apacheGH-2787: Deadlock in JenaSystem.init()

- printing an error to System.err, with a detailed message
  -> unable to fix the deadlock itself, that is all I came up with
- added integration tests for JenaSystem.init(), also for the parallel case.

jena-core/src/main/java/org/apache/jena/sys/JenaSystem.java

@@ -22,6 +22,8 @@
 import org.apache.jena.base.module.SubsystemRegistry;
 import org.apache.jena.base.module.SubsystemRegistryServiceLoader;
 
+import java.util.concurrent.Executors;
+
 /** Jena "system" - simple controls for ensuring components are loaded and initialized.
  * <p>
  * All initialization should be concurrent and thread-safe.  In particular,
@@ -67,6 +69,7 @@ public JenaSystem() { }
      */
     public static boolean DEBUG_INIT = false ;
     private static volatile boolean initialized = false ;
+    private static ThreadLocal<Boolean> isInitializing = ThreadLocal.withInitial(() -> false);
 
     /** Output a debugging message if DEBUG_INIT is set */
     public static void logLifecycle(String fmt, Object ...args) {
@@ -80,17 +83,37 @@ public static void init() {
         // Once jena is initialized, all calls are an immediate return.
         if ( initialized )
             return ;
+
+        // Avoid recursive initialization.
+        if( isInitializing.get() )
+            return ;
+
+        // Set the flag to prevent recursive initialization.
+        isInitializing.set(true);
+
         // Overlapping attempts to perform initialization will block on the synchronized.
         synchronized(JenaSystem.class) {
             if ( initialized )
                 return ;
+
+            // Timeout if initialization takes too long.
+            var future = Executors.newScheduledThreadPool(1).schedule(
+                    () -> {
+                        if (initialized)
+                            return;
+
+                        // This typically is a deadlock situation. The only thing we can do here is to print a message to System.err.
+                        System.err.println("Timeout while waiting for JenaSystem.init(), possibly due to deadlock while waiting on the Class initalization monitor . Please call JenaSystem.init() before working with multiple threads.");
+                    }, 5, java.util.concurrent.TimeUnit.SECONDS);
+
             setup();
             if ( DEBUG_INIT )
                 singleton.debug(DEBUG_INIT);
             singleton.initialize();
             singleton.debug(false);
             // Last so overlapping initialization waits on the synchronized
             initialized = true;
+            future.cancel(true);
         }
     }
 

jena-integration-tests/src/test/java/org/apache/jena/sys/TS_Sys.java

@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.jena.sys;
+
+import org.apache.jena.sparql.exec.http.*;
+import org.junit.runner.RunWith;
+import org.junit.runners.Suite;
+
+@RunWith(Suite.class)
+@Suite.SuiteClasses(
+    { TestJenaSystem.class
+    })
+
+public class TS_Sys { }

jena-integration-tests/src/test/java/org/apache/jena/sys/TestJenaSystem.java

@@ -0,0 +1,93 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.jena.sys;
+
+import org.apache.jena.rdf.model.ModelFactory;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.time.Duration;
+import java.time.temporal.ChronoUnit;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executors;
+import java.util.stream.IntStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class TestJenaSystem {
+
+    private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
+    private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();
+    private final PrintStream originalOut = System.out;
+    private final PrintStream originalErr = System.err;
+
+    @Before
+    public void setUpStreams() {
+        System.setOut(new PrintStream(outContent));
+        System.setErr(new PrintStream(errContent));
+    }
+
+    @After
+    public void restoreStreams() {
+        System.setOut(originalOut);
+        System.setErr(originalErr);
+    }
+
+    @Test
+    public void testInit() {
+        assertDoesNotThrow(() -> JenaSystem.init());
+
+        assertEquals("", errContent.toString(), "System.err should be empty");
+    }
+
+    @Test
+    public void testInitParallel() throws ExecutionException, InterruptedException {
+
+        // it is mandatory to init JenaSystem before running anything in parallel
+        JenaSystem.init();
+
+        var pool = Executors.newFixedThreadPool(8);
+
+        var futures = IntStream.range(0, 16)
+                .mapToObj(i -> pool.submit(() -> {
+                    if (i % 2 == 0)
+                        ModelFactory.createDefaultModel();
+                    else
+                        JenaSystem.init();
+
+                    return i;
+                }))
+                .toList();
+
+        assertTimeoutPreemptively(
+                Duration.of(5, ChronoUnit.SECONDS),
+                () -> {
+                    for (var future : futures) {
+                        System.out.println(future.get());
+                    }
+                });
+
+        var output = outContent.toString();
+
+        assertEquals(16, output.lines().count());
+    }
+}