Attaching from the loom jdb to a pre-loom jvm fails with:

com.sun.jdi.InternalException: Unexpected JDWP Error: 103
at jdk.jdi/com.sun.tools.jdi.JDWPException.toJDIException(JDWPException.java:80)
at jdk.jdi/com.sun.tools.jdi.EventRequestManagerImpl$EventRequestImpl.set(EventRequestManagerImpl.java:232)
at jdk.jdi/com.sun.tools.jdi.EventRequestManagerImpl$EventRequestImpl.setEnabled(EventRequestManagerImpl.java:194)
at jdk.jdi/com.sun.tools.jdi.EventRequestManagerImpl$EventRequestImpl.enable(EventRequestManagerImpl.java:179)
at jdk.jdi/com.sun.tools.example.debug.tty.VMConnection.setEventRequests(VMConnection.java:476)
at jdk.jdi/com.sun.tools.example.debug.tty.VMConnection.open(VMConnection.java:378)
at jdk.jdi/com.sun.tools.example.debug.tty.Env.init(Env.java:63)
at jdk.jdi/com.sun.tools.example.debug.tty.TTY.main(TTY.java:1176)

103 is ILLEGAL_ARGUMENT. The failure seems to be happening when enabling a ThreadStartRequest for which a PlatformThreadsOnlyFilter has been added. jdb needs to avoid adding this filter when attaching to pre-loom JVMs. The ThreadDeathRequest has the same issue.

A call from jdb to ThreadRefence.isVirtual() also causes an exception:

Exception in thread "event-handler" java.lang.UnsupportedOperationException
at jdk.jdi/com.sun.tools.jdi.JDWPException.toJDIException(JDWPException.java:69)
at jdk.jdi/com.sun.tools.jdi.ThreadReferenceImpl.isVirtual(ThreadReferenceImpl.java:619)
at jdk.jdi/com.sun.tools.example.debug.tty.ThreadInfo.addThread(ThreadInfo.java:83)
at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.threadStartEvent(EventHandler.java:315)
at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.handleEvent(EventHandler.java:149)
at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.run(EventHandler.java:78)
at java.base/java.lang.Thread.run(Thread.java:1532)