Investigate gap in instrumentation with Spring WebFlux/Reactor when using subscribeOn(Schedulers.boundedElastic())

[jasonjkeller]

This is a side issue that was noticed during the course of working on #1073

Below is a Spring controller that repros the problem and here is a zip of a repro app: newrelic-trace-on-util.zip

The problem is that the Transaction is lost and thus not linked when calling the response method via the /does-not-work route. Specifically this seems to be a result of using subscribeOn(Schedulers.boundedElastic()). In the past, we've seen cases where an unexpected thread hop occurred due to the use of subscribeOn(Schedulers.boundedElastic()) which caused the token linking failures.

package com.example.newrelictraceonutil;

import com.newrelic.api.agent.NewRelic;
import com.newrelic.api.agent.TracedMethod;
import com.newrelic.api.agent.Transaction;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

@RestController
public class ExampleController {

    @GetMapping(path = "/works")
    public Mono<String> works() {
        return Mono.fromCallable(() -> response("works"));
    }

    @GetMapping(path = "/does-not-work")
    public Mono<String> doesNotWork() {
        return Mono.fromCallable(() -> response("does-not-work"))
                .subscribeOn(Schedulers.boundedElastic());
    }

    /*
     * When hitting the /works route the transaction is properly renamed to works-custom-transaction-name as it
     * has reference to a valid TransactionApiImpl and DefaultTracer
     *
     * When hitting the /does-not-work route the transaction is NOT renamed as the Transaction context has been lost,
     * and it has reference to a NoOpTransaction and NoOpTracedMethod
     */
    private String response(String endpoint) {
        NewRelic.setTransactionName("custom", endpoint + "-custom-transaction-name");
        NewRelic.addCustomParameter("custom.param", endpoint);

        Transaction transaction = NewRelic.getAgent().getTransaction();
        TracedMethod tracedMethod = transaction.getTracedMethod();
        return endpoint
                + "\nTransaction:  " + transaction
                + "\nTracedMethod: " + tracedMethod;
    }

}

---

Run the repro app and hit http://localhost:8080/does-not-work. The response output will indicate that a NoOpTransaction was present when response was called. Also the transaction will remain named does-not-work (GET).

does-not-work Transaction: com.newrelic.agent.bridge.NoOpTransaction@65fbde12 TracedMethod: com.newrelic.agent.bridge.NoOpTracedMethod@46a1dc43

Compare this to the working route http://localhost:8080/works. The response output will indicate that a TransactionApiImpl was present when response was called. The transaction is renamed to works-custom-transaction-name as expected.

works Transaction: com.newrelic.agent.TransactionApiImpl@ec51b31 TracedMethod: com.newrelic.agent.tracers.DefaultTracer@5745ebc4

[workato-integration]

https://new-relic.atlassian.net/browse/NR-150619

[meiao]

The solution for this ticket should also be ported to the reactor-3.3.0 instrumentation module.