Add validation to CollectionDifference decoder (#76876)

The `CollectionDifference` type has a few different invariants
that were not being validated when initializing using the type's
`Decodable` conformance, since the type was using the
autogenerated `Codable` implementation. This change provides
manual implementations of the `Encodable` and `Decodable`
requirements, and adds tests that validate the failure when trying
to decode invalid JSON for CollectionDifference (and a few other
types).

stdlib/public/core/CollectionDifference.swift

@@ -63,6 +63,12 @@ public struct CollectionDifference<ChangeElement> {
         }
       }
     }
+    internal var _isRemoval: Bool {
+      switch self {
+      case .insert: false
+      case .remove: true
+      }
+    }
   }
 
   /// The insertions contained by this difference, from lowest offset to 
@@ -404,13 +410,7 @@ extension CollectionDifference.Change: Codable where ChangeElement: Codable {
 
   public func encode(to encoder: Encoder) throws {
     var container = encoder.container(keyedBy: _CodingKeys.self)
-    switch self {
+    try container.encode(_isRemoval, forKey: .isRemove)
-    case .remove(_, _, _):
-      try container.encode(true, forKey: .isRemove)
-    case .insert(_, _, _):
-      try container.encode(false, forKey: .isRemove)
-    }
-    
     try container.encode(_offset, forKey: .offset)
     try container.encode(_element, forKey: .element)
     try container.encode(_associatedOffset, forKey: .associatedOffset)
@@ -418,7 +418,37 @@ extension CollectionDifference.Change: Codable where ChangeElement: Codable {
 }
 
 @available(SwiftStdlib 5.1, *)
-extension CollectionDifference: Codable where ChangeElement: Codable {}
+extension CollectionDifference: Codable where ChangeElement: Codable {
+  private enum _CodingKeys: String, CodingKey {
+    case insertions
+    case removals
+  }
+  
+  public init(from decoder: Decoder) throws {
+    let container = try decoder.container(keyedBy: _CodingKeys.self)
+    var changes = try container.decode([Change].self, forKey: .removals)
+    let removalCount = changes.count
+    try changes.append(contentsOf: container.decode([Change].self, forKey: .insertions))
+
+    guard changes[..<removalCount].allSatisfy({ $0._isRemoval }),
+          changes[removalCount...].allSatisfy({ !$0._isRemoval }),
+          Self._validateChanges(changes)
+    else {
+      throw DecodingError.dataCorrupted(
+        DecodingError.Context(
+          codingPath: decoder.codingPath,
+          debugDescription: "Cannot decode an invalid collection difference"))
+    }
+
+    self.init(_validatedChanges: changes)
+  }
+  
+  public func encode(to encoder: Encoder) throws {
+    var container = encoder.container(keyedBy: _CodingKeys.self)
+    try container.encode(insertions, forKey: .insertions)
+    try container.encode(removals, forKey: .removals)
+  }
+}
 
 @available(SwiftStdlib 5.1, *)
 extension CollectionDifference: Sendable where ChangeElement: Sendable { }

test/stdlib/CodableTests.swift

@@ -118,6 +118,23 @@ func expectRoundTripEqualityThroughPlist<T : Codable>(for value: T, lineNumber:
     expectRoundTripEquality(of: value, encode: encode, decode: decode, lineNumber: lineNumber)
 }
 
+func expectDecodingErrorViaJSON<T : Codable>(
+    type: T.Type,
+    json: String,
+    errorKind: DecodingErrorKind,
+    lineNumber: Int = #line)
+{
+    let data = json.data(using: .utf8)!
+    do {
+        let value = try JSONDecoder().decode(T.self, from: data)
+        expectUnreachable(":\(lineNumber): Successfully decoded invalid \(T.self) <\(debugDescription(value))>")
+    } catch let error as DecodingError {
+        expectEqual(error.errorKind, errorKind, "\(#file):\(lineNumber): Incorrect error kind <\(error.errorKind)> not equal to expected <\(errorKind)>")
+    } catch {
+        expectUnreachableCatch(error, ":\(lineNumber): Unexpected error type when decoding \(T.self)")
+    }
+}
+
 // MARK: - Helper Types
 // A wrapper around a UUID that will allow it to be encoded at the top level of an encoder.
 struct UUIDCodingWrapper : Codable, Equatable, Hashable, CodingKeyRepresentable {
@@ -141,6 +158,24 @@ struct UUIDCodingWrapper : Codable, Equatable, Hashable, CodingKeyRepresentable
     }
 }
 
