Dissallow access and removal of Head and Tail ChannelHandlers

Motivation:

The Head and Tail ChannelHandlers should not be accessible as these are implementation details of the ChannelPipeline and removal of these will even make the whole ChannelPipeline not work anymore.

Modifications:

- Make it impossible to access / removal the Head and Tail ChannelHandlers
- Exclude the Head and Tail ChannelHandler from the debug String
- Add unit tests.

Result:

More robust ChannelPipeline.

Sources/NIO/ChannelPipeline.swift

@@ -446,9 +446,16 @@ public final class ChannelPipeline: ChannelInvoker {
         return promise.futureResult
     }
 
+    /// Returns a `ChannelHandlerContext` which matches.
+    ///
+    /// This skips head and tail (as these are internal and should not be accessible by the user.
+    ///
+    /// - parameters:
+    ///     - body: The predicate to execute per `ChannelHandlerContext` in the `ChannelPipeline`.
+    /// -returns: The `ChannelHandlerContext` that matches or `nil` if non did.
     private func contextForPredicate0(_ body: @escaping((ChannelHandlerContext) -> Bool)) -> ChannelHandlerContext? {
-        var curCtx: ChannelHandlerContext? = self.head
-        while let ctx = curCtx {
+        var curCtx: ChannelHandlerContext? = self.head?.next
+        while let ctx = curCtx, ctx !== self.tail {
             if body(ctx) {
                 return ctx
             }
@@ -1422,8 +1429,8 @@ public final class ChannelHandlerContext: ChannelInvoker {
 extension ChannelPipeline: CustomDebugStringConvertible {
     public var debugDescription: String {
         var desc = "ChannelPipeline (\(ObjectIdentifier(self))):\n"
-        var node = self.head
-        while let ctx = node {
+        var node = self.head?.next
+        while let ctx = node, ctx !== self.tail {
             let inboundStr = ctx.handler is _ChannelInboundHandler ? "I" : ""
             let outboundStr = ctx.handler is _ChannelOutboundHandler ? "O" : ""
             desc += "        \(ctx.name) (\(type(of: ctx.handler))) [\(inboundStr)\(outboundStr)]\n"

Tests/NIOTests/ChannelPipelineTest.swift

@@ -620,4 +620,48 @@ class ChannelPipelineTest: XCTestCase {
         XCTAssertTrue(try h1 === channel.pipeline.context(handlerType: TestHandler.self).wait().handler)
         XCTAssertFalse(try h2 === channel.pipeline.context(handlerType: TestHandler.self).wait().handler)
     }
+
+    func testContextForHeadOrTail() throws {
+        let channel = EmbeddedChannel()
+
+        defer {
+            XCTAssertFalse(try channel.finish())
+        }
+
+        do {
+            _ = try channel.pipeline.context(name: "head").wait()
+            XCTFail()
+        } catch let err as ChannelPipelineError where err == .notFound {
+            /// expected
+        }
+
+        do {
+            _ = try channel.pipeline.context(name: "tail").wait()
+            XCTFail()
+        } catch let err as ChannelPipelineError where err == .notFound {
+            /// expected
+        }
+    }
+
+    func testRemoveHeadOrTail() throws {
+        let channel = EmbeddedChannel()
+
+        defer {
+            XCTAssertFalse(try channel.finish())
+        }
+
+        do {
+            _ = try channel.pipeline.remove(name: "head").wait()
+            XCTFail()
+        } catch let err as ChannelPipelineError where err == .notFound {
+            /// expected
+        }
+
+        do {
+            _ = try channel.pipeline.remove(name: "tail").wait()
+            XCTFail()
+        } catch let err as ChannelPipelineError where err == .notFound {
+            /// expected
+        }
+    }
 }