Adopt `Sendable` in `ChannelPipeline.swift` and `ChannelInvoker.swift` (#2143)

Sources/NIOCore/ChannelInvoker.swift

@@ -231,7 +231,7 @@ public protocol ChannelInboundInvoker {
 public protocol ChannelInvoker: ChannelOutboundInvoker, ChannelInboundInvoker { }
 
 /// Specify what kind of close operation is requested.
-public enum CloseMode {
+public enum CloseMode: NIOSendable {
     /// Close the output (writing) side of the `Channel` without closing the actual file descriptor.
     /// This is an optional mode which means it may not be supported by all `Channel` implementations.
     case output

Sources/NIOCore/ChannelPipeline.swift

@@ -946,6 +946,10 @@ public final class ChannelPipeline: ChannelInvoker {
     }
 }
 
+#if swift(>=5.7)
+extension ChannelPipeline: @unchecked Sendable {}
+#endif
+
 extension ChannelPipeline {
     /// Adds the provided channel handlers to the pipeline in the order given, taking account
     /// of the behaviour of `ChannelHandler.add(first:)`.
@@ -1270,6 +1274,11 @@ extension ChannelPipeline {
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension ChannelPipeline.SynchronousOperations: Sendable {}
+#endif
+
 extension ChannelPipeline {
     /// A `Position` within the `ChannelPipeline` used to insert handlers into the `ChannelPipeline`.
     public enum Position {
@@ -1287,6 +1296,11 @@ extension ChannelPipeline {
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension ChannelPipeline.Position: Sendable {}
+#endif
+
 /// Special `ChannelHandler` that forwards all events to the `Channel.Unsafe` implementation.
 /* private but tests */ final class HeadChannelHandler: _ChannelOutboundHandler {
 
@@ -1839,11 +1853,16 @@ public final class ChannelHandlerContext: ChannelInvoker {
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension ChannelHandlerContext: Sendable {}
+#endif
+
 extension ChannelHandlerContext {
     /// A `RemovalToken` is handed to a `RemovableChannelHandler` when its `removeHandler` function is invoked. A
     /// `RemovableChannelHandler` is then required to remove itself from the `ChannelPipeline`. The removal process
     /// is finalized by handing the `RemovalToken` to the `ChannelHandlerContext.leavePipeline` function.
-    public struct RemovalToken {
+    public struct RemovalToken: NIOSendable {
         internal let promise: EventLoopPromise<Void>?
     }