+enum DecodingErrorKind {
+    case dataCorrupted
+    case keyNotFound
+    case typeMismatch
+    case valueNotFound
+}
+
+extension DecodingError {
+    var errorKind: DecodingErrorKind {
+        switch self {
+        case .dataCorrupted: .dataCorrupted
+        case .keyNotFound:   .keyNotFound
+        case .typeMismatch:  .typeMismatch
+        case .valueNotFound: .valueNotFound
+        }
+    }
+}
+
 // MARK: - Tests
 class TestCodable : TestCodableSuper {
     // MARK: - AffineTransform
@@ -392,6 +427,90 @@ class TestCodable : TestCodableSuper {
         expectEqual(value.upperBound, decoded.upperBound, "\(#file):\(#line): Decoded ClosedRange upperBound <\(debugDescription(decoded))> not equal to original <\(debugDescription(value))>")
         expectEqual(value.lowerBound, decoded.lowerBound, "\(#file):\(#line): Decoded ClosedRange lowerBound <\(debugDescription(decoded))> not equal to original <\(debugDescription(value))>")
     }
+    
+    func test_ClosedRange_JSON_Errors() {
+        expectDecodingErrorViaJSON(
+            type: ClosedRange<Int>.self,
+            json: "[5,0]",
+            errorKind: .dataCorrupted)
+        expectDecodingErrorViaJSON(
+            type: ClosedRange<Int>.self,
+            json: "[5,]",
+            errorKind: .valueNotFound)
+        expectDecodingErrorViaJSON(
+            type: ClosedRange<Int>.self,
+            json: "[0,Hello]",
+            errorKind: .dataCorrupted)
+    }
+
+    // MARK: - CollectionDifference
+    lazy var collectionDifferenceValues: [Int : CollectionDifference<Int>] = [
+        #line : [1, 2, 3].difference(from: [1, 2, 3]),
+        #line : [1, 2, 3].difference(from: [1, 2]),
+        #line : [1, 2, 3].difference(from: [2, 3, 4]),
+        #line : [1, 2, 3].difference(from: [6, 7, 8]),
+    ]
+    
+    func test_CollectionDifference_JSON() {
+        for (testLine, difference) in collectionDifferenceValues {
+            expectRoundTripEqualityThroughJSON(for: difference, lineNumber: testLine)
+        }
+    }
+    
+    func test_CollectionDifference_Plist() {
+        for (testLine, difference) in collectionDifferenceValues {
+            expectRoundTripEqualityThroughPlist(for: difference, lineNumber: testLine)
+        }
+    }
+
+    func test_CollectionDifference_JSON_Errors() {
+        // Valid serialization:
+        // {
+        //   "insertions" : [ { "associatedOffset" : null, "element" : 1, "isRemove" : false, "offset" : 0 } ],
+        //   "removals"   : [ { "associatedOffset" : null, "element" : 4, "isRemove" : true,  "offset" : 2 } ]
+        // }
+        
+        // Removal in insertion
+        expectDecodingErrorViaJSON(
+            type: CollectionDifference<Int>.self,
+            json: #"""
+                {
+                  "insertions" : [ { "associatedOffset" : null, "element" : 1, "isRemove" : true, "offset" : 0 } ],
+                  "removals"   : [ { "associatedOffset" : null, "element" : 4, "isRemove" : true,  "offset" : 2 } ]
+                }
+                """#,
+            errorKind: .dataCorrupted)
+        // Repeated offset
+        expectDecodingErrorViaJSON(
+            type: CollectionDifference<Int>.self,
+            json: #"""
+                {
+                  "insertions" : [ { "associatedOffset" : null, "element" : 1, "isRemove" : true, "offset" : 2 } ],
+                  "removals"   : [ { "associatedOffset" : null, "element" : 4, "isRemove" : true,  "offset" : 2 } ]
+                }
+                """#,
+            errorKind: .dataCorrupted)
+        // Invalid offset
+        expectDecodingErrorViaJSON(
+            type: CollectionDifference<Int>.self,
+            json: #"""
+                {
+                  "insertions" : [ { "associatedOffset" : null, "element" : 1, "isRemove" : true, "offset" : -2 } ],
+                  "removals"   : [ { "associatedOffset" : null, "element" : 4, "isRemove" : true,  "offset" : 2 } ]
+                }
+                """#,
+            errorKind: .dataCorrupted)
+        // Invalid associated offset
+        expectDecodingErrorViaJSON(
+            type: CollectionDifference<Int>.self,
+            json: #"""
+                {
+                  "insertions" : [ { "associatedOffset" : 2, "element" : 1, "isRemove" : true, "offset" : 0 } ],
+                  "removals"   : [ { "associatedOffset" : null, "element" : 4, "isRemove" : true,  "offset" : 2 } ]
+                }
+                """#,
+            errorKind: .dataCorrupted)
+    }
 
