Fix MDC context cleanup (micronaut-projects#3328)

Author: dstepanov

tracing/src/main/java/io/micronaut/tracing/instrument/util/MdcInstrumenter.java

@@ -20,8 +20,6 @@
 import io.micronaut.scheduling.instrument.InvocationInstrumenter;
 import io.micronaut.scheduling.instrument.InvocationInstrumenterFactory;
 import io.micronaut.scheduling.instrument.ReactiveInvocationInstrumenterFactory;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 import org.slf4j.MDC;
 
 import javax.inject.Singleton;
@@ -39,9 +37,6 @@
 @Internal
 public final class MdcInstrumenter implements InvocationInstrumenterFactory, ReactiveInvocationInstrumenterFactory {
 
-    private static final Logger LOG = LoggerFactory.getLogger(MdcInstrumenter.class);
-
-
     /**
      * Creates optional invocation instrumenter.
      *
@@ -57,10 +52,7 @@ public InvocationInstrumenter newInvocationInstrumenter() {
             @Override
             public void beforeInvocation() {
                 oldContextMap = MDC.getCopyOfContextMap();
-                if (oldContextMap == null || oldContextMap.isEmpty()) {
-                    LOG.debug("{} pushes data to MDC: {}", this, contextMap);
-                }
-                if (contextMap != null) {
+                if (contextMap != null && !contextMap.isEmpty()) {
                     MDC.setContextMap(contextMap);
                 }
             }
@@ -70,7 +62,6 @@ public void afterInvocation(boolean cleanup) {
                 if (oldContextMap != null && !oldContextMap.isEmpty()) {
                     MDC.setContextMap(oldContextMap);
                 } else {
-                    LOG.debug("{} clears MDC", this);
                     MDC.clear();
                 }
             }

tracing/src/test/groovy/io/micronaut/tracing/instrument/util/MdcInstrumenterSpec.groovy

@@ -112,6 +112,36 @@ class MdcInstrumenterSpec extends Specification {
         MDC.get(key) == value2
     }
 
+    def "empty context should't clean MDC"() {
+        when:
+        MDC.put(key, value)
+        MDC.remove(key) // Make empty context
+        def mdcBeforeNew = MDC.copyOfContextMap
+        def instrumenterWithEmptyContext = mdcInstrumenter.newInvocationInstrumenter()
+        MDC.put("contextValue", "contextValue")
+        def instrumenterWithContext = mdcInstrumenter.newInvocationInstrumenter()
+        MDC.clear()
+        Runnable runnable = InvocationInstrumenter.instrument({
+            MDC.put("inside1", "inside1")
+            InvocationInstrumenter.instrument({
+                assert MDC.get("contextValue") == "contextValue"
+                assert MDC.get("inside1") == "inside1"
+
+                MDC.put("inside2", "inside2")
+            } as Runnable, instrumenterWithEmptyContext).run()
+            assert MDC.get("contextValue") == "contextValue"
+            assert MDC.get("inside1") == "inside1"
+            assert MDC.get("inside2") == null
+        } as Runnable, instrumenterWithContext)
+        runnable.run()
+
+        then:
+        mdcBeforeNew.isEmpty()
+        MDC.get(key) == null
+        MDC.get("XXX") == null
+        MDC.get("aaa") == null
+    }
+
     void "test MDC instrumenter with Executor"() {
 
         given: