[Bug Report]: Application crash when producing second Kafka message – Assertion failed: (r == 0) in tinycthread_extra.c

[Oliverbyhring]

Prerequisites

• I have searched issues to ensure it has not already been reported

Description

We experienced a critical issue when producing Kafka messages where:

• The first message could sometimes be produced successfully.
• Producing a second message caused the application to crash.
• The application terminated with the following error:

Assertion failed: (r == 0), function rwlock_rdlock,
file tinycthread_extra.c, line 186.

No additional logs were emitted — even with debug logging enabled.

---

Environment

• .NET: version 10.0
• KafkaFlow: 4.1.0
• librdkafka version (if known): 12.13.1
• OS: Linux (Same behaviour on MacOS Tahoe 26.3)
• Hosting environment: AKS (Same behaviour locally)

Steps to reproduce

How We Were Registering Producers (Problematic Setup)

We were registering keyed producers like this:

static IServiceCollection AddKeyedProducer(this IServiceCollection services, string producerName)
{
    services.AddKeyedTransient<IMessageProducer>(producerName, (serviceProvider, _) =>
    {
        var producerAccessor = serviceProvider.GetRequiredService<IProducerAccessor>();
        var producer = producerAccessor.GetProducer(producerName);
        return producer;
    });
    return services;
}

Then adding the producer like this:

services.AddKeyedProducer(Constants.ProducerNames.ProducerName);

And in the service we injected the producer

public class Service(
    [FromKeyedServices(Constants.ProducerNames.ProducerName)]
    IMessageProducer producer)
{
    public async Task SendAsync(Message message)
    {
        await producer.ProduceAsync(null, message);
    }
}

---

Observed Problem

With this setup:

• The first ProduceAsync call would often succeed.
• A subsequent call (sometimes immediately after, sometimes later) would cause the application to terminate.
• The process crashed with:

Assertion failed: (r == 0), function rwlock_rdlock,
file tinycthread_extra.c, line 186.

No managed exception was thrown.
No additional logs were emitted — even with debug logging enabled.
The crash appeared to originate from the native layer (librdkafka).

The behavior was intermittent but reproducible under load or when producing multiple messages in sequence.

---

What We Changed (Working Setup)

Instead of injecting a keyed IMessageProducer, we changed the pattern to inject IProducerAccessor directly into the service and retrieve the producer explicitly when needed.

public class Service(IProducerAccessor producerAccessor)
{
    public async Task SendAsync(Message message)
    {
        var producer = producerAccessor.GetProducer(Constants.ProducerNames.ProducerName);
        await producer.ProduceAsync(null, message);
    }
}

After this change:

• The crash no longer occurs.
• Multiple sequential ProduceAsync calls work as expected.
• No native assertions are triggered.

Expected behavior

Producing multiple messages sequentially using an injected IMessageProducer should:

• Work reliably.
• Not terminate the process.
• Not trigger native assertion failures.
• Surface any failures as managed exceptions or proper logs.

Actual behavior

• First ProduceAsync call succeeds.
• A subsequent call causes the application to terminate immediately.
• No managed exception is thrown.
• No additional logs are emitted.
• The process crashes with:

Assertion failed: (r == 0), function rwlock_rdlock,
file tinycthread_extra.c, line 186.

The crash appears to originate from a native rwlock inside librdkafka.

KafkaFlow version

4.1.0