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/*
 * Copyright 2002-2021 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 io.micrometer.observation.contextpropagation;

import io.micrometer.common.util.internal.logging.InternalLogger;
import io.micrometer.common.util.internal.logging.InternalLoggerFactory;
import io.micrometer.context.ContextRegistry;
import io.micrometer.context.ThreadLocalAccessor;
import io.micrometer.observation.NullObservation;
import io.micrometer.observation.Observation;
import io.micrometer.observation.ObservationRegistry;

/**
 * A {@link ThreadLocalAccessor} to put and restore current {@link Observation}.
 *
 * @author Marcin Grzejszczak
 * @since 1.10.0
 */
public class ObservationThreadLocalAccessor implements ThreadLocalAccessor {

    private static final InternalLogger log = InternalLoggerFactory.getInstance(ObservationThreadLocalAccessor.class);

    /**
     * Key under which Micrometer Observation is being registered.
     */
    public static final String KEY = "micrometer.observation";

    /**
     * The default implementation of {@link ObservationRegistry} that Micrometer provides
     * comes with a static {@link ThreadLocal} instance for accessing the
     * {@link Observation.Scope}. That's why we can create an instance of the default
     * implementation of the registry just to call its
     * {@link ObservationRegistry#getCurrentObservation()} because that in turn will look
     * at the static ThreadLocal and will always return the proper scope.
     */
    private ObservationRegistry observationRegistry = ObservationRegistry.create();

    private static ObservationThreadLocalAccessor instance;

    /**
     * Creates a new instance of this class and stores a static handle to it. Remember to
     * call {@link ContextRegistry#getInstance()} to load all accessors which will call
     * this constructor.
     */
    public ObservationThreadLocalAccessor() {
        instance = this;
    }

    /**
     * Creates a new instance of this class.
     * @param observationRegistry observation registry
     * @since 1.10.8
     */
    public ObservationThreadLocalAccessor(ObservationRegistry observationRegistry) {
        this.observationRegistry = observationRegistry;
    }

    /**
     * Provide an {@link ObservationRegistry} to be used by
     * {@code ObservationThreadLocalAccessor}.
     * @param observationRegistry observation registry
     * @since 1.10.8
     */
    public void setObservationRegistry(ObservationRegistry observationRegistry) {
        this.observationRegistry = observationRegistry;
    }

    /**
     * Returns the provided {@link ObservationRegistry}.
     * @return observation registry
     * @since 1.10.8
     */
    public ObservationRegistry getObservationRegistry() {
        return this.observationRegistry;
    }

    /**
     * Return the singleton instance of this {@code ObservationThreadLocalAccessor}.
     * @return instance
     * @since 1.10.8
     */
    public static ObservationThreadLocalAccessor getInstance() {
        if (instance == null) {
            ContextRegistry.getInstance(); // this loads the instance
        }
        return instance;
    }

    @Override
    public Object key() {
        return KEY;
    }

    @Override
    public Observation getValue() {
        return observationRegistry.getCurrentObservation();
    }

    @Override
    public void setValue(Observation value) {
        // Iterate over all handlers and open a new scope. The created scope will put
        // itself to TL.
        value.openScope();
    }

    @Override
    public void setValue() {
        Observation currentObservation = observationRegistry.getCurrentObservation();
        if (currentObservation == null) {
            return;
        }
        // Since we can't fully rely on using the observation registry static instance
        // because you can have multiple ones being created in your code (e.g. tests,
        // production code, multiple contexts etc.) we need a way to use an existing,
        // pre-configured observation registry. What we're doing then is getting an OR
        // from an existing observation, and we pass it to the NullObservation. That
        // way we'll reuse its handlers.
        ObservationRegistry registryAttachedToCurrentObservation = currentObservation.getObservationRegistry();
        // Not closing a scope (we're not resetting)
        // Creating a new one with empty context and opens a new scope
        // This scope will remember the previously created one to
        // which we will revert once "null scope" is closed
        new NullObservation(registryAttachedToCurrentObservation).start().openScope();
    }

    private void closeCurrentScope() {
        Observation.Scope scope = observationRegistry.getCurrentObservationScope();
        if (scope != null) {
            scope.close();
        }
    }

    @Override
    public void restore() {
        closeCurrentScope();
    }

    @Override
    public void restore(Observation value) {
        Observation.Scope scope = observationRegistry.getCurrentObservationScope();
        if (scope == null) {
            String msg = "There is no current scope in thread local. This situation should not happen";
            log.warn(msg);
            assertFalse(msg);
        }
        Observation.Scope previousObservationScope = scope != null ? scope.getPreviousObservationScope() : null;
        if (previousObservationScope == null || value != previousObservationScope.getCurrentObservation()) {
            Observation previousObservation = previousObservationScope != null
                    ? previousObservationScope.getCurrentObservation() : null;
            String msg = "Observation <" + value
                    + "> to which we're restoring is not the same as the one set as this scope's parent observation <"
                    + previousObservation
                    + ">. Most likely a manually created Observation has a scope opened that was never closed. This may lead to thread polluting and memory leaks.";
            log.warn(msg);
            assertFalse(msg);
        }
        closeCurrentScope();
    }

    void assertFalse(String msg) {
        assert false : msg;
    }

    @Override
    @Deprecated
    public void reset() {
        ThreadLocalAccessor.super.reset();
    }

}