Fall back to global trace config when a span's own config is null

internal-api/src/main/java/datadog/trace/bootstrap/instrumentation/api/AgentTracer.java

@@ -163,7 +163,8 @@ public static AgentScope activateNext(final AgentSpan span) {
   }
 
   public static TraceConfig traceConfig(final AgentSpan span) {
-    return null != span ? span.traceConfig() : traceConfig();
+    final TraceConfig config = span == null ? null : span.traceConfig();
+    return config == null ? traceConfig() : config;
   }
 
   public static TraceConfig traceConfig() {

internal-api/src/test/java/datadog/trace/bootstrap/instrumentation/api/AgentTracerTest.java

@@ -0,0 +1,46 @@
+package datadog.trace.bootstrap.instrumentation.api;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import datadog.trace.api.TraceConfig;
+import org.junit.jupiter.api.Test;
+
+class AgentTracerTest {
+
+  @Test
+  void traceConfigOfNullSpanFallsBackToGlobalConfig() {
+    assertSame(AgentTracer.traceConfig(), AgentTracer.traceConfig((AgentSpan) null));
+  }
+
+  @Test
+  void traceConfigOfSpanWithOwnConfigReturnsThatConfig() {
+    TraceConfig ownConfig = mock(TraceConfig.class);
+    AgentSpan span = mock(AgentSpan.class);
+    when(span.traceConfig()).thenReturn(ownConfig);
+
+    assertSame(ownConfig, AgentTracer.traceConfig(span));
+  }
+
+  // Regression test: a span wrapping only an extracted remote context (e.g. built via
+  // AgentSpan.fromSpanContext for a propagated-but-not-yet-local trace) reports a null
+  // TraceConfig. Callers such as LogbackLoggerInstrumentation$CallAppendersAdvice do
+  // `traceConfig(span).isLogsInjectionEnabled()` without a further null check, so
+  // traceConfig(AgentSpan) must never return null for a non-null span.
+  @Test
+  void traceConfigOfExtractedSpanFallsBackToGlobalConfigInsteadOfNull() {
+    AgentSpan extractedSpan = AgentSpan.fromSpanContext(new TagContext());
+
+    assertNull(
+        extractedSpan.traceConfig(),
+        "test setup: expected the extracted span itself to report a null config");
+
+    TraceConfig config = AgentTracer.traceConfig(extractedSpan);
+
+    assertNotNull(config);
+    assertSame(AgentTracer.traceConfig(), config);
+  }
+}