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The isc_refcount_increment0() does two things and that's wrong.

1. The first purpose is to bump the value from 0 -> 1 making the object referenced.

2. The second purpose is to increment the reference counter.

This has several problems:

1. You can't check whether the previous value really was 0.
2. When object becomes dereferenced with isc_refcount_decrement() == 1, the isc_refcount_increment0() can make it referenced again while destroying the object.

There are two things that we could do about it:

1. Don't use isc_refcount API when it's not reference counting, prepare similar API for object counting (isc_objcount?)
2. Use isc_refcount_init() when initializing object for the first time (note that isc_refcount_init() is not atomic)
3. Always initialize the value to 1 and adjust the code that destroys the object
4. Split isc_refcount_increment0() to isc_refcount_incfirst() and existing isc_refcount_increment()

Nevertheless, the overloading of the API for <1, MAX> and <0, MAX> reference counting is wrong.