Throw an appropriate error from the writer when the channel closed (#…

…2744)

# Motivation
Currently, when the channel closes and a user tries to write something the writer throws an `AlreadyFinished` error. This error can also be thrown when calling `finish` on the writer and then trying to call `write` again. This makes it hard to distinguish if the thrown error was due to the channel being closed or due to a business logic error in handling the writer.

# Modification
This PR finishes the writer with a `ChannelError.ioOnClosedChannel` if the writer gets finished to due a channel inactive or handler removed.

# Result
Users can now distinguish if they did something wrong with the writer or if the channel closed.

Sources/NIOCore/AsyncChannel/AsyncChannelOutboundWriterHandler.swift

@@ -137,7 +137,7 @@ internal final class NIOAsyncChannelOutboundWriterHandler<OutboundOut: Sendable>
     @inlinable
     func handlerRemoved(context: ChannelHandlerContext) {
         self.context = nil
-        self.sink?.finish()
+        self.sink?.finish(error: ChannelError.ioOnClosedChannel)
         self.writer = nil
     }
 
@@ -150,7 +150,7 @@ internal final class NIOAsyncChannelOutboundWriterHandler<OutboundOut: Sendable>
 
     @inlinable
     func channelInactive(context: ChannelHandlerContext) {
-        self.sink?.finish()
+        self.sink?.finish(error: ChannelError.ioOnClosedChannel)
         context.fireChannelInactive()
     }
 

Tests/NIOCoreTests/AsyncChannel/AsyncChannelTests.swift

@@ -81,6 +81,24 @@ final class AsyncChannelTests: XCTestCase {
         }
     }
 
+    func testAsyncChannelThrowsWhenChannelClosed() async throws {
+        let channel = NIOAsyncTestingChannel()
+        let wrapped = try await channel.testingEventLoop.executeInContext {
+            return try NIOAsyncChannel<String, String>(wrappingChannelSynchronously: channel)
+        }
+
+        try await channel.close(mode: .all)
+
+        do {
+            try await wrapped.executeThenClose { _, outbound in
+                try await outbound.write("Test")
+            }
+            XCTFail("Expected an error to be thrown")
+        } catch {
+            XCTAssertEqual(error as? ChannelError, ChannelError.ioOnClosedChannel)
+        }
+    }
+
     func testFinishingTheWriterClosesTheWriteSideOfTheChannel() async throws {
         let channel = NIOAsyncTestingChannel()
         let closeRecorder = CloseRecorder()