Issue in pcm_dsnoop.c in alsa-lib

[GitHub issues - edited <github@alsa-project.org>]

alsa-project/alsa-lib issue #213 was edited from TE-N-ShengjiuWang:

Hi Takashi Iwai, Jaroslav Kysela

We encountered an issue in the pcm_dsnoop use case, could you please help to have a look? 

Issue description:
With two instances for dsnoop type device running in parallel, after suspend/resume,  one of the instances will be hung in memcpy because the very large copy size is obtained.

```
#3 0x0000ffffa78d5098 in snd_pcm_dsnoop_sync_ptr (pcm=0xaaab06563da0)
at pcm_dsnoop.c:158
dsnoop = 0xaaab06563c20
slave_hw_ptr = 64
old_slave_hw_ptr = 533120
avail = 187651522444320
```

   Reason analysis: 
   The root cause that I analysis is that after suspend/resume,  one instance will get the SND_PCM_STATE_SUSPENDED state from slave pcm device,   then it will do snd_pcm_prepare() and snd_pcm_start(),  which will reset the dsnoop->slave_hw_ptr and the hw_ptr of slave pcm device,  then the state of this instance is correct.  But another instance may not get the SND_PCM_STATE_SUSPENDED state from slave pcm device because slave device may have been recovered by first instance,  so the dsnoop->slave_hw_ptr is not reset.  but because hw_ptr of slave pcm device has been reset,  so there will be a very large "avail" size. 

   Solution:
   I didn't come up with a fix for this issue,  seems there is no easy way to let another instance know this case and reset the  dsnoop->slave_hw_ptr,  could you please help?

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/213
Repository URL: https://github.com/alsa-project/alsa-lib