[OpenThread]: When using WiFi DNS and Openthread DNS64 in the same code, an LwIP DNS Hook fail may happen (IDFGH-14276)

[SuGlider]

Answers checklist.

• I have read the documentation ESP-IDF Programming Guide and the issue is not addressed there.
• I have updated my IDF branch (master or release) to the latest version and checked that the issue is present there.
• I have searched the issue tracker for a similar issue and not found a similar issue.

General issue report

@wqx6 -

The issue is related to commit f62628d#diff-c64cd8bea4c6aa81e0c356a5f4e5dd0f4f8d9507fb7efee6a11d47f3dfcb3d76R58

Lines 58 and 64 have been added to use switching_lock Mutex.
Whenever the application tries to resolve DNS using, for instance WiFi, and OpenThread has not been initialized or has been deinitialized, the MUTEX will be NULL, therefore, lwip_hook_dns_external_resolve() that is called for any DNS query, will fail with these messages:

E (36035) OPENTHREAD: esp_openthread_task_switching_lock_acquire(38): Failed to acquire the lock because the mutex is not ready
E (36035) OPENTHREAD: esp_openthread_task_switching_lock_release(46): Failed to release the lock because the mutex is not ready
E (36046) OT_DNS64: Cannot find NAT64 prefix

This is impacting ESP 32 Arduino Framework:
espressif/arduino-esp32#10754

The error messages are caused by esp_openthread_task_switching_lock_acquire(portMAX_DELAY); and esp_openthread_task_switching_lock_release(); calls in esp_openthread_get_nat64_prefix(), which is called from lwip_hook_dns_external_resolve()

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A proposed solution would be to first check if OpenThread stack is Active before performing any operation in the lwip_hook_dns_external_resolve() routine.