False clang warning from -Wthread-safety-analysis

[gperciva]

Notes mainly to myself.

Clang's thread safety analysis warns that it cannot handle "conditionally held locks" [1]. They give the example of:

bool b = needsToLock();
if (b) mu.Lock();
...  // Warning!  Mutex 'mu' is not held on every path through here.
if (b) mu.Unlock();

This is rather unfortunate, because phread_mutex_lock() is documented as being able to fail and set errno [2]. Well-written code will therefore not assume that the lock is successful, and will instead check the returned value and jump to error handling if necessary. In other words, well-written code will include "conditionally held locks".

[1] https://clang.llvm.org/docs/ThreadSafetyAnalysis.html#conditional-locks
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_trylock.html

(BTW, we already have -Wno-pthread-safety-analys in spiped, for precisely this reason; the false warning arises from https://github.com/Tarsnap/spiped/blob/master/lib/dnsthread/dnsthread.c#L176)