RFR: JDK-8263558: Possible NULL dereference in fast path arena free if ZapResourceArea is true [v4]

[Thomas Stuefe]

On Tue, 16 Mar 2021 05:54:02 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Looks good.
>
> Thanks @coleenp and @kimbarrett . However, as I feared, with the assert alone I now see it firing both in our nightlies at SAP and in the GAs in compiler tests. 
> 
> Which is cool in a way since this is the first time SonarCloud reported anything demonstrably real.:) I'll take a look at the crashes and modify the patch.

I changed the patch to provide the same semantics on passed old ptr == NULL as standard `free(3)` and `realloc(3)` have. Which is to ignore it (free) or to revert to malloc (realloc).

The culprit in this case - calling realloc with a NULL pointer - was aot, `ImplicitExceptionTable::append( uint exec_off, uint cont_off )`. 

I considered fixing the caller, but I am used to the C-runtime semantics in free and realloc, and it looks like others are too; so this is the least surprising behavior for a realloc-like function.

Also, this fixes the subtle bug where, when passing NULL to Arealloc, we would return "false" to indicate that we have a lossfull realloc. The only case I can see where the return value was actually used was in `SymbolTable::delete_symbol()` which would print something about leaked symbols in that case. I did not investigate whether this has any practical relevance.

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PR: https://git.openjdk.java.net/jdk/pull/2995