[dart:io] Handle http timeout race

In http connection setup, when an explicit connection timeout is set on
the client, the onTimeout callback on the socket future caused the
future to be completed with null. If the 'then' callback on the socket
future ran before the error completer on the connection task, then
a NoSuchMethod error would result.

This CL addresses this issue by checking for null in the 'then'
callback on the socket future, and generating an appropriate
exception.

fixes flutter/flutter#32578

Change-Id: If2e60b1355aa3d80622c644dada8a62a24469567
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112742
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Zach Anderson <zra@google.com>

Author: zanderso

sdk/lib/_http/http_impl.dart

@@ -1990,14 +1990,26 @@ class _ConnectionTarget {
     return connectionTask.then((ConnectionTask task) {
       _socketTasks.add(task);
       Future socketFuture = task.socket;
-      if (client.connectionTimeout != null) {
-        socketFuture =
-            socketFuture.timeout(client.connectionTimeout, onTimeout: () {
+      final Duration connectionTimeout = client.connectionTimeout;
+      if (connectionTimeout != null) {
+        socketFuture = socketFuture.timeout(connectionTimeout, onTimeout: () {
           _socketTasks.remove(task);
           task.cancel();
+          return null;
         });
       }
       return socketFuture.then((socket) {
+        // When there is a timeout, there is a race in which the connectionTask
+        // Future won't be completed with an error before the socketFuture here
+        // is completed with 'null' by the onTimeout callback above. In this
+        // case, propagate a SocketException as specified by the
+        // HttpClient.connectionTimeout docs.
+        if (socket == null) {
+          assert(connectionTimeout != null);
+          throw new SocketException(
+              "HTTP connection timed out after ${connectionTimeout}, "
+              "host: ${host}, port: ${port}");
+        }
         _connecting--;
         socket.setOption(SocketOption.tcpNoDelay, true);
         var connection =