Try using the managed GC bridge.

	dotnet publish --self-contained -p:UseMonoRuntime=true -p:DotNetTargetFramework=net8.0 -p:UseAppHost=true -p:ErrorOnDuplicatePublishOutputFiles=false samples/Hello-Java.Base/Hello-Java.Base.csproj -r osx-x64 && \
	JAVA_INTEROP_GREF_LOG=g.txt  ./samples/Hello-Java.Base/bin/Release/osx-x64/publish/Hello-Java.Base

Does it work?  No quite.

What's present here "works", in that the managed `MarkCrossReferences`
callback *is* invoked, which just keeps all instances alive.

However, it *doesn't* fully work, for reasons I don't understand:
if `MarkCrossReferences` calls managed code, e.g.
`Console.WriteLine("here!")`, then it *hangs*.

Looks like this prototype approach can't work.

Author: jonpryor

src/Java.Runtime.Environment/Java.Interop/MonoRuntimeValueManager.cs

@@ -14,7 +14,7 @@ enum GCBridgeUseWeakReferenceKind {
 		Jni,
 	}
 
-	class MonoRuntimeValueManager : JniRuntime.JniValueManager {
+	partial class MonoRuntimeValueManager : JniRuntime.JniValueManager {
 
 		#pragma warning disable 0649
 		// This field is mutated by the java-interop native lib
@@ -46,6 +46,10 @@ public override void OnSetRuntime (JniRuntime runtime)
 					throw new NotSupportedException ("Could not register current AppDomain!");
 				if (JreNativeMethods.java_interop_gc_bridge_set_current_once (bridge) < 0)
 					throw new NotSupportedException ("Could not set GC Bridge instance!");
+				unsafe {
+					if (JreNativeMethods.java_interop_gc_bridge_set_mark_cross_references(bridge, &MarkCrossReferences) < 0)
+						throw new NotSupportedException("Could not set MarkCrossReferences!");
+				}
 			}
 			catch (Exception) {
 				JreNativeMethods.java_interop_gc_bridge_free (bridge);

src/java-interop/java-interop-gc-bridge-mono.cc

@@ -1288,6 +1288,7 @@ gc_cross_references (int num_sccs, MonoGCBridgeSCC **sccs, int num_xrefs, MonoGC
 static void
 managed_gc_cross_references (int num_sccs, MonoGCBridgeSCC **sccs, int num_xrefs, MonoGCBridgeXRef *xrefs)
 {
+	fprintf (stderr, "# jonp: managed_gc_cross_references: %d sccs and %d xrefs\n", num_sccs, num_xrefs);
 	if (mono_bridge->mark_cross_references == NULL) {
 		assert (!"mono_bridge->mark_cross_references is NULL; WE SHOULD NOT BE EXECUTING");
 		return;
@@ -1317,12 +1318,14 @@ managed_gc_cross_references (int num_sccs, MonoGCBridgeSCC **sccs, int num_xrefs
 		xref->DestinationGroupIndex = (void*) (intptr_t) xrefs [i].dst_scc_index;
 	}
 
+	fprintf (stderr, "# jonp: calling managed mark_cross_references\n");
 	mono_bridge->mark_cross_references (&cross_references);
 
 	for (i = 0; i < num_sccs; ++i) {
 		Srij_StronglyConnectedComponent *scc    = &cross_references.Components [i];
 		sccs [i]->is_alive = scc->IsAlive;
 	}
+	fprintf (stderr, "# jonp: calling done\n");
 }
 
 int