Fix crash issue at srs_app_encoder

[HungMingWu]

Root Cause:
On SrsEncoder::on_publish, if ret is equal to ERROR_ENCODER_LOOP and ffmpegs is empty.
It wouldn't create a trd object.
On SrsEncoder::on_unpublish:
Since the trd is NULL, it would crash.

[winlinvip]

I think it's better to create a dummy thread for classes like the SrsEncoder which need to create thread dynamically, for example:

SrsEncoder() {
    trd = new SrsDummyCoroutine();
}
~SrsEncoder() {
    srs_freep(trd);
}
int publish() {
    srs_freep(trd);
    trd = new SrsCoroutine();
    trd->start();
}
void unpublish() {
    srs_freep(trd);
}

class SrsDummyCoroutine {
    SrsDummyCoroutine() {
    }
    int start() {
        return ErrorEmptyObject;
    }
    void stop() {
    }
}

I recall it's a common design pattern named Empty Object. An empty object is always better than a NULL pointer, because the object has some behaviors, like the trd->stop(). In golang, all objects are initialized with zero values, like this pattern.

[winlinvip]

Please fix this issue at all places that use trd = NULL, we should use the dummy thread instead, which simply return failed when start and ignore when stop.

[HungMingWu]

This is my opinion. 1. 'trd = new SrsDummyThread();' appears twice, one is on constructor, and the other is on on_publish. On engineering's perceive, don't repeat yourself. 2. If you create a dummy thread, you gains the overhead on a constructor and a destructor. If you insist on the direction on dummy thread, I will follow your idea. 2017-06-03 17:07 GMT+08:00 winlin <notifications@github.com>:

Please fix this issue for all places use trd = NULL, use the dummy thread instead, which simply return failed when start and ignore when stop. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#908 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AA-lNEbvrUwGCaRKjO6O8qSSVNk51i_Bks5sASJkgaJpZM4Nt8-O> .

`TRANS_BY_GPT3`

[winlinvip]

That's sure the constructor is called multiple times in on_publish, but I think it's fine because it wouldn't hurt the performance(these functions is rarely called). Please use the empty object to solve this problem, thanks~

[HungMingWu]

I have update the commit.

[winlinvip]

Hi, thanks for your work, but I fixed it in 9ca3697