Use synchronized data structures for reachability handlers registration

Prevent data races in reachability handlers registration when using
`-H:-RunReachabilityHandlersConcurrently`.

Closes #5868

substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/ReachabilityHandlerFeature.java

@@ -32,6 +32,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 
@@ -44,6 +45,7 @@
 import com.oracle.svm.core.SubstrateOptions;
 import com.oracle.svm.core.feature.AutomaticallyRegisteredFeature;
 import com.oracle.svm.core.feature.InternalFeature;
+import com.oracle.svm.core.util.ConcurrentIdentityHashMap;
 import com.oracle.svm.core.util.UserError;
 import com.oracle.svm.core.util.VMError;
 import com.oracle.svm.hosted.FeatureImpl.BeforeAnalysisAccessImpl;
@@ -52,8 +54,8 @@
 @AutomaticallyRegisteredFeature
 public class ReachabilityHandlerFeature extends ReachabilityHandler implements InternalFeature {
 
-    private final IdentityHashMap<Object, Set<Object>> activeHandlers = new IdentityHashMap<>();
-    private final IdentityHashMap<Object, Map<Object, Set<Object>>> triggeredHandlers = new IdentityHashMap<>();
+    private final ConcurrentIdentityHashMap<Object, Set<Object>> activeHandlers = new ConcurrentIdentityHashMap<>();
+    private final ConcurrentIdentityHashMap<Object, Map<Object, Set<Object>>> triggeredHandlers = new ConcurrentIdentityHashMap<>();
 
     public static ReachabilityHandlerFeature singleton() {
         return ImageSingletons.lookup(ReachabilityHandlerFeature.class);
@@ -88,7 +90,7 @@ private void registerReachabilityHandler(BeforeAnalysisAccess a, Object callback
         BeforeAnalysisAccessImpl access = (BeforeAnalysisAccessImpl) a;
         AnalysisMetaAccess metaAccess = access.getMetaAccess();
 
-        Set<Object> triggerSet = activeHandlers.computeIfAbsent(callback, c -> new HashSet<>());
+        Set<Object> triggerSet = activeHandlers.computeIfAbsent(callback, c -> ConcurrentHashMap.newKeySet());
 
         for (Object trigger : triggers) {
             if (trigger instanceof Class) {