Fix lifecycle to support the Maven Daemon (#169)

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

@@ -19,7 +19,13 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-/** Hold the state of the spans in progress */
+/**
+ * Hold the state of the spans in progress.
+ *
+ * <p>As Maven processes, including the <a href="https://github.com/apache/maven-mvnd">Maven
+ * Daemon</a>, can't execute multiple builds concurrently, there is no need to differentiate spans
+ * per {@link org.apache.maven.execution.MavenSession}.
+ */
 @Component(role = SpanRegistry.class)
 public final class SpanRegistry {
 
@@ -75,6 +81,7 @@ public Span removeRootSpan() {
                   .map(MojoExecutionKey::toString)
                   .collect(Collectors.joining(", ")));
     }
+    this.rootSpan = null;
     return rootSpan;
   }
 

maven-extension/src/test/java/io/opentelemetry/maven/SpanRegistryTest.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.maven;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import io.opentelemetry.api.OpenTelemetry;
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.api.trace.Tracer;
+import org.junit.jupiter.api.Test;
+
+public class SpanRegistryTest {
+
+  /** MVND reuses the same Maven process and thus the Span Registry is reused. */
+  @Test
+  public void testSpanRegistryReuseWhenUsingMvnDaemon() {
+    SpanRegistry spanRegistry = new SpanRegistry();
+
+    Tracer tracer = OpenTelemetry.noop().getTracer("test");
+    Span firstRootSpan = tracer.spanBuilder("com.example:my-jar-1.1.0-SNAPSHOT").startSpan();
+    spanRegistry.setRootSpan(firstRootSpan);
+    Span firstRemovedRootSpan = spanRegistry.removeRootSpan();
+    assertThat(firstRemovedRootSpan).isEqualTo(firstRootSpan);
+
+    Span secondRootSpan = tracer.spanBuilder("com.example:my-jar-2.1.0-SNAPSHOT").startSpan();
+    spanRegistry.setRootSpan(secondRootSpan);
+    Span secondRemovedSpan = spanRegistry.removeRootSpan();
+    assertThat(secondRemovedSpan).isEqualTo(secondRootSpan);
+  }
+}