Make partial loading threadsafe across platforms

src/main/java/com/samskivert/mustache/Mustache.java

@@ -13,6 +13,8 @@ import java.util.Arrays;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 /**
  * Provides <a href="http://mustache.github.com/">Mustache</a> templating services.
@@ -1057,10 +1059,20 @@ public class Mustache {
         protected Template getTemplate () {
             // we compile our template lazily to avoid infinie recursion if a template includes
             // itself (see issue #13)
-            if (_template == null) {
+            Template t = _template;
-                _template = _comp.loadTemplate(_name).indent(_indent);
+            if (t == null) {
+                // We cannot use synchronized or a CAS operation here since loadTemplate might be an IO call
+                // and virtual threads prefer regular locks.
+                lock.lock();
+                try {
+                    if ((t = _template) == null) {
+                        _template = t = _comp.loadTemplate(_name).indent(_indent);
                     }
-            return _template;
+                } finally {
+                    lock.unlock();
+                }
+            }
+            return t;
         }
         protected IncludedTemplateSegment indent(String indent, boolean first, boolean last) {
             // Indent this partial based on the spacing provided.
@@ -1084,7 +1096,8 @@ public class Mustache {
         protected final Compiler _comp;
         protected final String _name;
         private final String _indent;
-        private Template _template;
+        private final Lock lock = new ReentrantLock();
+        private volatile Template _template;
         protected boolean _standalone = false;
     }
 

src/test/java/com/samskivert/mustache/PartialThreadSafeTest.java

@@ -0,0 +1,69 @@
+package com.samskivert.mustache;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.io.IOException;
+import java.io.Reader;
+import java.io.StringReader;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ConcurrentLinkedDeque;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.junit.Test;
+
+import com.samskivert.mustache.Mustache.TemplateLoader;
+
+public class PartialThreadSafeTest {
+    
+    @Test
+    public void testPartialThreadSafe() throws Exception {
+        long t = System.currentTimeMillis();
+        AtomicInteger loadCount = new AtomicInteger();
+        TemplateLoader loader = new TemplateLoader() {
+
+            @Override
+            public Reader getTemplate(String name) throws Exception {
+                if ("partial".equals(name)) {
+                    loadCount.incrementAndGet();
+                    TimeUnit.MILLISECONDS.sleep(20);
+                    return new StringReader("Hello");
+                }
+                throw new IOException(name);
+            }
+        };
+
+        Template template = Mustache.compiler().withLoader(loader).compile("{{stuff}}\n\t{{> partial }}");
+        ExecutorService executor = Executors.newFixedThreadPool(64);
+        ConcurrentLinkedDeque<Exception> q = new ConcurrentLinkedDeque<>();
+
+        Map<String, Object> m = new HashMap<>();
+        m.put("stuff", "Foo");
+        for (int i = 100; i > 0; i--) {
+            int ii = i;
+            executor.execute(() -> {
+                try {
+                    TimeUnit.MILLISECONDS.sleep(ii % 10);
+                    template.execute(m);
+                } catch (Exception e) {
+                    q.add(e);
+                }
+            });
+        }
+        executor.shutdown();
+        executor.awaitTermination(10_000, TimeUnit.MILLISECONDS);
+        if (!q.isEmpty()) {
+            System.out.println(q);
+        }
+        assertTrue(q.isEmpty());
+        assertEquals(1, loadCount.get());
+        System.out.println(loadCount);
+        System.out.println(System.currentTimeMillis() - t);
+
+    }
+
+}