Protocol version of Attach API

[David Holmes]

On 27/02/2019 7:50 am, Chris Plummer wrote:
> 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.

That depends on how well the code was written in 8, but I think it 
perfectly fine to get an error if you try to send a command that 8 
doesn't (fully) understand.

David


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