SR-12300: URLSession.dataTask doesn't follow redirects with lowercased location

- When initialising the HTTPURLResponse, store the header fields names
  capitalised except for X- headers. This matches Darwin.

- This allows responses with lowercase fields names to be read correctly
  and redirects to works if the field name is 'location' instead of
  'Location'.

- Add missing function HTTPURLResponse.value(forHTTPHeaderField:).

Author: spevans

Sources/FoundationNetworking/URLResponse.swift

@@ -186,7 +186,24 @@ open class HTTPURLResponse : URLResponse {
     /// - Returns: the instance of the object, or `nil` if an error occurred during initialization.
     public init?(url: URL, statusCode: Int, httpVersion: String?, headerFields: [String : String]?) {
         self.statusCode = statusCode
-        self.allHeaderFields = headerFields ?? [:]
+
+        self._allHeaderFields = {
+            // Canonicalize the header fields by capitalizing the field names, but not X- Headers
+            // This matches the behaviour of Darwin.
+            guard let headerFields = headerFields else { return [:] }
+            var canonicalizedFields: [String: String] = [:]
+
+            for (key, value) in headerFields  {
+                if key.isEmpty { continue }
+                if key.hasPrefix("x-") || key.hasPrefix("X-") {
+                    canonicalizedFields[key] = value
+                } else {
+                    canonicalizedFields[key.capitalized] = value
+                }
+            }
+            return canonicalizedFields
+        }()
+
         super.init(url: url, mimeType: nil, expectedContentLength: 0, textEncodingName: nil)
         expectedContentLength = getExpectedContentLength(fromHeaderFields: headerFields) ?? -1
         suggestedFilename = getSuggestedFilename(fromHeaderFields: headerFields) ?? "Unknown"
@@ -204,9 +221,9 @@ open class HTTPURLResponse : URLResponse {
         self.statusCode = aDecoder.decodeInteger(forKey: "NS.statusCode")
 
         if aDecoder.containsValue(forKey: "NS.allHeaderFields") {
-            self.allHeaderFields = aDecoder.decodeObject(of: NSDictionary.self, forKey: "NS.allHeaderFields") as! [AnyHashable: Any]
+            self._allHeaderFields = aDecoder.decodeObject(of: NSDictionary.self, forKey: "NS.allHeaderFields") as! [String: String]
         } else {
-            self.allHeaderFields = [:]
+            self._allHeaderFields = [:]
         }
 
         super.init(coder: aDecoder)
@@ -236,8 +253,15 @@ open class HTTPURLResponse : URLResponse {
     ///
     /// - Important: This is an *experimental* change from the
     /// `[NSObject: AnyObject]` type that Darwin Foundation uses.
-    public let allHeaderFields: [AnyHashable : Any]
-    
+    private let _allHeaderFields: [String: String]
+    public var allHeaderFields: [AnyHashable : Any] {
+        _allHeaderFields as [AnyHashable : Any]
+    }
+
+    public func value(forHTTPHeaderField field: String) -> String? {
+        return valueForCaseInsensitiveKey(field, fields: _allHeaderFields)
+    }
+
     /// Convenience method which returns a localized string
     /// corresponding to the status code for this response.
     /// - Parameter forStatusCode: the status code to use to produce a localized string.
@@ -315,8 +339,8 @@ open class HTTPURLResponse : URLResponse {
     /// This property is intended to produce readable output.
     override open var description: String {
         var result = "<\(type(of: self)) \(Unmanaged.passUnretained(self).toOpaque())> { URL: \(url!.absoluteString) }{ status: \(statusCode), headers {\n"
-        for(aKey, aValue) in allHeaderFields {
-            guard let key = aKey as? String, let value = aValue as? String else { continue } //shouldn't typically fail here 
+        for key in _allHeaderFields.keys.sorted() {
+            guard let value = _allHeaderFields[key] else { continue }
             if((key.lowercased() == "content-disposition" && suggestedFilename != "Unknown") || key.lowercased() == "content-type") {
                 result += "   \"\(key)\" = \"\(value)\";\n"
             } else {

