Filter out scalar Mono/Flux instances

instrumentation/reactor/reactor-3.1/library/src/main/java/io/opentelemetry/instrumentation/reactor/v3_1/ContextPropagationOperator.java

@@ -155,7 +155,8 @@ public void resetOnEachOperator() {
 
   private static <T> Function<? super Publisher<T>, ? extends Publisher<T>> tracingLift(
       ReactorAsyncOperationEndStrategy asyncOperationEndStrategy) {
-    return Operators.lift(new Lifter<>(asyncOperationEndStrategy));
+    return Operators.lift(
+        ContextPropagationOperator::shouldInstrument, new Lifter<>(asyncOperationEndStrategy));
   }
 
   /** Forces Mono to run in traceContext scope. */
@@ -220,7 +221,12 @@ public reactor.util.context.Context apply(reactor.util.context.Context context)
     }
   }
 
-  public static class Lifter<T>
+  private static boolean shouldInstrument(Scannable publisher) {
+    // skip if Flux/Mono #just, #empty, #error
+    return !(publisher instanceof Fuseable.ScalarCallable);
+  }
+
+  private static class Lifter<T>
       implements BiFunction<Scannable, CoreSubscriber<? super T>, CoreSubscriber<? super T>> {
 
     /** Holds reference to strategy to prevent it from being collected. */
@@ -233,10 +239,6 @@ public Lifter(ReactorAsyncOperationEndStrategy asyncOperationEndStrategy) {
 
     @Override
     public CoreSubscriber<? super T> apply(Scannable publisher, CoreSubscriber<? super T> sub) {
-      // if Flux/Mono #just, #empty, #error
-      if (publisher instanceof Fuseable.ScalarCallable) {
-        return sub;
-      }
       return new TracingSubscriber<>(sub, sub.currentContext());
     }
   }

instrumentation/reactor/reactor-3.1/library/src/test/java/io/opentelemetry/instrumentation/reactor/v3_1/HooksTest.java

@@ -7,10 +7,14 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+import java.util.concurrent.Callable;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReference;
 import org.junit.jupiter.api.Test;
 import reactor.core.CoreSubscriber;
+import reactor.core.Disposable;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 class HooksTest {
 
@@ -43,4 +47,28 @@ public void subscribe(CoreSubscriber<? super Integer> actual) {
       subscriber.set(actual);
     }
   }
+
+  @Test
+  void testInvalidBlockUsage() throws InterruptedException {
+    ContextPropagationOperator operator = ContextPropagationOperator.create();
+    operator.registerOnEachOperator();
+
+    Callable<String> callable =
+        () -> {
+          Mono.just("test1").block();
+          return "call1";
+        };
+
+    Disposable disposable =
+        Mono.defer(
+                () ->
+                    Mono.fromCallable(callable).publishOn(Schedulers.elastic()).flatMap(Mono::just))
+            .subscribeOn(Schedulers.single())
+            .subscribe();
+
+    TimeUnit.MILLISECONDS.sleep(100);
+
+    disposable.dispose();
+    operator.resetOnEachOperator();
+  }
 }