Avoid crashing when connecting to empty host. (#103)

Motivation:

Network.framework will crash when we attempt to connect to the host
string "". That's not ideal, so we should detect that case and avoid it.

Modifications:

- Add code to detect the empty host string.

Result:

No crashes when accidentally connecting to "".

Sources/NIOTransportServices/NIOTSConnectionChannel.swift

@@ -500,6 +500,16 @@ extension NIOTSConnectionChannel: StateManagedChannel {
     internal func beginActivating0(to target: NWEndpoint, promise: EventLoopPromise<Void>?) {
         assert(self.nwConnection == nil)
         assert(self.connectPromise == nil)
+
+        // Before we start, we validate that the target won't cause a crash: see
+        // https://github.com/apple/swift-nio/issues/1617.
+        if case .hostPort(host: let host, port: _) = target, host == "" {
+            // We don't pass the promise in here because we'll actually not complete it. We complete it manually ourselves.
+            self.close0(error: NIOTSErrors.InvalidHostname(), mode: .all, promise: nil)
+            promise?.fail(NIOTSErrors.InvalidHostname())
+            return
+        }
+
         self.connectPromise = promise
 
         let parameters = NWParameters(tls: self.tlsOptions, tcp: self.tcpOptions)

Sources/NIOTransportServices/NIOTSErrors.swift

@@ -69,5 +69,10 @@ public enum NIOTSErrors {
             self.timeout = timeout
         }
     }
+
+    /// `InvalidHostname` is thrown when attempting to connect to an invalid host.
+    public struct InvalidHostname: NIOTSError {
+        public init() { }
+    }
 }
 #endif

Tests/NIOTransportServicesTests/NIOTSConnectionChannelTests.swift

@@ -778,5 +778,12 @@ class NIOTSConnectionChannelTests: XCTestCase {
         // throw
         XCTAssertNoThrow(try eventPromise.futureResult.wait())
     }
+
+    func testConnectingToEmptyStringErrors() throws {
+        let connectBootstrap = NIOTSConnectionBootstrap(group: self.group)
+        XCTAssertThrowsError(try connectBootstrap.connect(host: "", port: 80).wait()) { error in
+            XCTAssertTrue(error is NIOTSErrors.InvalidHostname)
+        }
+    }
 }
 #endif