alsa-lib: snd_pcm_plugin_status() recently broken

[Mark Hills <mark@xwax.org>]

The following assertion:

  relay: pcm_plugin.c:570: snd_pcm_plugin_status: Assertion `status->appl_ptr == *pcm->appl.ptr' failed.

It bisects to 28cc099, the introduction of that assertion.

  #4  0x00007ffff7d0dd03 in snd_pcm_plugin_status (pcm=0x633560, 
      status=0x7fffffffe1e0) at pcm_plugin.c:570
  570                     assert(status->appl_ptr == *pcm->appl.ptr);
  (gdb) print status->appl_ptr
  $1 = 0
  (gdb) print pcm->appl.ptr
  $2 = (volatile snd_pcm_uframes_t *) 0x6334f8
  (gdb) print *pcm->appl.ptr
  $3 = 47936

The calling code is single threaded on a dmix device, with these 
operations in order:

  snd_pcm_avail_update();
  snd_pcm_status();
  snd_pcm_mmap_begin();

Commenting out the assertion leaves me with a credible result from the 
function as it did previously; snd_pcm_status_get_delay() == 64.

-- 
Mark


From 28cc099d9ea3962b033cb1cb9c3e07db828d9ff7 Mon Sep 17 00:00:00 2001
From: Jaroslav Kysela <perex@perex.cz>
Date: Sun, 3 Jan 2021 16:41:29 +0100
Subject: [PATCH] pcm: plugin - optimize sync in snd_pcm_plugin_status()

Do hw_ptr sync only once after the status call. This avoids
double update.

Also, the application pointer must not change when
the status is called. Add assert() call for this condition.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
---
 src/pcm/pcm_plugin.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/pcm/pcm_plugin.c b/src/pcm/pcm_plugin.c
index 83793397..6a815145 100644
--- a/src/pcm/pcm_plugin.c
+++ b/src/pcm/pcm_plugin.c
@@ -553,14 +553,11 @@ static int snd_pcm_plugin_status(snd_pcm_t *pcm, snd_pcm_status_t * status)
 	snd_pcm_plugin_t *plugin = pcm->private_data;
 	snd_pcm_sframes_t err;
 
-	/* sync with the latest hw and appl ptrs */
-	snd_pcm_plugin_avail_update(pcm);
-
 	err = snd_pcm_status(plugin->gen.slave, status);
 	if (err < 0)
 		return err;
-	status->appl_ptr = *pcm->appl.ptr;
-	status->hw_ptr = *pcm->hw.ptr;
+	assert(status->appl_ptr == *pcm->appl.ptr);
+	snd_pcm_plugin_sync_hw_ptr(pcm, status->hw_ptr, status->avail);
 	return 0;
 }
 
-- 
2.17.5