[FFmpeg-devel] [PATCH 1/2] avformat/network: fix timeout inaccurate in wait_fd_timeout

["zhilizhao(赵志立)"]

> On Feb 9, 2021, at 6:03 AM, Marton Balint <cus at passwd.hu> wrote:
> 
>> 
>> The wait_start was about POLLING_TIME larger which leads to timeout
>> 100ms late than the option setting.
>> ---
>> libavformat/network.c | 13 ++++++-------
>> 1 file changed, 6 insertions(+), 7 deletions(-)
>> 
>> diff --git a/libavformat/network.c b/libavformat/network.c
>> index 0f5a575f77..7a9a4be5bb 100644
>> --- a/libavformat/network.c
>> +++ b/libavformat/network.c
>> @@ -78,7 +78,10 @@ int ff_network_wait_fd(int fd, int write)
>> int ff_network_wait_fd_timeout(int fd, int write, int64_t timeout, AVIOInterruptCB *int_cb)
>> {
>>    int ret;
>> -    int64_t wait_start = 0;
>> +    int64_t wait_start;
>> +
>> +    if (timeout > 0)
>> +        wait_start = av_gettime_relative();
> 
> I think we intentionally wanted to avoid calling gettime on the fast path.
> 
>> 
>>    while (1) {
>>        if (ff_check_interrupt(int_cb))
>> @@ -86,12 +89,8 @@ int ff_network_wait_fd_timeout(int fd, int write, int64_t timeout, AVIOInterrupt
>>        ret = ff_network_wait_fd(fd, write);
>>        if (ret != AVERROR(EAGAIN))
>>            return ret;
>> -        if (timeout > 0) {
>> -            if (!wait_start)
>> -                wait_start = av_gettime_relative();
> 
> Why not simply wait_start = av_gettime_relative() - POLLING_TIME? It seems to achieve the same result.

Then it depends on the implementation of ff_network_wait_fd. After adding wait_fd as a callback,
there is no guarantee that it takes POLLING_TIME. It can be solved by adding a polling_time field
to NetworkWaitFdCB:

typedef struct NetworkWaitFdCB {
    int (*wait_fd)(int /*fd*/, int /*write*/, void* /*opaque*/);
    void *opaque;
    int polling_time;
} NetworkWaitFdCB;

This is trying to get rid of calling gettime once. We can go further and sacrifice the accuracy by:

int64_t loop_count = (timeout > 0) ? (timeout / polling_time) : timeout;
for (int64_t i = 0; loop_count <= 0 || i < loop_count; i++) {
    ...
}

In my opinion, either depends on gettime or loop_count, but not both.

> 
> Thanks,
> Marton
> 
>> -            else if (av_gettime_relative() - wait_start > timeout)
>> -                return AVERROR(ETIMEDOUT);
>> -        }
>> +        if (timeout > 0 && (av_gettime_relative() - wait_start > timeout))
>> +            return AVERROR(ETIMEDOUT);
>>    }
>> }
>> -- 
>> 2.28.0
>> 
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