KafkaConsumer: change locality of message error (#122)

Motivation:

If the `KafkaConsumer` receives an errorneous message, it should not
throw in the `func run()` method but rather in the
`KafkaConsumer.messages` `ThrowingSequence`.

Modifications:

* make `KafkaConsumerMessages` `AsyncSequence` wrap a sequence that
  yields a `Result<>` type -> `KafkaConsumerMessages` unwraps this
  result type and either yields or throws

Author: felixschlegel

Sources/Kafka/KafkaConsumer.swift

@@ -67,7 +67,7 @@ public struct KafkaConsumerMessages: Sendable, AsyncSequence {
     public typealias Element = KafkaConsumerMessage
     typealias BackPressureStrategy = NIOAsyncSequenceProducerBackPressureStrategies.NoBackPressure
     typealias WrappedSequence = NIOThrowingAsyncSequenceProducer<
-        Element,
+        Result<KafkaConsumerMessage, Error>,
         Error,
         BackPressureStrategy,
         KafkaConsumerCloseOnTerminate
@@ -80,24 +80,30 @@ public struct KafkaConsumerMessages: Sendable, AsyncSequence {
         var wrappedIterator: WrappedSequence.AsyncIterator?
 
         public mutating func next() async throws -> Element? {
-            guard let element = try await self.wrappedIterator?.next() else {
+            guard let result = try await self.wrappedIterator?.next() else {
                 self.deallocateIterator()
                 return nil
             }
 
-            let action = self.stateMachine.withLockedValue { $0.storeOffset() }
-            switch action {
-            case .storeOffset(let client):
-                do {
-                    try client.storeMessageOffset(element)
-                } catch {
+            switch result {
+            case .success(let message):
+                let action = self.stateMachine.withLockedValue { $0.storeOffset() }
+                switch action {
+                case .storeOffset(let client):
+                    do {
+                        try client.storeMessageOffset(message)
+                    } catch {
+                        self.deallocateIterator()
+                        throw error
+                    }
+                    return message
+                case .terminateConsumerSequence:
                     self.deallocateIterator()
-                    throw error
+                    return nil
                 }
-                return element
-            case .terminateConsumerSequence:
+            case .failure(let error):
                 self.deallocateIterator()
-                return nil
+                throw error
             }
         }
 
@@ -119,7 +125,7 @@ public struct KafkaConsumerMessages: Sendable, AsyncSequence {
 /// A ``KafkaConsumer `` can be used to consume messages from a Kafka cluster.
 public final class KafkaConsumer: Sendable, Service {
     typealias Producer = NIOThrowingAsyncSequenceProducer<
-        KafkaConsumerMessage,
+        Result<KafkaConsumerMessage, Error>,
         Error,
         NIOAsyncSequenceProducerBackPressureStrategies.NoBackPressure,
         KafkaConsumerCloseOnTerminate
@@ -156,7 +162,7 @@ public final class KafkaConsumer: Sendable, Service {
         self.logger = logger
 
         let sourceAndSequence = NIOThrowingAsyncSequenceProducer.makeSequence(
-            elementType: KafkaConsumerMessage.self,
+            elementType: Result<KafkaConsumerMessage, Error>.self,
             backPressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.NoBackPressure(),
             delegate: KafkaConsumerCloseOnTerminate(stateMachine: self.stateMachine)
         )
@@ -333,14 +339,8 @@ public final class KafkaConsumer: Sendable, Service {
                 for event in events {
                     switch event {
                     case .consumerMessages(let result):
-                        switch result {
-                        case .success(let message):
-                            // We do not support back pressure, we can ignore the yield result
-                            _ = source.yield(message)
-                        case .failure(let error):
-                            source.finish()
-                            throw error
-                        }
+                        // We do not support back pressure, we can ignore the yield result
+                        _ = source.yield(result)
                     default:
                         break // Ignore
                     }