`NullPointerException` and `ArrayIndexOutOfBoundsException` with pekko-http

### Describe the bug

We get intermittent NPE and array out of bounds exceptions in processes that uses Pekko HTTP.

The interesting part of the stack trace:
```scala
java.lang.ArrayIndexOutOfBoundsException: arraycopy: last source index 36 out of bounds for byte[29]
	at java.base/java.lang.System.arraycopy(Native Method)
	at java.base/java.lang.AbstractStringBuilder.getBytes(AbstractStringBuilder.java:1669)
	at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:613)
	at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:629)
	at java.base/java.lang.StringBuilder.append(StringBuilder.java:209)
	at java.base/java.lang.StringBuilder.append(StringBuilder.java:91)
	at java.base/java.lang.AbstractStringBuilder.<init>(AbstractStringBuilder.java:161)
	at java.base/java.lang.StringBuilder.<init>(StringBuilder.java:143)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteHolder.save(PekkoRouteHolder.java:69)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper.apply(PekkoRouteWrapper.java:29)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper.apply(PekkoRouteWrapper.java:15)
	at org.apache.pekko.http.scaladsl.server.RouteConcatenation$RouteWithConcatenation.$anonfun$$tilde$2(RouteConcatenation.scala:58)
	at org.apache.pekko.http.scaladsl.util.FastFuture$.strictTransform$1(FastFuture.scala:49)
	at org.apache.pekko.http.scaladsl.util.FastFuture$.$anonfun$transformWith$3(FastFuture.scala:59)
	at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:517)
	at org.apache.pekko.dispatch.BatchingExecutor$AbstractBatch.processBatch(BatchingExecutor.scala:73)
	at org.apache.pekko.dispatch.BatchingExecutor$BlockableBatch.$anonfun$run$1(BatchingExecutor.scala:110)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
	at scala.concurrent.BlockContext$.withBlockContext(BlockContext.scala:94)
	at org.apache.pekko.dispatch.BatchingExecutor$BlockableBatch.run(BatchingExecutor.scala:110)
	at org.apache.pekko.dispatch.TaskInvocation.run(AbstractDispatcher.scala:59)
	at org.apache.pekko.dispatch.ForkJoinExecutorConfigurator$PekkoForkJoinTask.exec(ForkJoinExecutorConfigurator.scala:61)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)


java.lang.NullPointerException: null
	at java.base/java.lang.System.arraycopy(Native Method)
	at java.base/java.lang.AbstractStringBuilder.getBytes(AbstractStringBuilder.java:1669)
	at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:613)
	at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:629)
	at java.base/java.lang.StringBuilder.append(StringBuilder.java:209)
	at java.base/java.lang.StringBuilder.append(StringBuilder.java:91)
	at java.base/java.lang.AbstractStringBuilder.<init>(AbstractStringBuilder.java:161)
	at java.base/java.lang.StringBuilder.<init>(StringBuilder.java:143)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteHolder.save(PekkoRouteHolder.java:69)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper.apply(PekkoRouteWrapper.java:29)
	at io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper.apply(PekkoRouteWrapper.java:15)
	at org.apache.pekko.http.scaladsl.server.RouteConcatenation$RouteWithConcatenation.$anonfun$$tilde$2(RouteConcatenation.scala:58)
	at org.apache.pekko.http.scaladsl.util.FastFuture$.flatMap$extension(FastFuture.scala:49)
	at org.apache.pekko.http.scaladsl.server.RouteConcatenation$RouteWithConcatenation.$anonfun$$tilde$1(RouteConcatenation.scala:55)
	at org.apache.pekko.http.scaladsl.server.directives.BasicDirectives.$anonfun$mapRouteResult$2(BasicDirectives.scala:80)
	at org.apache.pekko.http.scaladsl.server.directives.BasicDirectives.$anonfun$textract$2(BasicDirectives.scala:173)
	at org.apache.pekko.http.scaladsl.server.directives.BasicDirectives.$anonfun$mapRouteResultWith$2(BasicDirectives.scala:86)
	at org.apache.pekko.http.scaladsl.server.directives.BasicDirectives.$anonfun$textract$2(BasicDirectives.scala:173)
	at org.apache.pekko.http.scaladsl.server.directives.ExecutionDirectives.$anonfun$handleExceptions$2(ExecutionDirectives.scala:42)
	at org.apache.pekko.http.scaladsl.server.Route$.$anonfun$createAsyncHandler$1(Route.scala:127)
	at org.apache.pekko.stream.impl.fusing.MapAsyncUnordered$$anon$29.onPush(Ops.scala:1435)
	at org.apache.pekko.stream.impl.fusing.GraphInterpreter.processPush(GraphInterpreter.scala:556)
	at org.apache.pekko.stream.impl.fusing.GraphInterpreter.execute(GraphInterpreter.scala:434)
	at org.apache.pekko.stream.impl.fusing.GraphInterpreterShell.runBatch(ActorGraphInterpreter.scala:662)
	at org.apache.pekko.stream.impl.fusing.GraphInterpreterShell$AsyncInput.execute(ActorGraphInterpreter.scala:532)
	at org.apache.pekko.stream.impl.fusing.GraphInterpreterShell.processEvent(ActorGraphInterpreter.scala:637)
	at org.apache.pekko.stream.impl.fusing.ActorGraphInterpreter.org$apache$pekko$stream$impl$fusing$ActorGraphInterpreter$$processEvent(ActorGraphInterpreter.scala:813)
	at org.apache.pekko.stream.impl.fusing.ActorGraphInterpreter$$anonfun$receive$1.applyOrElse(ActorGraphInterpreter.scala:831)
	at org.apache.pekko.actor.Actor.aroundReceive(Actor.scala:547)
	at org.apache.pekko.actor.Actor.aroundReceive$(Actor.scala:545)
	at org.apache.pekko.stream.impl.fusing.ActorGraphInterpreter.aroundReceive(ActorGraphInterpreter.scala:729)
	at org.apache.pekko.actor.ActorCell.receiveMessage(ActorCell.scala:590)
	at org.apache.pekko.actor.ActorCell.invoke(ActorCell.scala:557)
	at org.apache.pekko.dispatch.Mailbox.processMailbox(Mailbox.scala:272)
	at org.apache.pekko.dispatch.Mailbox.run(Mailbox.scala:233)
	at org.apache.pekko.dispatch.Mailbox.exec(Mailbox.scala:245)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)
```

