Protocol version of Attach API

[David Holmes]

On 26/02/2019 5:01 pm, Yasumasa Suenaga wrote:
> 2019年2月26日(火) 15:47 Thomas Stüfe <thomas.stuefe at gmail.com>:
>>
>> 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 with Thomas,
> but I think we can update protocol version and reject request(s) from
> jcmd and other tools on earlier release.

That is not a decision to be taken lightly and one that requires a CSR 
request. Simply allowing for an extra arg on the jmap histo subcommand 
should not break all backward compatability.

I think this is a "simple" bug in the Linux (and possibly other) attach 
listener logic. It should be reading a protocol "packet" with up to 4 
arguments, but it should be perfectly fine to get a packet with fewer 
than 4. The current code doesn't do that, but assumes the "packet" is 
always the maximum size.

David
-----

> 
> I guess serviceability tools like a jhsdb are recommended in use with
> same version.
> 
> 
> Thanks,
> 
> Yasumasa
> 
> 
>> ..Thomas
>>