RFR 8223065: Add jcmd to get the listen address of the debugger

[David Holmes]

Responding to Chris's question below ...

On 30/04/2019 3:58 am, Chris Plummer wrote:
> Hi Ralf,
> 
> I think print_debug_listen_address() should have some exception checking 
> added after the java calls.
> 
> I'm a little unsure why you modified DebugOnCmdStartDCmd to use 
> print_debug_listen_address(), but still have a fallback to print the 
> specified transport and address. If anything I would have written a 
> get_debug_listen_address() and used it to verify that the specified and 
> actual addresses end up being the same (and then also make 
> print_debug_listen_address() use this API).
> 
> I'm also unsure of your ThreadToNativeFromVM change. This is not an area 
> I understand well, so best to get someone else to ok it.

I can see that the new code for print_debug_listen_address needs to be 
executed whilst still _thread_in_vm so the ThreadToNativeFromVM helper 
can't extend over that part. However, I would suggest that you otherwise 
keep it covering as much of the existing code as possible - specifically 
the chunk that calls os::find_agent_function. I would be concerned that 
the eventual calls to dlsym etc may potentially take some time and this 
would prevent safepoints from occurring as this thread is still "in VM".

This might be overly conservative but it avoids any possibility of 
introducing a change in behaviour for the existing code.

Cheers,
David
-----

> You need to update copyright date to 2019.
> 
> Can you write a test for this new dcmd. You can probably just extend 
> OnJcmdTest.java.
> 
> thanks,
> 
> Chris
> 
> On 4/29/19 8:31 AM, Schmelter, Ralf wrote:
>> Thanks for the review.
>>
>> I've update the webrev to use explicit NULL checks: 
>> https://bugs.openjdk.java.net/browse/JDK-8223065
>>
>> And I now use the pointer to the first byte in the result to split the 
>> property value, since I might need the calculate the pointer past the 
>> last character (if the prop ends with ':').
>>
>> I cannot see the SEGV, but I've scheduled the patch to be tested in 
>> our nightly build again, so maybe I can reproduce it there.
>>
>> Best regards,
>> Ralf
> 
>