Remove agressive precondition in zero-length-writes. (#108)

Motivation:

The zero-length writes handler will crash if it receives channelInactive
before channelActive. Sadly, that's totally possible if a channel is
closed from the connect promise.

Modifications:

- Tone back the preconditions a bit.
- Add a test that triggers this path.

Result:

Channel handler behaves better on early close

Sources/NIOTransportServices/NIOFilterEmptyWritesHandler.swift

@@ -88,9 +88,16 @@ public final class NIOFilterEmptyWritesHandler: ChannelDuplexHandler {
 // Connection state management
 extension NIOFilterEmptyWritesHandler {
     public func channelActive(context: ChannelHandlerContext) {
-        assert(self.state == .notActiveYet)
-        self.state = .open
-        context.fireChannelActive()
+        switch self.state {
+        case .notActiveYet:
+            self.state = .open
+            context.fireChannelActive()
+        case .closedFromLocal:
+            // Closed before we activated, not a problem.
+            assert(self.prefixEmptyWritePromise == nil)
+        case .open, .closedFromRemote, .error:
+            preconditionFailure()
+        }
     }
     
     public func channelInactive(context: ChannelHandlerContext) {
@@ -117,15 +124,17 @@ extension NIOFilterEmptyWritesHandler {
 
         switch (mode, self.state) {
         case (.all, .open),
-             (.output, .open):
+             (.output, .open),
+
+             // We allow closure in .notActiveYet because it is possible to close before the channelActive fires.
+             (.all, .notActiveYet),
+             (.output, .notActiveYet):
             self.state = .closedFromLocal
             save?.fail(ChannelError.outputClosed)
         case (.all, .closedFromLocal),
              (.output, .closedFromLocal),
              (.all, .closedFromRemote),
              (.output, .closedFromRemote),
-             (.all, .notActiveYet),
-             (.output, .notActiveYet),
              (.all, .error),
              (.output, .error):
             assert(save == nil)

Tests/NIOTransportServicesTests/NIOFilterEmptyWritesHandlerTests.swift

@@ -260,5 +260,19 @@ class NIOFilterEmptyWritesHandlerTests: XCTestCase {
         )
         thenEmptyWritePromise = nil
     }
+
+    func testCloseOnConnect() {
+        /// This test reproduces https://github.com/grpc/grpc-swift/issues/1014. It does so by testing
+        /// that a precondition _does not fire_. This test makes no assertions, it just shouldn't crash.
+        let channel = EmbeddedChannel(handler: NIOFilterEmptyWritesHandler())
+        let promise = channel.eventLoop.makePromise(of: Void.self)
+        let future: EventLoopFuture<Void> = promise.futureResult.flatMap {
+            XCTAssertTrue(channel.isActive)
+            return channel.close()
+        }
+        channel.connect(to: try! .init(ipAddress: "1.1.1.1", port: 1), promise: promise)
+        XCTAssertNoThrow(try future.wait())
+        XCTAssertFalse(channel.isActive)
+    }
 }
 #endif