snd_pcm_drain stucked.

[GitHub issues - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-lib issue #457 was opened from yangyagami:

When snd_pcm_drain is called, if snd_pcm_drop is called in another thread, snd_pcm_drain will get stuck.
```cpp
#include <iostream>
#include <cmath>
#include <thread>

#include <alsa/asoundlib.h>

#define SAMPLE_RATE 44100
#define FREQUENCY 440.0 // A4 note
#define DURATION 5.0    // seconds

using namespace std::chrono_literals;

int main() {
    snd_pcm_t *handle;
    snd_pcm_hw_params_t *params;
    unsigned int rate = SAMPLE_RATE;
    int dir;
    int pcm;

    pcm = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
    if (pcm < 0) {
        std::cerr << "Unable to open PCM device: " << snd_strerror(pcm) << std::endl;
        return 1;
    }

    snd_pcm_hw_params_alloca(&params);
    snd_pcm_hw_params_any(handle, params);

    snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
    snd_pcm_hw_params_set_rate_near(handle, params, &rate, &dir);
    snd_pcm_hw_params_set_channels(handle, params, 1);

    pcm = snd_pcm_hw_params(handle, params);
    if (pcm < 0) {
        std::cerr << "Unable to set HW parameters: " << snd_strerror(pcm) << std::endl;
        return 1;
    }

    int samples = static_cast<int>(DURATION * SAMPLE_RATE);
    int16_t *buffer = new int16_t[samples];

    for (int i = 0; i < samples; ++i) {
        buffer[i] = static_cast<int16_t>(32767 * sin(2 * M_PI * FREQUENCY * i / SAMPLE_RATE));
    }

    pcm = snd_pcm_writei(handle, buffer, samples);
    if (pcm < 0) {
        std::cerr << "Playback error: " << snd_strerror(pcm) << std::endl;
    }

    std::thread t([handle](){
      std::this_thread::sleep_for(500ms);
      snd_pcm_drop(handle);
    });

    snd_pcm_drain(handle);

    t.join();

    delete[] buffer;
    snd_pcm_close(handle);

    return 0;
}
```

gdb output
```shell
Thread 1 "main" received signal SIGINT, Interrupt.
0x00007ffff71b5e22 in ?? () from /usr/lib/libc.so.6
(gdb) bt
#0  0x00007ffff71b5e22 in ?? () from /usr/lib/libc.so.6
#1  0x00007ffff71a9fda in ?? () from /usr/lib/libc.so.6
#2  0x00007ffff71aa64c in ?? () from /usr/lib/libc.so.6
#3  0x00007ffff71acd1e in pthread_cond_wait () from /usr/lib/libc.so.6
#4  0x00007ffff6b9e2b9 in pw_thread_loop_wait () from /usr/lib/libpipewire-0.3.so.0
#5  0x00007ffff7f2ec8f in ?? () from /usr/lib/alsa-lib/libasound_module_pcm_pipewire.so
#6  0x00007ffff777c2b8 in ?? () from /usr/lib/libasound.so.2
#7  0x0000555555558d6f in main () at main.cc:116
(gdb) 
```

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/457
Repository URL: https://github.com/alsa-project/alsa-lib