AcceptHandler: don't swallow close errors when quiescing (#1015)

Motivation:

When receiving the ChannelShouldQuiesceEvent event, the AcceptHandler
correctly closes the ServerChannel. However, if that fails, it swallows
the error which is incorrect.

Modifications:

Don't swallow the error, send it through the pipeline.

Result:

More correct.

Package.swift

@@ -62,7 +62,7 @@ var targets: [PackageDescription.Target] = [
     .target(name: "NIOTestUtils",
             dependencies: ["NIO"]),
     .testTarget(name: "NIOTests",
-                dependencies: ["NIO", "NIOFoundationCompat"]),
+                dependencies: ["NIO", "NIOFoundationCompat", "NIOTestUtils", "NIOConcurrencyHelpers"]),
     .testTarget(name: "NIOConcurrencyHelpersTests",
                 dependencies: ["NIOConcurrencyHelpers"]),
     .testTarget(name: "NIOHTTP1Tests",

Sources/NIO/Bootstrap.swift

@@ -249,7 +249,9 @@ public final class ServerBootstrap {
 
         func userInboundEventTriggered(context: ChannelHandlerContext, event: Any) {
             if event is ChannelShouldQuiesceEvent {
-                context.channel.close(promise: nil)
+                context.channel.close().whenFailure { error in
+                    context.fireErrorCaught(error)
+                }
             }
             context.fireUserInboundEventTriggered(event)
         }

Tests/NIOTests/ChannelTests+XCTest.swift

@@ -77,6 +77,7 @@ extension ChannelTests {
                 ("testCloseInReadTriggeredByDrainingTheReceiveBufferBecauseOfWriteError", testCloseInReadTriggeredByDrainingTheReceiveBufferBecauseOfWriteError),
                 ("testApplyingTwoDistinctSocketOptionsOfSameTypeWorks", testApplyingTwoDistinctSocketOptionsOfSameTypeWorks),
                 ("testUnprocessedOutboundUserEventFailsOnServerSocketChannel", testUnprocessedOutboundUserEventFailsOnServerSocketChannel),
+                ("testAcceptHandlerDoesNotSwallowCloseErrorsWhenQuiescing", testAcceptHandlerDoesNotSwallowCloseErrorsWhenQuiescing),
            ]
    }
 }

Tests/NIOTests/ChannelTests.swift

@@ -15,6 +15,7 @@
 import XCTest
 @testable import NIO
 import NIOConcurrencyHelpers
+import NIOTestUtils
 import Dispatch
 
 class ChannelLifecycleHandler: ChannelInboundHandler {
@@ -2671,6 +2672,48 @@ public final class ChannelTests: XCTestCase {
             }
         }
     }
+
+    func testAcceptHandlerDoesNotSwallowCloseErrorsWhenQuiescing() {
+        // AcceptHandler is a `private class` so I can only implicitly get it by creating the real thing
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            XCTAssertNoThrow(try group.syncShutdownGracefully())
+        }
+
+        let counter = EventCounterHandler()
+
+        struct DummyError: Error {}
+        class MakeFirstCloseFailAndDontActuallyCloseHandler: ChannelOutboundHandler {
+            typealias OutboundIn = Any
+
+            var closes = 0
+
+            func close(context: ChannelHandlerContext, mode: CloseMode, promise: EventLoopPromise<Void>?) {
+                self.closes += 1
+                if self.closes == 1 {
+                    promise?.fail(DummyError())
+                } else {
+                    context.close(mode: mode, promise: promise)
+                }
+            }
+        }
+
+        let channel = try! assertNoThrowWithValue(ServerBootstrap(group: group).serverChannelInitializer { channel in
+            channel.pipeline.addHandler(MakeFirstCloseFailAndDontActuallyCloseHandler(), position: .first)
+        }.bind(host: "localhost", port: 0).wait())
+        defer {
+            XCTAssertNoThrow(try channel.close().wait())
+        }
+
+        XCTAssertNoThrow(try channel.pipeline.addHandler(counter).wait())
+
+        XCTAssertNoThrow(try channel.eventLoop.submit {
+            // this will trigger a close (which will fail and also not actually close)
+            channel.pipeline.fireUserInboundEventTriggered(ChannelShouldQuiesceEvent())
+        }.wait())
+        XCTAssertEqual(["userInboundEventTriggered", "close", "errorCaught"], counter.allTriggeredEvents())
+        XCTAssertEqual(1, counter.errorCaughtCalls)
+    }
 }
 
 fileprivate final class FailRegistrationAndDelayCloseHandler: ChannelOutboundHandler {