_0_NoReentryScope.m

void _NoReentryScopeEnter(BOOL *aVar)
{
    if (*aVar)
        [NSException raise:NSInternalInconsistencyException format:@"the method is not reentrable"];

    *aVar = YES;
}

void _NoReentryScopeLeave(BOOL **aVar)
{
    **aVar = NO;
}

#define _NoReentryScope(var, aVarPtr) \
__attribute__((__cleanup__(_NoReentryScopeLeave))) BOOL *var = aVarPtr; \
_NoReentryScopeEnter(var);

#define NoReentryScope(aVarPtr) _NoReentryScope(OS_CONCAT(NoReentryScope, __COUNTER__), aVarPtr)

_1_Example.m

@implementation MyObject
{
    BOOL _canary;
}

- (void)stateMutation
{
    NoReentryScope(&_canary);
    // Mutate State
}

- (void)stateMutationWithKVO
{
    {
        NoReentryScope(&_canary);
        [self willChangeValueForKey:@"..."]; // calling -stateMutation / stateMutationWithKVO is an error
    }
    
    // Mutate the state
    
    {
        NoReentryScope(&_canary);
        [self didChangeValueForKey:@"..."]; // calling -stateMutation / stateMutationWithKVO is an error
    }
    
    // Continue State Mutation
}

@end