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Lifecycle checks

To assist developers in catching some simple mistakes, there are a couple of checks which are run by fbl:: containers when node state structure and containers are destroyed. They are implemented using ZX_DEBUG_ASSERT and are therefor only present when debug asserts are enabled.

First, it is illegal for a node state structure to be destroyed while it exists in a container. Doing so is considered to be an error and will trigger an assert. Always make sure that objects are not allowed to be destroyed while still in a container. This is usually fairly easy to arrange when using managed pointers to track object lifecycle as objects in a container will need to have given ownership of at least one object reference to the container, and so the object cannot destruct until it leaves the container and can have its final reference go away.

Second, it is illegal for a container of unmanaged pointers to be destroyed while there are still objects in it. Containers of managed pointers will automatically clear() themselves upon destruction dropping the managed references that they hold, but containers of unmanaged pointers will not do the same. If there are still unmanaged pointers in the container when it is destroyed, it is likely that this was an error and that memory may have been leaked. Therefore, allowing a non-empty container of unmanaged pointers to be destroyed will trigger an assert.