/*
 * Copyright 2013-2021 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
 *
 *      https://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;

import java.io.Closeable;

import org.springframework.cloud.sleuth.propagation.Propagator;
import org.springframework.lang.Nullable;

/**
 * This API was heavily influenced by Brave. Parts of its documentation were taken
 * directly from Brave.
 *
 * Using a tracer, you can create a root span capturing the critical path of a request.
 * Child spans can be created to allocate latency relating to outgoing requests.
 *
 * When tracing single-threaded code, just run it inside a scoped span: <pre>{@code
 * // Start a new trace or a span within an existing trace representing an operation
 * ScopedSpan span = tracer.startScopedSpan("encode");
 * try {
 *   // The span is in "scope" so that downstream code such as loggers can see trace IDs
 *   return encoder.encode();
 * } catch (RuntimeException | Error e) {
 *   span.error(e); // Unless you handle exceptions, you might not know the operation failed!
 *   throw e;
 * } finally {
 *   span.end();
 * }
 * }</pre>
 *
 * When you need more features, or finer control, use the {@linkplain Span} type:
 * <pre>{@code
 * // Start a new trace or a span within an existing trace representing an operation
 * Span span = tracer.nextSpan().name("encode").start();
 * // Put the span in "scope" so that downstream code such as loggers can see trace IDs
 * try (SpanInScope ws = tracer.withSpanInScope(span)) {
 *   return encoder.encode();
 * } catch (RuntimeException | Error e) {
 *   span.error(e); // Unless you handle exceptions, you might not know the operation failed!
 *   throw e;
 * } finally {
 *   span.end(); // note the scope is independent of the span. Always finish a span.
 * }
 * }</pre>
 *
 * Both of the above examples report the exact same span on finish!
 *
 * @author OpenZipkin Brave Authors
 * @author Marcin Grzejszczak
 * @since 3.0.0
 * @see Span
 * @see ScopedSpan
 * @see Propagator
 */
public interface Tracer extends BaggageManager {

	/**
	 * This creates a new span based on the current span in scope. If there's no such span
	 * a new trace will be created.
	 * @return a child span or a new trace if no span was present
	 */
	Span nextSpan();

	/**
	 * This creates a new span whose parent is {@link Span}. If parent is {@code null}
	 * then will create act as {@link #nextSpan()}.
	 * @param parent parent span
	 * @return a child span for the given parent, {@code null} if context was empty.
	 */
	Span nextSpan(@Nullable Span parent);

	/**
	 * Makes the given span the "current span" and returns an object that exits that scope
	 * on close. Calls to {@link #currentSpan()} and {@link #currentSpanCustomizer()} will
	 * affect this span until the return value is closed.
	 *
	 * The most convenient way to use this method is via the try-with-resources idiom.
	 *
	 * When tracing in-process commands, prefer {@link #startScopedSpan(String)} which
	 * scopes by default.
	 *
	 * Note: While downstream code might affect the span, calling this method, and calling
	 * close on the result have no effect on the input. For example, calling close on the
	 * result does not finish the span. Not only is it safe to call close, you must call
	 * close to end the scope, or risk leaking resources associated with the scope.
	 * @param span span to place into scope or null to clear the scope
	 * @return scope with span in it
	 */
	Tracer.SpanInScope withSpan(@Nullable Span span);

	/**
	 * Returns a new child span if there's a {@link #currentSpan()} or a new trace if
	 * there isn't. The result is the "current span" until {@link ScopedSpan#end()} ()} is
	 * called.
	 *
	 * Here's an example: <pre>{@code
	 * ScopedSpan span = tracer.startScopedSpan("encode");
	 * try {
	 *   // The span is in "scope" so that downstream code such as loggers can see trace IDs
	 *   return encoder.encode();
	 * } catch (RuntimeException | Error e) {
	 *   span.error(e); // Unless you handle exceptions, you might not know the operation failed!
	 *   throw e;
	 * } finally {
	 *   span.end();
	 * }
	 * }</pre>
	 * @param name of the span in scope
	 * @return span in scope
	 */
	ScopedSpan startScopedSpan(String name);

	/**
	 * In some cases (e.g. when dealing with
	 * {@link Propagator#extract(Object, Propagator.Getter)}'s we want to create a span
	 * that has not yet been started, yet it's heavily configurable (some options are not
	 * possible to be set when a span has already been started). We can achieve that by
	 * using a builder.
	 * @return a span builder
	 */
	Span.Builder spanBuilder();

	/**
	 * Builder for {@link TraceContext}.
	 * @return a trace context builder
	 */
	TraceContext.Builder traceContextBuilder();

	/**
	 * Returns the {@link CurrentTraceContext}. Can be {@code null} so that we don't break
	 * backward compatibility.
	 * @return current trace context
	 */
	@Nullable
	CurrentTraceContext currentTraceContext();

	/**
	 * Allows to customize the current span in scope.
	 * @return current span customizer
	 */
	@Nullable
	SpanCustomizer currentSpanCustomizer();

	/**
	 * Retrieves the current span in scope or {@code null} if one is not available.
	 * @return current span in scope
	 */
	@Nullable
	Span currentSpan();

	/**
	 * Scope of a span. Needs to be closed so that resources are let go (e.g. MDC is
	 * cleared).
	 */
	interface SpanInScope extends Closeable {

		@Override
		void close();

	}

}

@@ -5440,16 +5440,24 @@
 llable%0A%09
+default 
 CurrentT
@@ -5489,17 +5489,36 @@
 ontext()
-;
+ %7B%0A%09%09return null;%0A%09%7D
 %0A%0A%09/**%0A%09

Add default implementation to Tracer#currentTraceContext