### Steps to reproduce

I'm at a loss trying to reproduce this in a controlled project, it happens very rarely for us in production. The only thing I have found is that I suspect that there is a race condition in `io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper#apply` between `routeHolder.save()` and `routeHolder.restore()` and this messes up `StringBuilder route` in `PekkoRouteHolder`. This is mostly a guess on my part, perhaps this is not meant to be thread safe, and instead should never be called from different threads concurrently.

I have not been able to get it to throw NPE or out of bound exceptions in a controlled project, but I have managed to get `routeHolder.restore()` to run on a different thread than `routeHolder.save()` which I guess should be bad since `PekkoRouteHolder` is not thread safe. This together with other concurrent calls to `push` or `pushIfNotCompletelyMatched` could lead to 

This simple Pekko HTTP server tries to force the `map` in `io.opentelemetry.javaagent.instrumentation.pekkohttp.v1_0.server.route.PekkoRouteWrapper#apply` to run on a different thread than the one that called `save`:

```scala
object PekkoHttpServer {

  def main(args: Array[String]): Unit = {
    implicit val system: ActorSystem[Nothing] = ActorSystem(Behaviors.empty, "my-system")
    implicit val executionContext: ExecutionContextExecutor = system.executionContext

    val wrappedResponse: Route = { ctx =>
      ctx
        .reject()
        .flatMap { result =>
          // tries to force context switch
          Future { result }(ctx.executionContext)
        }(ctx.executionContext)
    }

    val route =
      wrappedResponse ~ path("hello") {
        complete(HttpResponse(StatusCodes.OK, entity = HttpEntity(ContentTypes.`text/plain(UTF-8)`, "Hello World")))
      }

    Http().newServerAt("localhost", 8080).bind(route)
  }
}
```

Running the agent with this patch somewhat consistently prints "UH OH" when `save` and `restore` is run on different threads. Again, this might be a red herring, but I can't think of any other reasons for the `StringBuilder` to be corrupted in this way.

[uh-oh-logger.patch](https://github.com/user-attachments/files/24199484/uh-oh-logger.patch)

### Expected behavior

No NPE and OOB exceptions.

### Actual behavior

Calls from multiple threads calling into code that modifies state that is probably not thread safe.

### Javaagent or library instrumentation version

2.23.0

### Environment

**JDK**: openjdk 21.0.9 2025-10-21
**OS**: arch linux


### Additional context

_No response_

### Tip

<sub>[React](https://github.blog/news-insights/product-news/add-reactions-to-pull-requests-issues-and-comments/) with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding `+1` or `me too`, to help us triage it. Learn more [here](https://opentelemetry.io/community/end-user/issue-participation/).</sub>

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

`NullPointerException` and `ArrayIndexOutOfBoundsException` with pekko-http #15681

Describe the bug

Steps to reproduce

Expected behavior

Actual behavior

Javaagent or library instrumentation version

Environment

Additional context

Tip

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

NullPointerException and ArrayIndexOutOfBoundsException with pekko-http #15681

Description

Describe the bug

Steps to reproduce

Expected behavior

Actual behavior

Javaagent or library instrumentation version

Environment

Additional context

Tip

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

`NullPointerException` and `ArrayIndexOutOfBoundsException` with pekko-http #15681