[maven-extension] Propagate OTel context to plugin mojos (#786)

**Description:**

Propagate OTel context to plugin mojos so they can add their own
attributes and spans to traces.

**Existing Issue(s):**

None

**Testing:**

No unit test framework compatible with JUnit 5 for Maven builds yet.

**Documentation:**

See updated README.md

**Outstanding items:**

None

maven-extension/README.md

@@ -36,7 +36,7 @@ Add the Maven OpenTelemetry Extension in the `pom.xml` file:
       <extension>
           <groupId>io.opentelemetry.contrib</groupId>
           <artifactId>opentelemetry-maven-extension</artifactId>
-          <version>1.10.0-alpha</version>
+          <version>1.23.0-alpha</version>
       </extension>
     </extensions>
   </build>
@@ -175,6 +175,70 @@ The [Jenkins OpenTelemetry Plugin](https://plugins.jenkins.io/opentelemetry/) ex
 
 The [`otel-cli`](https://github.com/equinix-labs/otel-cli) is a command line wrapper to observe the execution of a shell command as an OpenTelemetry trace.
 
+## Instrumenting Maven Mojos for better visibility in Maven builds
+
+Maven plugin authors can instrument Mojos for better visibility in Maven builds.
+
+Common instrumentation patterns include:
+
+* Adding contextual data as attributes on the spans created by the OpenTelemetry Maven Extension,
+* Creating additional sub spans to breakdown long mojo goal executions in finer grained steps
+
+Note that the instrumentation of a plugin is enabled when the OpenTelemetry Maven extension is added to the build and activated.
+Otherwise, the instrumentation of the Maven plugin is noop.
+
+It is recommended to enrich spans using the [OpenTelemetry Semantic Conventions](https://opentelemetry.io/docs/concepts/semantic-conventions/)
+to improve the visualization and analysis in Observability products.
+The [HTTP](https://opentelemetry.io/docs/reference/specification/trace/semantic_conventions/http/)
+and [database client calls](https://opentelemetry.io/docs/reference/specification/trace/semantic_conventions/database/)
+conventions are particularly useful when  invoking external systems.
+
+Steps to instrument a Maven Mojo:
+
+* Add the OpenTelemetry API dependency in the `pom.xml` of the Maven plugin.
+
+```xml
+<project>
+    ...
+    <dependencies>
+        <dependency>
+            <groupId>io.opentelemetry</groupId>
+            <artifactId>opentelemetry-api</artifactId>
+            <version>LATEST</version>
+        </dependency>
+        ...
+    </dependencies>
+</project>
+````
+
+* Instrument the Mojo:
+
+```java
+@Mojo(name = "test", defaultPhase = LifecyclePhase.PROCESS_SOURCES)
+public class TestMojo extends AbstractMojo {
+
+  @Override
+  public void execute() {
+    Span mojoExecuteSpan = Span.current();
+
+    // ENRICH THE DEFAULT SPAN OF THE MOJO EXECUTION
+    // (this span is created by the opentelemetry-maven-extension)
+    mojoExecuteSpan.setAttribute("an-attribute", "a-value");
+
+    // ... some logic
+
+    // CREATE SUB SPANS TO CAPTURE FINE GRAINED DETAILS OF THE MOJO EXECUTION
+    Tracer tracer = GlobalOpenTelemetry.get().getTracer("com.example.maven.otel_aware_plugin");
+    Span childSpan = tracer.spanBuilder("otel-aware-goal-sub-span").setAttribute("another-attribute", "another-value").startSpan();
+    try (Scope ignored2 = childSpan.makeCurrent()) {
+      // ... mojo sub operation
+    } finally {
+      childSpan.end();
+    }
+  }
+}
+```
+
 ## Component owners
 
 - [Cyrille Le Clerc](https://github.com/cyrille-leclerc), Elastic

maven-extension/src/main/java/io/opentelemetry/maven/OtelExecutionListener.java

@@ -11,13 +11,15 @@
 import io.opentelemetry.api.trace.StatusCode;
 import io.opentelemetry.api.trace.Tracer;
 import io.opentelemetry.context.Context;
+import io.opentelemetry.context.ContextStorage;
+import io.opentelemetry.context.Scope;
 import io.opentelemetry.context.propagation.TextMapGetter;
 import io.opentelemetry.maven.handler.MojoGoalExecutionHandler;
 import io.opentelemetry.maven.handler.MojoGoalExecutionHandlerConfiguration;
 import io.opentelemetry.maven.semconv.MavenOtelSemanticAttributes;
-import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 import org.apache.maven.execution.AbstractExecutionListener;
@@ -45,6 +47,15 @@ public final class OtelExecutionListener extends AbstractExecutionListener {
 
   private static final Logger logger = LoggerFactory.getLogger(OtelExecutionListener.class);
 
+  /**
+   * Note that using a thread local around the mojo goal execution to carry the {@link Scope } works
+   * even when using Maven build parallelization. {@link Span#current()} invoked in {@link
+   * org.apache.maven.plugin.Mojo#execute()} returns as expected the span set in {@link
+   * ExecutionListener#mojoStarted(ExecutionEvent)} using {@link Span#makeCurrent()}. For this
+   * reason, we can carry over the {@link Scope} in a thread local variable.
+   */
+  private static final ThreadLocal<Scope> MOJO_EXECUTION_SCOPE = new ThreadLocal<>();
+
   @SuppressWarnings("NullAway") // Automatically initialized by DI
   @Requirement
   private SpanRegistry spanRegistry;
@@ -53,7 +64,7 @@ public final class OtelExecutionListener extends AbstractExecutionListener {
   @Requirement
   private OpenTelemetrySdkService openTelemetrySdkService;
 
