[Ovmsdev] MDNS crash

[Mark Webb-Johnson]

Messy. The issue is the restarting of mdns, coupled with AP/STA coming up/down in different threads. That could result in a mdns_free while another mdns_free or mdns_init is in progress. The mdns library is not thread safe. In particular, they mutex protect some internal stuff but not all of mdns_free or any of mdns_init.

I introduced an ovms_mutex library to main. This contains some helpful wrapper functions to make mutexes easy and safe to use:

class OvmsMutex
Encapsulates a mutex.
Provides Lock() and Unlock() functions to access it.

class OvmsMutexLock
Encapsulates a mutex lock in a safer manner.
Constructor locks it, and destructor unlocks it.
Using this is as simple as creating a mutex for the object to be protected, and then creating a OvmsMutexLock(&mutex) over that. When the OvmsMutexLock comes in scope, it will lock the mutex, and when it goes out of scope it will unlock it.

I’ve done this in other projects, with great success. Using OvmsMutexLock objects means you never forget to unlock the mutex (no matter how weird and wonderful your code paths are).

I then used OvmsMutex and OvmsMutexLock in ovms_mdns. The mdns stuff seems ok for me now.

Regards, Mark.

> On 19 Mar 2018, at 9:51 AM, Mark Webb-Johnson <mark at webb-johnson.net> wrote:
> 
> 
> It seems that mdns_free is being called, while mdns_free is running…
> 
> It is trivial to fix it in the timer deletion code, but that just delays the problem until a bit later (the hostname free).
> 
> I am looking at this now.
> 
> Regards, Mark.
> 
>> On 19 Mar 2018, at 9:40 AM, Stephen Casner <casner at acm.org> wrote:
>> 
>> On Sun, 18 Mar 2018, Michael Balzer wrote:
>> 
>>> From a first look at the mdns code in your backtrace, I would guess
>>> that's a double free() for the timer, it seems the timer_handle
>>> never gets NULLed.
>> 
>> That may be a bug (deficiency) in the esp-idf mDNS code.
>> 
>> Perhaps I hit it because OvmsMDNS::StopMDNS was called without
>> everything coming up correctly first?
>> 
>>                                                       -- Steve
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