RFR: 8263670: pmap and pstack in jhsdb do not work on debug server

[Yasumasa Suenaga]

On Wed, 17 Mar 2021 04:55:27 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>>> If I understand correctly, this is very different than the way we normally implement remote debugging support. Normally pages are read in from the remote VM process and into the local SA client, where it can then implement SA support in much the same way as it normally does. For PStack and PMap, you've chosen to instead just execute the commands in the remote debugd server, and return the (text) result to the client.
>> 
>> Yes.
>> 
>>> It's not clear to me why the normal model (of reading in the remote pages and leveraging existing PStack and PMAp supprt) can't instead be used so we can keep the remote debugging support consistent. How are PStack and PMap different from other SA support that does not require remote execution of commands.
>> 
>> SA would set various information when it attaches to debuggee, so it is difficult to catch up them when remote debugger attached because only debugd can attach to debuggee directly.
>> As I said in the description of this PR, both PMap and PStack depend on CDebugger. CDebugger holds low-level information such as library list and native thread list. They would be initialized when it attached - it is startup of debugd, and we cannot get them when we connect to debugd.
>> 
>> I thought to export CDebugger as RMI object, but it seems to complex. So I decided to run both pmap and pstack on debugd.
>
> Do we have any issues with any other clhsdb commands that rely CDebugger. I haven't looked into the implementation, but I'm suspicious of `thread`, `threads`, `findpc` of an address in the executable, and `findsym`.
> 
> And speaking of CDebugger, the SA "debugger" hierarchy is too heavily abstracted. I counted about 18 classes and interfaces that have "Debugger" in their name, and some of the interfaces get inherited more than once. I really wonder if it has to be that complex. My first adventure into this mess was for [JDK-8239062](https://bugs.openjdk.java.net/browse/JDK-8239062), a time when I knew close to nothing about SA. Was not fun and I eventually gave up. You might want to have a look at the CR, if not just to add some comments and maybe enlighten me on some things.

Thanks @plummercj for the comment! I updated testcases.

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PR: https://git.openjdk.java.net/jdk/pull/3027