RFR: 8301639: JDI and JDWP specs should clarify potential deadlock issues with method invocation

[Chris Plummer]

On Wed, 13 Sep 2023 13:01:32 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Of course as soon as I posted that I now realize the issue. There are more things that could "deadlock" virtual threads when all other threads are suspended - such as calling sleep, because it requires a "system" thread to be active. This suggest to me that we need modifications to JDWP/JDI to support effective debugging of virtual threads, by allowing for the notion of "system threads" that by default are not suspended (or else always resumed for an invoke).
>
> A general point here is that there many cases that are unrelated to virtual threads where suspending all, except one, thread will deadlock/hang if other threads can't make progress. This includes several standard (java.*) APIs.
> 
> It is true that there are a couple of additional blocking operations with virtual threads that require I/O or timer threads to make progress in order to avoid a virtual thread being parked or blocked indefinitely. It feels more like something for  "@apiNote" for the invokeMethod/etc. javadoc, not clear to me that it's worth adding as a note in the description of the commands in the JDWP spec.

I can change it to `@apiNote`. Since it is not really part of the spec, I can see leaving it out of the JDWP spec. It's mostly important to get this info to JDI users.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15695#discussion_r1324751119