SR-12275: JSONEncoder on Linux can't encode number JSON fragments

- JSONEncoder now encodes fragments by default to match Darwin.

- JSONDecoder now allows fragments when decoding to match Darwin.

- Update the following TestJSONEncoder tests now that fragments encode,
  these tests now pass on Darwin:

  test_encodingTopLevelSingleValueEnum
  test_encodingTopLevelSingleValueStruct
  test_encodingTopLevelSingleValueClass

Author: spevans

Sources/Foundation/JSONEncoder.swift

@@ -238,15 +238,8 @@ open class JSONEncoder {
             throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) did not encode any values."))
         }
 
-        if topLevel is NSNull {
-            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as null JSON fragment."))
-        } else if topLevel is NSNumber {
-            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as number JSON fragment."))
-        } else if topLevel is NSString {
-            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: [], debugDescription: "Top-level \(T.self) encoded as string JSON fragment."))
-        }
+        let writingOptions: JSONSerialization.WritingOptions = [JSONSerialization.WritingOptions(rawValue: self.outputFormatting.rawValue), .fragmentsAllowed]
 
-        let writingOptions = JSONSerialization.WritingOptions(rawValue: self.outputFormatting.rawValue)
         do {
             return try JSONSerialization.data(withJSONObject: topLevel, options: writingOptions)
         } catch {
@@ -1155,7 +1148,7 @@ open class JSONDecoder {
     open func decode<T : Decodable>(_ type: T.Type, from data: Data) throws -> T {
         let topLevel: Any
         do {
-            topLevel = try JSONSerialization.jsonObject(with: data)
+            topLevel = try JSONSerialization.jsonObject(with: data, options: .allowFragments)
         } catch {
             throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: error))
         }

Tests/Foundation/Tests/TestJSONEncoder.swift

@@ -21,6 +21,40 @@ struct TopLevelObjectWrapper<T: Codable & Equatable>: Codable, Equatable {
 
 class TestJSONEncoder : XCTestCase {
 
+    // MARK: - Encoding Top-Level fragments
+    func test_encodingTopLevelFragments() {
+
+        func _testFragment<T: Codable & Equatable>(value: T, fragment: String) {
+            let data: Data
+            let payload: String
+
+            do {
+                data = try JSONEncoder().encode(value)
+                payload = try XCTUnwrap(String.init(decoding: data, as: UTF8.self))
+                XCTAssertEqual(fragment, payload)
+            } catch {
+                XCTFail("Failed to encode \(T.self) to JSON: \(error)")
+                return
+            }
+            do {
+                let decodedValue = try JSONDecoder().decode(T.self, from: data)
+                XCTAssertEqual(value, decodedValue)
+            } catch {
+                XCTFail("Failed to decode \(payload) to \(T.self): \(error)")
+            }
+        }
+
+        _testFragment(value: 2, fragment: "2")
+        _testFragment(value: false, fragment: "false")
+        _testFragment(value: true, fragment: "true")
+        _testFragment(value: Float(1), fragment: "1")
+        _testFragment(value: Double(2), fragment: "2")
+        _testFragment(value: Decimal(Double.leastNormalMagnitude), fragment: "0.0000000000000000000000000000000000000000000000000002225073858507201792")
+        _testFragment(value: "test", fragment: "\"test\"")
+        let v: Int? = nil
+        _testFragment(value: v, fragment: "null")
+    }
+
     // MARK: - Encoding Top-Level Empty Types
     func test_encodingTopLevelEmptyStruct() {
         let empty = EmptyStruct()
@@ -34,20 +68,20 @@ class TestJSONEncoder : XCTestCase {
 
     // MARK: - Encoding Top-Level Single-Value Types
     func test_encodingTopLevelSingleValueEnum() {
-        _testEncodeFailure(of: Switch.off)
-        _testEncodeFailure(of: Switch.on)
+        _testRoundTrip(of: Switch.off)
+        _testRoundTrip(of: Switch.on)
 
         _testRoundTrip(of: TopLevelArrayWrapper(Switch.off))
         _testRoundTrip(of: TopLevelArrayWrapper(Switch.on))
     }
 
     func test_encodingTopLevelSingleValueStruct() {
-        _testEncodeFailure(of: Timestamp(3141592653))
+        _testRoundTrip(of: Timestamp(3141592653))
         _testRoundTrip(of: TopLevelArrayWrapper(Timestamp(3141592653)))
     }
 
     func test_encodingTopLevelSingleValueClass() {
-        _testEncodeFailure(of: Counter())
+        _testRoundTrip(of: Counter())
         _testRoundTrip(of: TopLevelArrayWrapper(Counter()))
     }
 
@@ -492,6 +526,11 @@ class TestJSONEncoder : XCTestCase {
         }
     }
 
+    func test_codingOfNil() {
+        let x: Int? = nil
+        test_codingOf(value: x, toAndFrom: "null")
+    }
+
     func test_codingOfInt8() {
         test_codingOf(value: Int8(-42), toAndFrom: "-42")
     }
@@ -1364,6 +1403,7 @@ fileprivate struct JSON: Equatable {
 extension TestJSONEncoder {
     static var allTests: [(String, (TestJSONEncoder) -> () throws -> Void)] {
         return [
+            ("test_encodingTopLevelFragments", test_encodingTopLevelFragments),
             ("test_encodingTopLevelEmptyStruct", test_encodingTopLevelEmptyStruct),
             ("test_encodingTopLevelEmptyClass", test_encodingTopLevelEmptyClass),
             ("test_encodingTopLevelSingleValueEnum", test_encodingTopLevelSingleValueEnum),
@@ -1393,6 +1433,7 @@ extension TestJSONEncoder {
             ("test_nestedContainerCodingPaths", test_nestedContainerCodingPaths),
             ("test_superEncoderCodingPaths", test_superEncoderCodingPaths),
             ("test_codingOfBool", test_codingOfBool),
+            ("test_codingOfNil", test_codingOfNil),
             ("test_codingOfInt8", test_codingOfInt8),
             ("test_codingOfUInt8", test_codingOfUInt8),
             ("test_codingOfInt16", test_codingOfInt16),