samples: bluetooth: central_iso: unchecked buffer allocation

[msmttchr]

Describe the bug
In central_iso sample (function iso_timer_timeout), no check on buffer allocation failure is performed.
See code snippet:

zephyr/samples/bluetooth/central_iso/src/main.c

Lines 62 to 68 in d9b3ffa

	buf = net_buf_alloc(&tx_pool, K_FOREVER);
	net_buf_reserve(buf, BT_ISO_CHAN_SEND_RESERVE);
	net_buf_add_mem(buf, buf_data, len_to_send);
	ret = bt_iso_chan_send(&iso_chan, buf, seq_num++);

As a consequence, a system crash is observed when buffer allocation fails.

To Reproduce
Not so easy to reproduce and it depends on the buffer allocation policy.
Error observed while running a new platform against Nordic nRF52840 board.

Expected behavior
Sample should be robust against buffer allocation failure, e.g. throwing an error or a warning an retrying.

Impact
Annoyance, especially when validating new platform prone to have genuine bugs.

Logs and console output
Zephyr crash log.

Environment (please complete the following information):

• OS: Windows
• Toolchain Zephyr SDK

Additional context
Actually, from quick check, having K_FOREVER as timeout should avoid to get NULL as return value, but indeed this observed.

[cvinayak]

Please provide details on how to build and test in the To Reproduce section, and also mention the commit SHA used to build.

Actually, from quick check, having K_FOREVER as timeout should avoid to get NULL as return value, but indeed this observed.

Lowering the priority of the issue, as steps to reproduce is unclear.

[msmttchr]

It looks like related/introduced to/by #71697

[jori-nordic]

Yep. A bunch of (mostly audio or iso-related) samples do this.

Samples should've been programmed better, and not block the whole syswq by allocating with K_FOREVER in a callback.
I was not familiar enough with them to propose a patch when I did #71697 . Maybe someone else is?

Note that getting NULL was probably possible before:
https://github.com/zephyrproject-rtos/zephyr/pull/71697/files#diff-7d20cda516d057def150f03b4d2cdce79dc768dbc8ec7c3fab9d463265572466L1322-L1325