On 02/26/2019 13:58, David Holmes wrote:
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.
Looks like AttachOperation::arg_count_max is used not just as max arg
count, but also to calculate packet size which needs to be read from the
socket on unix systems (it's expected to be the same for all
operations). So it cannot be easily fixed.
BTW as far as I got Windows if not affected. Looking into the code
(src/hotspot/os/windows/attachListener_windows.cpp) I suppose the
feature implemented by JDK-8215622 does not work at Windows as
Win32AttachListener still support only 3 args.
In my opinion it would be better to revert fix for JDK-8215622 and
rethink how to implement it without changing AttachOperation::arg_count_max
--alex
David
Chris
David
-----
Chris
..Thomas
