Surface potential root cause when agent initialization errors

dd-java-agent/src/main/java/datadog/trace/bootstrap/BootstrapInitializationTelemetry.java

@@ -1,11 +1,15 @@
 package datadog.trace.bootstrap;
 
 import datadog.json.JsonWriter;
+import datadog.trace.bootstrap.environment.EnvironmentVariables;
 import de.thetaphi.forbiddenapis.SuppressForbidden;
 import java.io.Closeable;
 import java.io.OutputStream;
 import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Map;
 
 /** Thread safe telemetry class used to relay information about tracer activation. */
 public abstract class BootstrapInitializationTelemetry {
@@ -88,15 +92,15 @@ public static final class JsonBased extends BootstrapInitializationTelemetry {
     private final JsonSender sender;
 
     private final List<String> meta;
-    private final List<String> points;
+    private final Map<String, List<String>> points;
 
     // one way false to true
     private volatile boolean incomplete = false;
 
     JsonBased(JsonSender sender) {
       this.sender = sender;
       this.meta = new ArrayList<>();
-      this.points = new ArrayList<>();
+      this.points = new LinkedHashMap<>();
     }
 
     @Override
@@ -115,7 +119,35 @@ public void onAbort(String reasonCode) {
 
     @Override
     public void onError(Throwable t) {
-      onPoint("library_entrypoint.error", "error_type:" + t.getClass().getName());
+      List<String> causes = new ArrayList<>();
+
+      Throwable cause = t.getCause();
+      while (cause != null) {
+        causes.add("error_type:" + cause.getClass().getName());
+        cause = cause.getCause();
+      }
+      causes.add("error_type:" + t.getClass().getName());
+
+      // Limit the number of tags to avoid overpopulating the JSON payload.
+      int maxTags = maxTags(5);
+      int numCauses = causes.size();
+      if (numCauses > maxTags) {
+        causes = causes.subList(numCauses - maxTags, numCauses);
+      }
+
+      onPoint("library_entrypoint.error", causes);
+    }
+
+    private int maxTags(int defaultValue) {
+      try {
+        String s =
+            EnvironmentVariables.getOrDefault(
+                "DD_TELEMETRY_FORWARDER_MAX_TAGS", String.valueOf(defaultValue));
+
+        return Integer.parseInt(s);
+      } catch (Exception e) {
+        return defaultValue;
+      }
     }
 
     @Override
@@ -130,9 +162,12 @@ public void onError(String reasonCode) {
     }
 
     private void onPoint(String name, String tag) {
+      onPoint(name, Collections.singletonList(tag));
+    }
+
+    private void onPoint(String name, List<String> tags) {
       synchronized (this.points) {
-        this.points.add(name);
-        this.points.add(tag);
+        this.points.put(name, tags);
       }
     }
 
@@ -156,10 +191,14 @@ public void finish() {
 
         writer.name("points").beginArray();
         synchronized (this.points) {
-          for (int i = 0; i + 1 < this.points.size(); i = i + 2) {
+          for (Map.Entry<String, List<String>> entry : points.entrySet()) {
             writer.beginObject();
-            writer.name("name").value(this.points.get(i));
-            writer.name("tags").beginArray().value(this.points.get(i + 1)).endArray();
+            writer.name("name").value(entry.getKey());
+            writer.name("tags").beginArray();
+            for (String tag : entry.getValue()) {
+              writer.value(tag);
+            }
+            writer.endArray();
             writer.endObject();
           }
           this.points.clear();

dd-java-agent/src/test/groovy/datadog/trace/bootstrap/BootstrapInitializationTelemetryTest.groovy

@@ -76,6 +76,18 @@ class BootstrapInitializationTelemetryTest extends Specification {
     !capture.json().contains("library_entrypoint.complete")
   }
 
+  def "unwind root cause"() {
+    when:
+    initTelemetry.initMetaInfo("runtime_name", "java")
+    initTelemetry.initMetaInfo("runtime_version", "1.8.0_382")
+
+    initTelemetry.onError(new Exception("top cause", new FileNotFoundException("root cause")))
+    initTelemetry.finish()
+
+    then:
+    capture.json() == '{"metadata":{"runtime_name":"java","runtime_version":"1.8.0_382"},"points":[{"name":"library_entrypoint.error","tags":["error_type:java.io.FileNotFoundException","error_type:java.lang.Exception"]},{"name":"library_entrypoint.complete"}]}'
+  }
+
   static class Capture implements BootstrapInitializationTelemetry.JsonSender {
     String json