fix: refactor engine creation in JSPlugin and JSInterceptorFactory to use PolyglotThreadUtils for platform thread compatibility and add test for virtual thread offloading

#663

Author: mkjsix

polyglot/src/main/java/org/restheart/polyglot/JSPlugin.java

@@ -20,7 +20,6 @@
  */
 package org.restheart.polyglot;
 
-import java.io.IOException;
 import java.util.Map;
 import java.util.Optional;
 
@@ -40,11 +39,11 @@ public abstract class JSPlugin {
 
     static {
         try {
-            // use PluginsClassloader so ServiceLoader can find js-language's TruffleLanguageProvider
-            engine = PolyglotClassloaderHelper.withPluginsClassloaderResult(Engine::create);
-        } catch (IOException e) {
-            // Engine.create() does not throw a checked exception; this cannot happen
-            throw new IllegalStateException(e);
+            // Engine.create() touches Truffle thread locals, must run on a platform thread (see PolyglotThreadUtils)
+            // and needs PluginsClassloader so ServiceLoader can find js-language's TruffleLanguageProvider
+            engine = PolyglotThreadUtils.onPlatformThread(() -> PolyglotClassloaderHelper.withPluginsClassloaderResult(Engine::create));
+        } catch (Exception e) {
+            throw new IllegalStateException("Error creating polyglot Engine", e);
         }
     }
 

polyglot/src/main/java/org/restheart/polyglot/interceptors/JSInterceptorFactory.java

@@ -66,11 +66,11 @@ public JSInterceptorFactory(Optional<MongoClient> mclient, Configuration config)
         this.mclient = mclient;
         this.config = config;
         try {
-            // use PluginsClassloader so ServiceLoader can find js-language's TruffleLanguageProvider
-            this.engine = PolyglotClassloaderHelper.withPluginsClassloaderResult(Engine::create);
-        } catch (IOException e) {
-            // Engine.create() does not throw a checked exception; this cannot happen
-            throw new IllegalStateException(e);
+            // Engine.create() touches Truffle thread locals, must run on a platform thread (see PolyglotThreadUtils)
+            // and needs PluginsClassloader so ServiceLoader can find js-language's TruffleLanguageProvider
+            this.engine = PolyglotThreadUtils.onPlatformThread(() -> PolyglotClassloaderHelper.withPluginsClassloaderResult(Engine::create));
+        } catch (Exception e) {
+            throw new IllegalStateException("Error creating polyglot Engine", e);
         }
     }
 

polyglot/src/test/java/org/restheart/polyglot/ContextQueueVirtualThreadTest.java

@@ -112,4 +112,25 @@ void executeWithContextSucceedsWhenCalledFromVirtualThread() throws Exception {
         assertNull(failure.get(), () -> String.valueOf(failure.get()));
         assertEquals(42, result.get());
     }
+
+    @Test
+    void engineCreateSucceedsWhenOffloadedFromVirtualThread() throws Exception {
+        // mirrors how JSPlugin/JSInterceptorFactory build their static Engine field
+        var result = new AtomicReference<Engine>();
+        var failure = new AtomicReference<Throwable>();
+
+        var vt = Thread.ofVirtual().unstarted(() -> {
+            try {
+                result.set(PolyglotThreadUtils.onPlatformThread(Engine::create));
+            } catch (Throwable t) {
+                failure.set(t);
+            }
+        });
+
+        vt.start();
+        vt.join();
+
+        assertNull(failure.get(), () -> String.valueOf(failure.get()));
+        result.get().close();
+    }
 }