SR-7455: Ask for ASCII when calling CFStringGetCStringPtr()

- Add tests for multibyte UTF-8/ UTF-16.

Author: spevans

Foundation/String.swift

@@ -32,7 +32,12 @@ extension String : _ObjectiveCBridgeable {
             let length = CFStringGetLength(cf)
             if length == 0 {
                 result = ""
-            } else if let ptr = CFStringGetCStringPtr(cf, CFStringEncoding(kCFStringEncodingUTF8)) {
+            } else if let ptr = CFStringGetCStringPtr(cf, CFStringEncoding(kCFStringEncodingASCII)) {
+                // ASCII encoding has 1 byte per code point and CFStringGetLength() returned the length in
+                // codepoints so length should be the length of the ASCII string in bytes. We cant ask for the UTF-8
+                // encoding as some codepoints are multi-byte in UTF8 so the buffer length wouldn't be known.
+                // Note: CFStringGetCStringPtr(cf, CFStringEncoding(kCFStringEncodingUTF8)) does seems to return NULL
+                // for strings with multibyte UTF-8 but this isnt guaranteed or documented so ASCII is safer.
                 result = ptr.withMemoryRebound(to: UInt8.self, capacity: length) {
                     return String(decoding: UnsafeBufferPointer(start: $0, count: length), as: UTF8.self)
                 }

TestFoundation/TestNSString.swift

@@ -116,6 +116,49 @@ class TestNSString: LoopbackServerTest {
         } else {
             XCTFail("Cant convert Data to String")
         }
+
+        // Test multibyte UTF8 and UTF16
+        // kra ("ĸ") has codepoint value 312,
+        // as UTF-8  bytes it is 0xC4 0xB8
+        // as UTF-16 bytes it is 0x1, 0x38
+        let kra = "ĸ"
+        let utf8KraData = Data(bytes: [0xc4, 0xb8])
+        if let utf8kra = utf8KraData.withUnsafeBytes( { (bytes: UnsafePointer<UInt8>) in
+            return NSString(bytes: bytes, length: utf8KraData.count, encoding: String.Encoding.utf8.rawValue)
+        }) {
+            XCTAssertEqual(kra.count, 1)
+            XCTAssertEqual(kra.utf8.count, 2)
+            XCTAssertEqual(kra.utf16.count, 1)
+            XCTAssertEqual(kra, utf8kra as String)
+        } else {
+            XCTFail("Cant create UTF8 kra")
+        }
+
+        let utf16KraData = Data(bytes: [0x1, 0x38])
+        if let utf16kra = utf16KraData.withUnsafeBytes( { (bytes: UnsafePointer<UInt8>) in
+            return NSString(bytes: bytes, length: utf16KraData.count, encoding: String.Encoding.utf16.rawValue)
+        }) {
+            XCTAssertEqual(kra.count, 1)
+            XCTAssertEqual(kra.utf8.count, 2)
+            XCTAssertEqual(kra.utf16.count, 1)
+            XCTAssertEqual(kra, utf16kra as String)
+        } else {
+            XCTFail("Cant create UTF16 kra")
+        }
+
+        // Test a large string > 255 characters
+        let largeString = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut at tincidunt arcu. Suspendisse nec sodales erat, sit amet imperdiet ipsum. Etiam sed ornare felis. Nunc mauris turpis, bibendum non lectus quis, malesuada placerat turpis. Nam adipiscing non massa et semper. Nulla convallis semper bibendum."
+        XCTAssertTrue(largeString.count > 255)
+        let largeData = largeString.data(using: .utf8)!
+        if let largeText = largeData.withUnsafeBytes( { (bytes: UnsafePointer<UInt8>) in
+            return NSString(bytes: bytes, length: largeData.count, encoding: String.Encoding.ascii.rawValue)
+        }) {
+            XCTAssertEqual(largeText.length, largeString.count)
+            XCTAssertEqual(largeText.length, largeData.count)
+            XCTAssertEqual(largeString, largeText as String)
+        } else {
+            XCTFail("Cant convert large Data string to String")
+        }
     }
 
     func test_boolValue() {