We have just identified a Metaspace leak in a very specific case when
a class has a method using a try-with-resources construct (or similar
with try-catch) and re-transforming this class in a loop. It is
reproducible from jdk8 to jdk20.
Here the steps to reproduce:

 1. create a java file with following content:

public class RetransformLeak {
    public static void main(String[] args) throws Exception {
        new MyClass();
        while (true) {
            Thread.sleep(1000);
        }
    }
}

class MyClass {
    private static void writeFile() {
        TWR var0 = new TWR();
        try {
            var0.process();
        } catch (Throwable var4) {
            try {
                var0.close();
            } catch (Throwable var3) {
                var4.addSuppressed(var3);
            }
            throw var4;
        }
        var0.close();
// try (TWR twr = new TWR()) {
// twr.process();
// }
    }

    static class TWR implements AutoCloseable {
        public void process() {}
        @Override
        public void close() {}
    }
}

2. compile it: javac RetransformLeak.java
3. create a java file Agent.java with the following content that will
be our java agent performing re-transformation:

public class Agent {
    public static void premain(String arg, Instrumentation inst) {
        new Thread(() -> retransformLoop(inst, arg)).start();
    }

    private static void retransformLoop(Instrumentation
instrumentation, String className) {
        Class<?> classToRetransform = null;
        while (true) {
            if (classToRetransform == null) {
                for (Class<?> clazz : instrumentation.getAllLoadedClasses()) {
                    if (clazz.getName().equals(className)) {
                        System.out.println("found class: " + className);
                        classToRetransform = clazz;
                        break;
                    }
                }
            }
            if (classToRetransform != null) {
                try {
                    instrumentation.retransformClasses(classToRetransform);
                    //Thread.sleep(1);
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

 4. Compile it: javac Agent.java
 5. create a Manifest.txt file for the java agent:

Premain-Class: Agent
Can-Retransform-Classes: true

 6. create java agent jar: jar cfm agent.jar Manifest.txt Agent.class
 7. execute the RetransformLeak class with a max metaspace size:
java -javaagent:agent.jar=MyClass -XX:MaxMetaspaceSize=128M -cp .
RetransformLeak

output:
found class: MyClass
Exception in thread "Thread-0" java.lang.OutOfMemoryError
  at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses0(Native
Method)
  at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:169)
  at Agent.retransformLoop(Agent.java:22)
  at Agent.lambda$premain$0(Agent.java:5)
  at java.base/java.lang.Thread.run(Thread.java:1623)

If you comment the line:
                var4.addSuppressed(var3);
in MyClass#writeFile method, no OOME will be thrown and Metaspace will
remain stable.
You can also directly use a try-with-resources construct to reproduce
the leak but I have decomposed it with try catch to be able to
pinpoint more precisely which bytecode may generate the leak.

Here the output of VM.metaspace after the OOME:

$ jcmd 6940 VM.metaspace
6940:

Total Usage - 6 loaders, 809 classes (776 shared):
  Non-Class: 2605 chunks, 131.55 MB capacity, 127.87 MB ( 97%)
committed, 127.83 MB ( 97%) used, 37.94 KB ( <1%) free, 8
bytes ( <1%) waste , deallocated: 11783 blocks with 127.36 MB
      Class: 8 chunks, 266.00 KB capacity, 74.00 KB ( 28%)
committed, 17.48 KB ( 7%) used, 56.52 KB ( 21%) free, 0
bytes ( 0%) waste , deallocated: 2 blocks with 512 bytes
       Both: 2613 chunks, 131.81 MB capacity, 127.94 MB ( 97%)
committed, 127.85 MB ( 97%) used, 94.45 KB ( <1%) free, 8
bytes ( <1%) waste , deallocated: 11785 blocks with 127.36 MB


Virtual space:
  Non-class space: 192.00 MB reserved, 127.88 MB ( 67%)
committed, 3 nodes.
      Class space: 104.00 MB reserved, 128.00 KB ( <1%)
committed, 1 nodes.
             Both: 296.00 MB reserved, 128.00 MB ( 43%) committed.


