Protocol version of Attach API

[David Holmes]

Hi Yasumasa,

On 27/02/2019 1:05 pm, Yasumasa Suenaga wrote:
> Hi Lin,
> 
> My proposal is a just idea, so you need to tweak it.
> 
> AttachListener receives raw command.
> For example, jcmd is `jcmd\0<arg strings>`. Please see 
> HotSpotVirtualMachine and extended classes.
> 
> In case of jcmd, I guess AttachListener will receive message 
> `<version>\0jcmd\0<args>\0\0\0` (I did not check it well).
> So I guess we can add '\0' when `str_count` does not reach to maximum.

I don't see how this approach can work. You have to know how many 
arguments are coming in the "packet", but that information is not 
available in the current Linux implementation.Without it you can't know 
when to stop calling read().

The protocol would need to be changed to send the "packet" size, but 
that's not compatible with older JDKs.

Otherwise I think we have no choice but to use a non-blocking read ... 
though I'm still unsure if you can know for certain that you've reached 
the end of the "packet" ...

Thanks,
David

> 
> Thanks,
> 
> Yasumasa
> 
> 
> On 2019/02/27 11:50, zanglin5 at jd.com wrote:
>> Dear  Yasumasa,
>>    The fix you mentioned seems not work as expected.
>>    I have done an experiment that use jdk1.8's "jcmd <pid> help" to 
>> attach to latest jdk.
>>    it seem the whole "jcmd <pid> help"  buffer is not
>>    read completely at one time. in my case it is "jcmd", "<pid>", 
>> "help", and then block at while().
>>    After applied your change, it seems only the "jcmd" is processed, 
>> the "<pid>", "help" is replaced by '\0'.
>>
>> BRs,
>> Lin
>>