting = false;
    }
    else
        vlc_testcancel();

On Tue, Feb 25, 2020, at 12:47, quentin.chateau at deepskycorp.com wrote:
>> 
>>> From: Quentin Chateau <quentin.chateau at deepskycorp.com>

When ThreadDisplayPicture does not return VLC_SUCCESS,
explicitely check for thread cancellation.

There are cases where a video filter can fail
systematically without hitting any cancellation point
after the video output thread is cancelled, resulting
in an infinite loop if there is no explicit check.
---
 src/video_output/video_output.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/video_output/video_output.c b/src/video_output/video_output.c
index 229b010a96..f7aac554e4 100644
--- a/src/video_output/video_output.c
+++ b/src/video_output/video_output.c
@@ -1703,6 +1703,7 @@ noreturn static void *Thread(void *object)
 
         if (wait)
         {
+            vlc_testcancel();
             const vlc_tick_t max_deadline = vlc_tick_now() + 
VLC_TICK_FROM_MS(100);
             deadline = deadline == VLC_TICK_INVALID ? max_deadline : 
__MIN(deadline, max_deadline);
         } else {
-- 
2.17.1

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