Protocol version of Attach API

[Chris Plummer]

On 2/26/19 1:30 PM, David Holmes wrote:
> On 27/02/2019 5:52 am, Chris Plummer wrote:
>> On 2/26/19 9:34 AM, Erik Gahlin wrote:
>>> On 2019-02-26 07:47, Thomas Stüfe wrote:
>>>> Hi David, Yasumasa,
>>>>
>>>>
>>>>
>>>>     > Do we support connection to later VMs from earlier JDK tools?
>>>>
>>>>     I could not find the spec about this.
>>>>     So I asked to serviceability folks before filing this to JBS :-)
>>>>
>>>>
>>>> Just to chime in on that, I do not know if it is specified but it 
>>>> is certainly very handy in daily use. I often use old jcmd tools to 
>>>> connect to newer VMs. I always thought that was a neat design.
>>>
>>> I agree.
>>>
>>> The tool was designed to be dumb, so it can connect to JVMs 
>>> regardless of release. If something has changed so that is no longer 
>>> true, it should be fixed (and backported if needed).
>>>
>>> Erik
>>
>> So do we all agree that backporting is acceptable for fixing 
>> backwards compatibility here? Just want to be clear on that. You 
>> would need to get the latest 8u to interact with 13 and later, and 7 
>> and earlier would not work. I'd prefer to keep 8 (and earlier) 
>> working without any changes to it if possible.
>
> EEk No! You don't fix this by trying to change all the old versions! 
> Simply impossible - only the latest updates in those trains would 
> work. Logistical nightmare as well.
That was my thought as well, which is why I called attention to this.
>
> This is a very simple logic error in the attach logic on Linux (and 
> possibly elsewhere)! We just need to fix that bug in 13. Yasumasa 
> already suggested one fix that seems quite simple - it adjusts the 
> expected amount of data to read if it encounters a logical "EOF". (I 
> think it can be simplified further but the gist of it seems fine.)
>
>>
>> Also want to point out that this issue might be two-way: 8 can't 
>> attach to 13 and 13 may have issues attaching to 8 (what happens with 
>> the extra argument that is sent?).
>
> You can't expect an old release to accept new and unknown args. That 
> is perfectly fine.

But we should expect that you can attach from 13 to 8. If unknown args 
are sent, we would need for them to be quietly ignored.

Chris

>
> David
> -----
>
>> Chris
>>
>>>
>>>>
>>>> ..Thomas
>>>
>>