Make source file tracking asynchronous

Source file tracking is enqueueing classfile buffer to be processed
by a background thread avoiding startup delay.

Author: jpbempel

dd-java-agent/agent-debugger/src/main/java/com/datadog/debugger/agent/DebuggerAgent.java

@@ -74,10 +74,7 @@ public static synchronized void run(Instrumentation inst, SharedCommunicationObj
     Config config = Config.get();
     DebuggerContext.initProductConfigUpdater(new DefaultProductConfigUpdater());
     classesToRetransformFinder = new ClassesToRetransformFinder();
-    if (config.isDynamicInstrumentationEnabled() || config.isDebuggerExceptionEnabled()) {
-      // only activate Source File Tracking if DI or ER is enabled from the start
-      setupSourceFileTracking(instrumentation, classesToRetransformFinder);
-    }
+    setupSourceFileTracking(instrumentation, classesToRetransformFinder);
     if (config.isDebuggerCodeOriginEnabled()) {
       startCodeOriginForSpans();
     }
@@ -314,7 +311,10 @@ private static String getDiagnosticEndpoint(
 
   private static void setupSourceFileTracking(
       Instrumentation instrumentation, ClassesToRetransformFinder finder) {
-    instrumentation.addTransformer(new SourceFileTrackingTransformer(finder));
+    SourceFileTrackingTransformer sourceFileTrackingTransformer =
+        new SourceFileTrackingTransformer(finder);
+    sourceFileTrackingTransformer.start();
+    instrumentation.addTransformer(sourceFileTrackingTransformer);
   }
 
   private static void loadFromFile(

dd-java-agent/agent-debugger/src/main/java/com/datadog/debugger/agent/SourceFileTrackingTransformer.java

@@ -4,22 +4,57 @@
 import static com.datadog.debugger.util.ClassFileHelper.stripPackagePath;
 
 import com.datadog.debugger.util.ClassFileHelper;
+import datadog.trace.util.AgentTaskScheduler;
 import java.lang.instrument.ClassFileTransformer;
 import java.lang.instrument.IllegalClassFormatException;
 import java.security.ProtectionDomain;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.TimeUnit;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Permanent Transformer to track all Inner or Top-Level classes associated with the same SourceFile
  * (String.java) Allows to get all classes that are dependent from a source file and be able to
  * trigger {@link java.lang.instrument.Instrumentation#retransformClasses(Class[])} on them
  */
 public class SourceFileTrackingTransformer implements ClassFileTransformer {
+  private static final Logger LOGGER = LoggerFactory.getLogger(SourceFileTrackingTransformer.class);
+
   private final ClassesToRetransformFinder finder;
+  private final Queue<SourceFileItem> queue = new ConcurrentLinkedQueue<>();
+  private final AgentTaskScheduler scheduler = AgentTaskScheduler.INSTANCE;
+  private AgentTaskScheduler.Scheduled<Runnable> scheduled;
 
   public SourceFileTrackingTransformer(ClassesToRetransformFinder finder) {
     this.finder = finder;
   }
 
+  public void start() {
+    scheduled = scheduler.scheduleAtFixedRate(this::flush, 0, 1, TimeUnit.SECONDS);
+  }
+
+  public void stop() {
+    if (scheduled != null) {
+      scheduled.cancel();
+    }
+  }
+
+  void flush() {
+    if (queue.isEmpty()) {
+      return;
+    }
+    int size = queue.size();
+    long start = System.nanoTime();
+    SourceFileItem item;
+    while ((item = queue.poll()) != null) {
+      registerSourceFile(item.className, item.classfileBuffer);
+    }
+    LOGGER.debug(
+        "flushing {} source file items in {}ms", size, (System.nanoTime() - start) / 1_000_000);
+  }
+
   @Override
   public byte[] transform(
       ClassLoader loader,
@@ -31,16 +66,30 @@ public byte[] transform(
     if (className == null) {
       return null;
     }
+    queue.add(new SourceFileItem(className, classfileBuffer));
+    return null;
+  }
+
+  private void registerSourceFile(String className, byte[] classfileBuffer) {
     String sourceFile = ClassFileHelper.extractSourceFile(classfileBuffer);
     if (sourceFile == null) {
-      return null;
+      return;
     }
     String simpleClassName = stripPackagePath(className);
     String simpleSourceFile = removeExtension(sourceFile);
     if (simpleClassName.equals(simpleSourceFile)) {
-      return null;
+      return;
     }
     finder.register(sourceFile, className);
-    return null;
+  }
+
+  private static class SourceFileItem {
+    final String className;
+    final byte[] classfileBuffer;
+
+    public SourceFileItem(String className, byte[] classfileBuffer) {
+      this.className = className;
+      this.classfileBuffer = classfileBuffer;
+    }
   }
 }