     // MARK: - ContiguousArray
     lazy var contiguousArrayValues: [Int : ContiguousArray<String>] = [
@@ -789,6 +908,21 @@ class TestCodable : TestCodableSuper {
         expectEqual(value.upperBound, decoded.upperBound, "\(#file):\(#line): Decoded Range upperBound<\(debugDescription(decoded))> not equal to original <\(debugDescription(value))>")
         expectEqual(value.lowerBound, decoded.lowerBound, "\(#file):\(#line): Decoded Range lowerBound<\(debugDescription(decoded))> not equal to original <\(debugDescription(value))>")
     }
+    
+    func test_Range_JSON_Errors() {
+        expectDecodingErrorViaJSON(
+            type: Range<Int>.self,
+            json: "[5,0]",
+            errorKind: .dataCorrupted)
+        expectDecodingErrorViaJSON(
+            type: Range<Int>.self,
+            json: "[5,]",
+            errorKind: .valueNotFound)
+        expectDecodingErrorViaJSON(
+            type: Range<Int>.self,
+            json: "[0,Hello]",
+            errorKind: .dataCorrupted)
+    }
 
     // MARK: - TimeZone
     lazy var timeZoneValues: [Int : TimeZone] = [
@@ -808,7 +942,7 @@ class TestCodable : TestCodableSuper {
             expectRoundTripEqualityThroughPlist(for: timeZone, lineNumber: testLine)
         }
     }
-
+    
     // MARK: - URL
     lazy var urlValues: [Int : URL] = {
         var values: [Int : URL] = [
@@ -845,7 +979,7 @@ class TestCodable : TestCodableSuper {
             expectRoundTripEqualityThroughPlist(for: url, lineNumber: testLine)
         }
     }
-
+    
     // MARK: - URLComponents
     lazy var urlComponentsValues: [Int : URLComponents] = [
         #line : URLComponents(),
@@ -1016,6 +1150,10 @@ var tests = [
     "test_CGVector_Plist" : TestCodable.test_CGVector_Plist,
     "test_ClosedRange_JSON" : TestCodable.test_ClosedRange_JSON,
     "test_ClosedRange_Plist" : TestCodable.test_ClosedRange_Plist,
+    "test_ClosedRange_JSON_Errors" : TestCodable.test_ClosedRange_JSON_Errors,
+    "test_CollectionDifference_JSON" : TestCodable.test_CollectionDifference_JSON,
+    "test_CollectionDifference_Plist" : TestCodable.test_CollectionDifference_Plist,
+    "test_CollectionDifference_JSON_Errors" : TestCodable.test_CollectionDifference_JSON_Errors,
     "test_ContiguousArray_JSON" : TestCodable.test_ContiguousArray_JSON,
     "test_ContiguousArray_Plist" : TestCodable.test_ContiguousArray_Plist,
     "test_DateComponents_JSON" : TestCodable.test_DateComponents_JSON,
@@ -1038,6 +1176,7 @@ var tests = [
     "test_PartialRangeUpTo_Plist" : TestCodable.test_PartialRangeUpTo_Plist,
     "test_Range_JSON" : TestCodable.test_Range_JSON,
     "test_Range_Plist" : TestCodable.test_Range_Plist,
+    "test_Range_JSON_Errors" : TestCodable.test_Range_JSON_Errors,
     "test_TimeZone_JSON" : TestCodable.test_TimeZone_JSON,
     "test_TimeZone_Plist" : TestCodable.test_TimeZone_Plist,
     "test_URL_JSON" : TestCodable.test_URL_JSON,