Disable heap iteration for Shenandoah in SA

[Roman Kennke]

> On 11/28/18 6:59 PM, Aleksey Shipilev wrote:
>> On 11/28/18 6:38 PM, Roman Kennke wrote:
>>> As Per pointed out in reviews, we cannot safely iterate objects in SA,
>>> because current implementation is size-based. This might just crash.
>>> Let's do the same as ZGC for now and disable it, and revert the other
>>> pieces.
>>>
>>> http://cr.openjdk.java.net/~rkennke/disable-sa-heapdump/webrev.00/
>>
>> Comment is incorrect, should be "Shenandoah":
>>
>>  372        // Operation (currently) not supported with ZGC. Print
>>
>> Then what Zhengyu said, we can remove cell_header_size completely?
> 
> Also, wait. This screws up jstack and Java monitor detection, right? I vaguely remember we added
> this to satisfy adopter's request for this feature. I wonder if we can somehow check that class
> unloading is not enabled before traversing.

I have no idea. How about I cut this out of the upstreaming patch, and
see into fixing it properly (for ZGC too)? We don't need to push it to
shenandoah/jdk for that (but we could). WDYT.

http://cr.openjdk.java.net/~rkennke/disable-sa-heapdump/webrev.02/

Roman