Protocol version of Attach API

[臧琳]

Another solution I can figure out is try to add timeout for socket read. I will also investigate whether is works.

Cheers,
Lin


> -----Original Message-----
> From: 臧琳
> Sent: Wednesday, February 27, 2019 1:52 PM
> To: 'David Holmes' <david.holmes at oracle.com>; Yasumasa Suenaga
> <yasuenag at gmail.com>
> Cc: serviceability-dev at openjdk.java.net
> Subject: RE: Protocol version of Attach API
> 
> 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
> > >>