RFR: 8290200: com/sun/jdi/InvokeHangTest.java fails with "Debuggee appears to be hung" [v2]

[Chris Plummer]

> The debuggee main method creates two threads and then starts them:
> 
> 
>     public static void main(String[] args) {
>         System.out.println("Howdy!");
>         Thread t1 = TestScaffold.newThread(new InvokeHangTarg(), name1);
>         Thread t2 = TestScaffold.newThread(new InvokeHangTarg(), name2);
> 
>         t1.start();
>         t2.start();
>     }
> 
> 
> These threads will hit breakpoints which the debugger handles and issues an invoke on the breakpoint thread. The threads run until they generate 100 breakpoints. There is an issue when these two threads are virtual threads. Virtual threads are daemon threads. That means the JVM can exit while they are still running. The above main() method is not waiting for these two threads to exit, so main() exits immediately and the JVM starts the shutdown process. It first must wait for all non-daemon threads to exit, but there are none, so the JVM exits right away before the two threads are completed.  The end result of this early exit is that sometimes the invoke done by the debugger never completes because the JVM has already issued a VMDeath event and the debuggee has been disconnected.
> 
> When these two threads are platform threads, the JVM has to wait until they complete before it exits, so they will always complete. The fix for virtual threads is to do a join with t1 and t2. This forces the main() method to block until they have completed.

Chris Plummer has updated the pull request incrementally with three additional commits since the last revision:

 - fix spelling error
 - add comment
 - minor formatting fix

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13068/files
  - new: https://git.openjdk.org/jdk/pull/13068/files/4d28b92e..58ca27b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13068&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13068&range=00-01

  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13068.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13068/head:pull/13068

PR: https://git.openjdk.org/jdk/pull/13068