remove bogus 'is active' assertion in channelRead0 (#739)

Motivation:

BaseSocketChannel used to assert that `channelRead0` is only called when
the channel is active. That's bogus and it's trivial to hit this issue
when calling `fireChannelRead` in the last `ChannelHandler` when the
channel has gone inactive.

Modifications:

remove assertion, add comment

Result:

fewer crashes

Sources/NIO/BaseSocketChannel.swift

@@ -1076,8 +1076,8 @@ class BaseSocketChannel<T: BaseSocket>: SelectableChannel, ChannelCore {
     }
 
     public func channelRead0(_ data: NIOAny) {
-        assert(self.lifecycleManager.isActive)
         // Do nothing by default
+        // note: we can't assert that we're active here as TailChannelHandler will call this on channelRead
     }
 
     public func errorCaught0(error: Error) {

Tests/NIOTests/ChannelPipelineTest+XCTest.swift

@@ -51,6 +51,7 @@ extension ChannelPipelineTest {
                 ("testRemovingByNameWithFutureNotInChannel", testRemovingByNameWithFutureNotInChannel),
                 ("testRemovingByReferenceWithPromiseStillInChannel", testRemovingByReferenceWithPromiseStillInChannel),
                 ("testRemovingByReferenceWithFutureNotInChannel", testRemovingByReferenceWithFutureNotInChannel),
+                ("testFireChannelReadInInactiveChannelDoesNotCrash", testFireChannelReadInInactiveChannelDoesNotCrash),
            ]
    }
 }

Tests/NIOTests/ChannelPipelineTest.swift

@@ -902,4 +902,31 @@ class ChannelPipelineTest: XCTestCase {
         XCTAssertNil(channel.readOutbound())
         XCTAssertNoThrow(try channel.throwIfErrorCaught())
     }
+
+    func testFireChannelReadInInactiveChannelDoesNotCrash() throws {
+        class FireWhenInactiveHandler: ChannelInboundHandler {
+            typealias InboundIn = ()
+            typealias InboundOut = ()
+
+            func channelInactive(ctx: ChannelHandlerContext) {
+                ctx.fireChannelRead(self.wrapInboundOut(()))
+            }
+        }
+        let handler = FireWhenInactiveHandler()
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            XCTAssertNoThrow(try group.syncShutdownGracefully())
+        }
+        let server = try assertNoThrowWithValue(ServerBootstrap(group: group).bind(host: "127.0.0.1", port: 0).wait())
+        defer {
+            XCTAssertNoThrow(try server.close().wait())
+        }
+        let client = try assertNoThrowWithValue(ClientBootstrap(group: group)
+            .channelInitializer { channel in
+                channel.pipeline.add(handler: handler)
+            }
+            .connect(to: server.localAddress!)
+            .wait())
+        XCTAssertNoThrow(try client.close().wait())
+    }
 }