Tests/Foundation/HTTPServer.swift

@@ -643,7 +643,7 @@ public class TestURLSessionServer {
         }
 
         if uri == "/redirectToEchoHeaders" {
-            return _HTTPResponse(response: .REDIRECT, headers: "Location: /echoHeaders\r\nSet-Cookie: redirect=true; Max-Age=7776000; path=/", body: "")
+            return _HTTPResponse(response: .REDIRECT, headers: "location: /echoHeaders\r\nSet-Cookie: redirect=true; Max-Age=7776000; path=/", body: "")
         }
 
         if uri == "/UnitedStates" {

Tests/Foundation/Tests/TestHTTPURLResponse.swift

@@ -173,6 +173,57 @@ class TestHTTPURLResponse: XCTestCase {
         XCTAssertEqual(sut?.textEncodingName, "iso-8859-4")
     }
 
+    func test_fieldCapitalisation() throws {
+        let f = [
+            "location": "/newLocation",
+            "conTent-lenGTH": "123",
+            "CONTENT-type": "text/plAIn; charset=ISO-8891-1",
+            "x-extra-HEADER": "my Header",
+            "X-UPPERCASE": "UPPERCASE",
+            "x-lowercase": "lowercase",
+            "X-mixedCASE": "MIXEDcase",
+            "vary": "much",
+            "X-xss-protection": "1; mode=block",
+
+        ]
+        guard let sut = HTTPURLResponse(url: url, statusCode: 302, httpVersion: "HTTP/1.1", headerFields: f) else {
+            XCTFail("Cant create HTTPURLResponse")
+            return
+        }
+        XCTAssertEqual(sut.statusCode, 302)
+        XCTAssertEqual(sut.expectedContentLength, 123)
+        XCTAssertEqual(sut.mimeType, "text/plain")
+        XCTAssertEqual(sut.textEncodingName, "iso-8891-1")
+
+        guard let ahf = sut.allHeaderFields as? [String: String] else {
+            XCTFail("Cant read .allheaderFields")
+            return
+        }
+
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "location"), "/newLocation")
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "LOcation"), "/newLocation")
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "locATIon"), "/newLocation")
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "x-extra-HEADER"), "my Header")
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "X-EXTRA-HEADER"), "my Header")
+        XCTAssertEqual(sut.value(forHTTPHeaderField: "x-ExTrA-header"), "my Header")
+
+        XCTAssertEqual(ahf["Location"], "/newLocation")
+        XCTAssertEqual(ahf["Content-Length"], "123")
+        XCTAssertEqual(ahf["Content-Type"], "text/plAIn; charset=ISO-8891-1")
+        XCTAssertEqual(ahf["x-extra-HEADER"], "my Header")
+        XCTAssertEqual(ahf["X-UPPERCASE"], "UPPERCASE")
+        XCTAssertEqual(ahf["x-lowercase"], "lowercase")
+        XCTAssertEqual(ahf["X-mixedCASE"], "MIXEDcase")
+
+        XCTAssertNil(ahf["location"])
+        XCTAssertNil(ahf["conTent-lenGTH"])
+        XCTAssertNil(ahf["CONTENT-type"])
+        XCTAssertNil(ahf["X-Extra-Header"])
+        XCTAssertNil(ahf["X-Uppercase"])
+        XCTAssertNil(ahf["X-Lowercase"])
+        XCTAssertNil(ahf["X-Mixedcase"])
+    }
+
     // NSCoding
 
     func test_NSCoding() {
@@ -230,6 +281,7 @@ class TestHTTPURLResponse: XCTestCase {
             ("test_MIMETypeAndCharacterEncoding_2", test_MIMETypeAndCharacterEncoding_2),
             ("test_MIMETypeAndCharacterEncoding_3", test_MIMETypeAndCharacterEncoding_3),
 
+            ("test_fieldCapitalisation", test_fieldCapitalisation),
             ("test_NSCoding", test_NSCoding),
         ]
     }