Don't fail a URLSessionTask on a 401 response

The basic authentication implementation fails a URLSessionTask with an error if
a status code of 401 is received but we aren't able to build a
URLProtectionSpace, possibly due to the absence of a the WWW-Authenticate
response header. A valid HTTP response object is received in this case. It may
not possible to go ahead with the `urlSession(_,task: task, didReceive:)`
callback, either because a delegate isn't being used or because the
`WWW-Authenticate` response header is absent. In such cases, the completion
handler is invoked with the received response, NOT with an error.

Note that basic authentication
(swiftlang@4c1e8c4271c4e21000528
d3910f20a958d03f308#diff-14c6cb724eead661596e4468ae5ca638) wasn't available
with Swift 4.x. It was first made available in development builds for Swift 5.
Initially, when we failed to build a URLProtectionSpace, we failed with an
error, which resulted in removing the task from the TaskRegistry. Further we
went ahead and invoked the completion handler, which again tried to remove the
same task from the TaskRegistry, resulting in a fatal error reported against
Swift 5.0. As a fix to this, we decided to simply return after failing with an
error (see
swiftlang@69abd6dd396e1985a882a9
a39079801f84658fb3#diff-14c6cb724eead661596e4468ae5ca638) which means we
skipped invoking the completion handler.

It seems that the right thing to do is NOT fail with an error, but simply
invoke the completion handler with the 401 response received.

Author: Pushkar N Kulkarni

Foundation/URLSession/URLSessionTask.swift

@@ -574,11 +574,8 @@ extension _ProtocolClient : URLProtocolClient {
                                                                      sender: `protocol` as! _HTTPURLProtocol)
                 task.previousFailureCount += 1
                 urlProtocol(`protocol`, didReceive: authenticationChallenge)
-            } else {
-                let urlError = URLError(_nsError: NSError(domain: NSURLErrorDomain, code: NSURLErrorUserAuthenticationRequired, userInfo: nil))
-                urlProtocol(`protocol`, didFailWithError: urlError)
+                return
             }
-            return
         }
         switch session.behaviour(for: task) {
         case .taskDelegate(let delegate):

TestFoundation/TestURLSession.swift

@@ -766,9 +766,10 @@ class TestURLSession : LoopbackServerTest {
         let expect = expectation(description: "GET \(urlString): with a completion handler")
         var expectedResult = "unknown"
         let session = URLSession(configuration: URLSessionConfiguration.default)
-        let task = session.dataTask(with: url) { _, _, error in
+        let task = session.dataTask(with: url) { _, response, error in
             defer { expect.fulfill() }
-            XCTAssertNotNil(error)
+            XCTAssertNotNil(response)
+            XCTAssertNil(error)
         }
         task.resume()
         waitForExpectations(timeout: 12, handler: nil)