-  private Map<MavenGoal, MojoGoalExecutionHandler> mojoGoalExecutionHandlers = new HashMap<>();
+  private final Map<MavenGoal, MojoGoalExecutionHandler> mojoGoalExecutionHandlers;
 
   public OtelExecutionListener() {
     this.mojoGoalExecutionHandlers =
@@ -66,6 +77,28 @@ public OtelExecutionListener() {
               + mojoGoalExecutionHandlers.entrySet().stream()
                   .map(entry -> entry.getKey().toString() + ": " + entry.getValue().toString())
                   .collect(Collectors.joining(", ")));
+
+      // help debugging class loader issues when the OTel APIs used in
+      // Maven plugin mojos are mistakenly not loaded by the OTel Maven extension
+      // causing the lack of context propagation from the OTel Maven extension to the plugin mojos
+      ContextStorage contextStorage = ContextStorage.get();
+      logger.debug(
+          "ContextStorage: "
+              + contextStorage
+              + ", identity="
+              + System.identityHashCode(contextStorage));
+      Class<? extends ContextStorage> contextStorageClass = contextStorage.getClass();
+      logger.debug(
+          "ContextStorageClass="
+              + contextStorageClass.getName()
+              + ", identity="
+              + System.identityHashCode(contextStorageClass)
+              + " classloader="
+              + contextStorageClass.getClassLoader()
+              + " codeLocation="
+              + Optional.of(contextStorageClass.getProtectionDomain().getCodeSource())
+                  .map(source -> source.getLocation().toString())
+                  .orElse("#unknown#"));
     }
   }
 
@@ -232,7 +265,28 @@ public void mojoStarted(ExecutionEvent executionEvent) {
     }
 
     Span span = spanBuilder.startSpan();
+    @SuppressWarnings("MustBeClosedChecker")
+    Scope scope = span.makeCurrent();
     spanRegistry.putSpan(span, mojoExecution, executionEvent.getProject());
+    Optional.ofNullable(MOJO_EXECUTION_SCOPE.get())
+        .ifPresent(
+            previousScope ->
+                logger.warn(
+                    "OpenTelemetry: Scope "
+                        + System.identityHashCode(previousScope)
+                        + "already attached to thread '"
+                        + Thread.currentThread().getName()
+                        + "'"));
+    MOJO_EXECUTION_SCOPE.set(scope);
+    if (logger.isDebugEnabled()) {
+      logger.debug(
+          "OpenTelemetry: Attach scope "
+              + System.identityHashCode(scope)
+              + " to thread '"
+              + Thread.currentThread().getName()
+              + "' for "
+              + mojoExecution);
+    }
   }
 
   @Override
@@ -247,8 +301,27 @@ public void mojoSucceeded(ExecutionEvent executionEvent) {
         executionEvent.getProject());
     Span mojoExecutionSpan = spanRegistry.removeSpan(mojoExecution, executionEvent.getProject());
     mojoExecutionSpan.setStatus(StatusCode.OK);
-
     mojoExecutionSpan.end();
+    Scope scope = MOJO_EXECUTION_SCOPE.get();
+    if (scope == null) {
+      logger.warn(
+          "OpenTelemetry: No scope found on thread '"
+              + Thread.currentThread().getName()
+              + "' for succeeded "
+              + mojoExecution);
+    } else {
+      scope.close();
+      MOJO_EXECUTION_SCOPE.remove();
+      if (logger.isDebugEnabled()) {
+        logger.debug(
+            "OpenTelemetry: Remove scope "
+                + System.identityHashCode(scope)
+                + " on thread '"
+                + Thread.currentThread().getName()
+                + "' for succeeded "
+                + mojoExecution);
+      }
+    }
   }
 
   @Override
@@ -273,6 +346,26 @@ public void mojoFailed(ExecutionEvent executionEvent) {
       mojoExecutionSpan.recordException(exception);
     }
     mojoExecutionSpan.end();
+    Scope scope = MOJO_EXECUTION_SCOPE.get();
+    if (scope == null) {
+      logger.warn(
+          "OpenTelemetry: No scope found on thread '"
+              + Thread.currentThread().getName()
+              + "' for failed "
+              + mojoExecution);
+    } else {
+      scope.close();
+      MOJO_EXECUTION_SCOPE.remove();
+      if (logger.isDebugEnabled()) {
+        logger.debug(
+            "OpenTelemetry: Remove scope "
+                + System.identityHashCode(scope)
+                + " on thread '"
+                + Thread.currentThread().getName()
+                + "' for failed "
+                + mojoExecution);
+      }
+    }
   }
 
   @Override

maven-extension/src/main/resources/META-INF/maven/extension.xml

@@ -0,0 +1,14 @@
+<extension>
+    <exportedPackages>
+        <!-- opentelemetry.api.* -->
+        <exportedPackage>io.opentelemetry.api</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.*</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.baggage</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.baggage.propagation</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.common</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.internal</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.metrics</exportedPackage>
+        <exportedPackage>io.opentelemetry.api.trace</exportedPackage>
+        <exportedPackage>io.opentelemetry.context</exportedPackage>
+    </exportedPackages>
+</extension>