Reusing reactor context for span continuing subscriber

without this change we were not reusing a span context if there was no previous span. Instead we were overwrriting it with an empty one

with this change, if the context is there we are reusing it instead of overwriting it

fixes gh-897

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ScopePassingSpanSubscriber.java

@@ -46,8 +46,8 @@ final class ScopePassingSpanSubscriber<T> extends AtomicBoolean implements SpanS
 		Span root = ctx != null ?
 				ctx.getOrDefault(Span.class, this.tracer.currentSpan()) : null;
 		this.span = root;
-		this.context = root != null ?
-				ctx.put(Span.class, root): Context.empty();
+		this.context = ctx != null && root != null ? ctx.put(Span.class, root) :
+				ctx != null ? ctx : Context.empty();
 	}
 
 	@Override public void onSubscribe(Subscription subscription) {

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/ScopePassingSpanSubscriberTests.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2013-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.sleuth.instrument.reactor;
+
+import brave.Span;
+import brave.Tracer;
+import brave.Tracing;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.junit.MockitoJUnitRunner;
+import reactor.util.context.Context;
+
+import static org.assertj.core.api.BDDAssertions.then;
+
+/**
+ * @author Marcin Grzejszczak
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class ScopePassingSpanSubscriberTests {
+
+	Tracing tracing = Tracing.newBuilder().build();
+
+	@Test
+	public void should_propagate_current_context() {
+		ScopePassingSpanSubscriber subscriber = new ScopePassingSpanSubscriber(
+				null, Context.of("foo", "bar"), this.tracing);
+
+		then((String) subscriber.currentContext().get("foo")).isEqualTo("bar");
+	}
+
+	@Test
+	public void should_set_empty_context_when_context_is_null() {
+		ScopePassingSpanSubscriber subscriber = new ScopePassingSpanSubscriber(
+				null, null, this.tracing);
+
+		then(subscriber.currentContext().isEmpty()).isTrue();
+	}
+
+	@Test
+	public void should_put_current_span_to_context() {
+		Span span = this.tracing.tracer().nextSpan();
+		try (Tracer.SpanInScope ws = this.tracing.tracer().withSpanInScope(span.start())) {
+			ScopePassingSpanSubscriber subscriber = new ScopePassingSpanSubscriber(
+					null, Context.empty(), this.tracing);
+
+			then(subscriber.currentContext().get(Span.class)).isEqualTo(span);
+		}
+
+	}
+}