The following test failed in the JDK21 CI:

applications/runthese/RunThese8H.java

Here's snippets from the hs_err_pid file:

# Internal Error (/opt/mach5/mesos/work_dir/slaves/741e9afd-8c02-45c3-b2e2-9db1450d0832-S100001/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/4bffef37-b607-4e11-a688-94ad227a52f8/runs/1328c16f-d0c3-4a7c-9b54-4d8f083bcdb6/workspace/open/src/hotspot/share/utilities/ostream.cpp:1046), pid=3024, tid=3061
# assert(false) failed: Exceeded max buffer size for this string.
#
# JRE version: Java(TM) SE Runtime Environment (21.0+23) (fastdebug build 21-ea+23-LTS-1920)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 21-ea+23-LTS-1920, mixed mode, sharing, tiered, compressed class ptrs, z gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x14a1359] bufferedStream::write(char const*, unsigned long)+0x1a9

<snip>

--------------- T H R E A D ---------------

Current thread (0x00007f111c23d960): VMThread "VM Thread" [id=3061, stack(0x00007f11043d9000,0x00007f11044d9000) (1024K)]

Stack: [0x00007f11043d9000,0x00007f11044d9000], sp=0x00007f11044d6d50, free space=1015k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x14a1359] bufferedStream::write(char const*, unsigned long)+0x1a9 (ostream.cpp:1046)
V [libjvm.so+0x14a17c9] outputStream::do_vsnprintf_and_write_with_automatic_buffer(char const*, __va_list_tag*, bool)+0x109 (ostream.cpp:130)
V [libjvm.so+0x14a1ffc] outputStream::print(char const*, ...)+0x16c (ostream.cpp:143)
V [libjvm.so+0xdbb948] print_classname(outputStream*, Klass*)+0x78 (heapInspection.cpp:442)
V [libjvm.so+0xdbd444] KlassHierarchy::print_class(outputStream*, KlassInfoEntry*, bool)+0x194 (heapInspection.cpp:472)
V [libjvm.so+0xdbf005] KlassHierarchy::print_class_hierarchy(outputStream*, bool, bool, char*)+0x5b5 (heapInspection.cpp:386)
V [libjvm.so+0x1872de7] VM_Operation::evaluate()+0x137 (vmOperations.cpp:71)
V [libjvm.so+0x1895aeb] VMThread::evaluate_operation(VM_Operation*)+0x10b (vmThread.cpp:281)
V [libjvm.so+0x18965ff] VMThread::inner_execute(VM_Operation*)+0x35f (vmThread.cpp:428)
V [libjvm.so+0x18967d4] VMThread::loop()+0x84 (vmThread.cpp:495)
V [libjvm.so+0x18968d7] VMThread::run()+0x87 (vmThread.cpp:175)
V [libjvm.so+0x17946d8] Thread::call_run()+0xb8 (thread.cpp:217)
V [libjvm.so+0x149138a] thread_native_entry(Thread*)+0x11a (os_linux.cpp:740)

Since we're trying to do a KlassHierarchy::print_class_hierarchy from
the VMThread, I'm starting this off in hotspot/runtime for initial triage.