Chunk freelists:
   Non-Class:

  4m: (none)
  2m: (none)
  1m: (none)
512k: (none)
256k: 2, capacity=512.00 KB, committed=0 bytes ( 0%)
128k: 2, capacity=256.00 KB, committed=0 bytes ( 0%)
 64k: 2, capacity=128.00 KB, committed=0 bytes ( 0%)
 32k: (none)
 16k: (none)
  8k: (none)
  4k: 2, capacity=8.00 KB, committed=8.00 KB (100%)
  2k: 6, capacity=12.00 KB, committed=12.00 KB (100%)
  1k: (none)
Total word size: 916.00 KB, committed: 20.00 KB ( 2%)

       Class:

  4m: (none)
  2m: 1, capacity=2.00 MB, committed=0 bytes ( 0%)
  1m: 1, capacity=1.00 MB, committed=0 bytes ( 0%)
512k: 1, capacity=512.00 KB, committed=0 bytes ( 0%)
256k: (none)
128k: 1, capacity=128.00 KB, committed=0 bytes ( 0%)
 64k: 1, capacity=64.00 KB, committed=0 bytes ( 0%)
 32k: 1, capacity=32.00 KB, committed=0 bytes ( 0%)
 16k: 1, capacity=16.00 KB, committed=0 bytes ( 0%)
  8k: (none)
  4k: (none)
  2k: 1, capacity=2.00 KB, committed=2.00 KB (100%)
  1k: 3, capacity=3.00 KB, committed=3.00 KB (100%)
Total word size: 3.74 MB, committed: 5.00 KB ( <1%)

        Both:

  4m: (none)
  2m: 1, capacity=2.00 MB, committed=0 bytes ( 0%)
  1m: 1, capacity=1.00 MB, committed=0 bytes ( 0%)
512k: 1, capacity=512.00 KB, committed=0 bytes ( 0%)
256k: 2, capacity=512.00 KB, committed=0 bytes ( 0%)
128k: 3, capacity=384.00 KB, committed=0 bytes ( 0%)
 64k: 3, capacity=192.00 KB, committed=0 bytes ( 0%)
 32k: 1, capacity=32.00 KB, committed=0 bytes ( 0%)
 16k: 1, capacity=16.00 KB, committed=0 bytes ( 0%)
  8k: (none)
  4k: 2, capacity=8.00 KB, committed=8.00 KB (100%)
  2k: 7, capacity=14.00 KB, committed=14.00 KB (100%)
  1k: 3, capacity=3.00 KB, committed=3.00 KB (100%)
Total word size: 4.63 MB, committed: 25.00 KB ( <1%)



Waste (unused committed space):(percentages refer to total committed
size 128.00 MB):
        Waste in chunks in use: 8 bytes ( <1%)
        Free in chunks in use: 94.45 KB ( <1%)
                In free chunks: 25.00 KB ( <1%)
Deallocated from chunks in use: 127.36 MB (>99%) (11785 blocks)
                       -total-: 127.48 MB (>99%)

chunk header pool: 2632 items, 189.33 KB.

Internal statistics:

num_allocs_failed_limit: 23.
num_arena_births: 22.
num_arena_deaths: 10.
num_vsnodes_births: 4.
num_vsnodes_deaths: 0.
num_space_committed: 2048.
num_space_uncommitted: 0.
num_chunks_returned_to_freelist: 33.
num_chunks_taken_from_freelist: 2624.
num_chunk_merges: 11.
num_chunk_splits: 1456.
num_chunks_enlarged: 276.
num_inconsistent_stats: 0.


Settings:
MaxMetaspaceSize: 128.00 MB
CompressedClassSpaceSize: 104.00 MB
Initial GC threshold: 21.00 MB
Current GC threshold: 128.00 MB
CDS: on
MetaspaceReclaimPolicy: balanced
 - commit_granule_bytes: 65536.
 - commit_granule_words: 8192.
 - virtual_space_node_default_size: 8388608.
 - enlarge_chunks_in_place: 1.
 - new_chunks_are_fully_committed: 0.
 - uncommit_free_chunks: 1.
 - use_allocation_guard: 0.

