RFR: 8309396: com/sun/jdi/JdbMethodExitTest.java fails with virtual threads due to a bug in determining the main thread id

[Chris Plummer]

JdbMethodExitTest.java tries to determine the jdb threadID for the "main" thread, and then later use it in jdb commands that require a threadID. It does this by first having the debuggee execute the following:

        System.out.println("threadid="+Thread.currentThread().getId());

And then later on the test parses the threadID from this output. The problem is that the id returned by getId() has no relation to threadIDs used by jdb, which are actually JDWP ObjectIDs. In the past this has worked due to some dumb luck. getID() always returns 1 for the main thread, which is always the thread we are executing in. Coincidentally the JDWP ObjectID for the main Thread object is also always 1 because this is the first ObjectID that the debug agent ever needs to create. However, when the debuggee main thread is a virtual thread, neither getId() nor JDWP assign 1 to the threadID, and in fact both will end up with very different values for the threadID. The end result is errors from jdb for using an invalid threadID.

The correct threadID can be obtained by executing the jdb "threads" command and parsing it from a line that looks like the following:

   (java.lang.VirtualThread)694 main running (at breakpoint)

Note this test will also fail due to [JDK-8309334](https://bugs.openjdk.org/browse/JDK-8309334) and [JDK-8309397](https://bugs.openjdk.org/browse/JDK-8309397), which should be fixed first. 

I've tested with mach5 tier5 in a workspace that has integrated the various CRs mentioned. Once JDK-8309334 and JDK-8309397 are fixed, before integrating this PR I'll first merge and verify that the test being removed from the problem list by this PR also passes.

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Commit messages:
 - remove test from problem list
 - properly determine the main threadId

Changes: https://git.openjdk.org/jdk/pull/14294/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14294&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309396
  Stats: 11 lines in 2 files changed: 5 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14294.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14294/head:pull/14294

PR: https://git.openjdk.org/jdk/pull/14294