RFR: 8135322: ConstantPool::release_C_heap_structures not run in some circumstances

[Andreas Eriksson]

Hi,

Please review this fix for 8135322: 
ConstantPool::release_C_heap_structures not run in some circumstances.

Bug: https://bugs.openjdk.java.net/browse/JDK-8135322
Webrev: http://cr.openjdk.java.net/~aeriksso/8135322/webrev.00

The way this occurs is:

 1. A ConstantPool, /merge_cp/, without a corresponding InstanceKlass is
    created during class redefinition.
 2. /Merge_cp/ is added to ClassLoaderData::_deallocate_list, but is not
    cleaned up by ClassLoaderDataGraph::do_unloading because the holding
    ClassLoaderData is already dead.
 3. ClassLoaderData::~ClassLoaderData is called, which usually runs
    InstanceKlass::release_C_heap_structures, which in turn will call
    ConstantPool::release_C_heap_structures. But since there is no
    corresponding InstanceKlass for /merge_cp/ it will never release its
    C heap structures.
 4. For JDK 8 this means we leak a Monitor (ConstantPool::_lock). For
    JDK 9 the Monitor has been removed, but we will miss doing a call to
    ConstantPool::unreference_symbols, which is probably not good either.


This change adds a call to free everything in the _deallocate_list first 
thing in ClassLoaderData::~ClassLoaderData.

Note:
Running deallocate_contents() on everything in the list might be doing 
unnecessary work:
What we really would like to do is run release_C_heap_structures() on 
constant pools in the list that do not have a corresponding 
InstanceKlass. Not sure it's worth the effort to do the filtering, since 
I believe the _deallocate_list is usually not that long.

Note 2:
After fixing in JDK 9 I'll backport this to JDK 8, where it has a more 
immediately visible effect.

Regards,
Andreas