Also I have run it with -XX:+CrashOnOutOfMemoryError to have stacktrace:
Stack: [0x00007f8d9c400000,0x00007f8d9c500000],
sp=0x00007f8d9c4fdb80, free space=1014k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0xf9f4e7] VMError::report_and_die(int, char const*,
char const*, __va_list_tag*, Thread*, unsigned char*, void*, void*,
char const*, int, unsigned long)+0x197 (debug.cpp:368)
V [libjvm.so+0x672eab] report_fatal(VMErrorType, char const*, int,
char const*, ...)+0x12b
V [libjvm.so+0x67315d] report_java_out_of_memory(char const*)+0xed
V [libjvm.so+0xc238d0]
Metaspace::report_metadata_oome(ClassLoaderData*, unsigned long,
MetaspaceObj::Type, Metaspace::MetadataType, JavaThread*)+0x260
V [libjvm.so+0xc23b33] Metaspace::allocate(ClassLoaderData*,
unsigned long, MetaspaceObj::Type, JavaThread*)+0x133
V [libjvm.so+0x650e8b] ConstantPool::allocate(ClassLoaderData*, int,
JavaThread*)+0x7b
V [libjvm.so+0xae518c]
VM_RedefineClasses::merge_cp_and_rewrite(InstanceKlass*,
InstanceKlass*, JavaThread*)+0x4c
V [libjvm.so+0xae6e6d] VM_RedefineClasses::load_new_class_versions()
[clone .part.0]+0x34d
V [libjvm.so+0xae794f] VM_RedefineClasses::doit_prologue()+0x17f
V [libjvm.so+0xfa8ee0] VMThread::execute(VM_Operation*)+0x40
V [libjvm.so+0xab416f] JvmtiEnv::RetransformClasses(int, _jclass*
const*)+0x2bf
V [libjvm.so+0xa5f04c] jvmti_RetransformClasses+0xfc
C [libinstrument.so+0x4e86] retransformClasses+0x1b6
J 182 sun.instrument.InstrumentationImpl.retransformClasses0(J[Ljava/lang/Class;)V
java.instrument@20.0.1 (0 bytes) @ 0x00007f8d886d8077
[0x00007f8d886d7fa0+0x00000000000000d7]
J 181 c1 sun.instrument.InstrumentationImpl.retransformClasses([Ljava/lang/Class;)V
java.instrument@20.0.1 (33 bytes) @ 0x00007f8d80c29a5c
[0x00007f8d80c29860+0x00000000000001fc]
j Agent.retransformLoop(Ljava/lang/instrument/Instrumentation;Ljava/lang/String;)V+82
j Agent.lambda$premain$0(Ljava/lang/instrument/Instrumentation;Ljava/lang/String;)V+2
j Agent$$Lambda$14+0x0000000801002c00.run()V+8
j java.lang.Thread.runWith(Ljava/lang/Object;Ljava/lang/Runnable;)V+5
java.base@20.0.1
j java.lang.Thread.run()V+19 java.base@20.0.1
v ~StubRoutines::call_stub 0x00007f8d88138cc6
V [libjvm.so+0x8c9c85] JavaCalls::call_helper(JavaValue*,
methodHandle const&, JavaCallArguments*, JavaThread*)+0x315
V [libjvm.so+0x8cb5f2] JavaCalls::call_virtual(JavaValue*, Handle,
Klass*, Symbol*, Symbol*, JavaThread*)+0x1d2
V [libjvm.so+0x99debe] thread_entry(JavaThread*, JavaThread*)+0x8e
V [libjvm.so+0x8e1578] JavaThread::thread_main_inner() [clone .part.0]+0xb8
V [libjvm.so+0xf18736] Thread::call_run()+0xa6
V [libjvm.so+0xcae8d8] thread_native_entry(Thread*)+0xd8
Java frames: (J=compiled Java code, j=interpreted, Vv=VM code)
J 182 sun.instrument.InstrumentationImpl.retransformClasses0(J[Ljava/lang/Class;)V
java.instrument@20.0.1 (0 bytes) @ 0x00007f8d886d8004
[0x00007f8d886d7fa0+0x0000000000000064]
J 181 c1 sun.instrument.InstrumentationImpl.retransformClasses([Ljava/lang/Class;)V
java.instrument@20.0.1 (33 bytes) @ 0x00007f8d80c29a5c
[0x00007f8d80c29860+0x00000000000001fc]
j Agent.retransformLoop(Ljava/lang/instrument/Instrumentation;Ljava/lang/String;)V+82
j Agent.lambda$premain$0(Ljava/lang/instrument/Instrumentation;Ljava/lang/String;)V+2
j Agent$$Lambda$14+0x0000000801002c00.run()V+8
j java.lang.Thread.runWith(Ljava/lang/Object;Ljava/lang/Runnable;)V+5
java.base@20.0.1
j java.lang.Thread.run()V+19 java.base@20.0.1
v ~StubRoutines::call_stub 0x00007f8d88138cc6

I also have run with "-Xlog:metaspace=trace" and the log is attached as metaspace.log.gz

Running with "-Xlog:redefine+class+iklass+purge=trace" gives:

[0.705s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false
[0.963s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false
[1.425s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false
[1.914s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false
[2.318s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false
[2.323s][trace][redefine,class,iklass,purge] Class unloading: has_previous_versions = false