Protocol version of Attach API

[臧琳]

Dear David, Yasumasa, 
       I think it is hard to know how long the buffer is passed from socket without changing the info of the message from the receiver side. 
       So how about when str_count reach to 3, we test it with non_blocking read?  


Cheers, 
Lin
> -----Original Message-----
> From: David Holmes <david.holmes at oracle.com>
> Sent: Wednesday, February 27, 2019 1:44 PM
> To: Yasumasa Suenaga <yasuenag at gmail.com>; 臧琳 <zanglin5 at jd.com>
> Cc: serviceability-dev at openjdk.java.net
> Subject: Re: Protocol version of Attach API
> 
> 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
> >>