Handle circular reference in snap function (#942)

* handle circular references during reccursive child search

* add tests for circular reference snapshotting as dump

* fixes

---------

Co-authored-by: Oguz Yuksel <[email protected]>
Co-authored-by: Brandon Williams <[email protected]>
Co-authored-by: Brandon Williams <[email protected]>

Author: 3 people

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -502,6 +502,8 @@ func sanitizePathComponent(_ string: String) -> String {
 }
 
 #if !os(Linux) && !os(Windows)
+  import CoreServices
+
   func uniformTypeIdentifier(fromExtension pathExtension: String) -> String? {
     // This can be much cleaner in macOS 11+ using UTType
     let unmanagedString = UTTypeCreatePreferredIdentifierForTag(

Sources/SnapshotTesting/Snapshotting/Any.swift

@@ -66,23 +66,29 @@ extension Snapshotting where Format == String {
   }
 }
 
-private func snap<T>(_ value: T, name: String? = nil, indent: Int = 0) -> String {
+private func snap<T>(
+  _ value: T,
+  name: String? = nil,
+  indent: Int = 0,
+  visitedValues: Set<ObjectIdentifier> = .init()
+) -> String {
   let indentation = String(repeating: " ", count: indent)
   let mirror = Mirror(reflecting: value)
   var children = mirror.children
   let count = children.count
   let bullet = count == 0 ? "-" : "▿"
+  var visitedValues = visitedValues
 
   let description: String
   switch (value, mirror.displayStyle) {
   case (_, .collection?):
     description = count == 1 ? "1 element" : "\(count) elements"
   case (_, .dictionary?):
     description = count == 1 ? "1 key/value pair" : "\(count) key/value pairs"
-    children = sort(children)
+    children = sort(children, visitedValues: visitedValues)
   case (_, .set?):
     description = count == 1 ? "1 member" : "\(count) members"
-    children = sort(children)
+    children = sort(children, visitedValues: visitedValues)
   case (_, .tuple?):
     description = count == 1 ? "(1 element)" : "(\(count) elements)"
   case (_, .optional?):
@@ -95,10 +101,19 @@ private func snap<T>(_ value: T, name: String? = nil, indent: Int = 0) -> String
     return "\(indentation)- \(name.map { "\($0): " } ?? "")\(value.snapshotDescription)\n"
   case (let value as CustomStringConvertible, _):
     description = value.description
-  case (_, .class?), (_, .struct?):
+  case let (value as AnyObject, .class?):
+    let objectID = ObjectIdentifier(value)
+    if visitedValues.contains(objectID) {
+      return "\(indentation)\(bullet) \(name ?? "value") (circular reference detected)\n"
+    }
+    visitedValues.insert(objectID)
+    description = String(describing: mirror.subjectType)
+      .replacingOccurrences(of: " #\\d+", with: "", options: .regularExpression)
+    children = sort(children, visitedValues: visitedValues)
+  case (_, .struct?):
     description = String(describing: mirror.subjectType)
       .replacingOccurrences(of: " #\\d+", with: "", options: .regularExpression)
-    children = sort(children)
+    children = sort(children, visitedValues: visitedValues)
   case (_, .enum?):
     let subjectType = String(describing: mirror.subjectType)
       .replacingOccurrences(of: " #\\d+", with: "", options: .regularExpression)
@@ -109,15 +124,15 @@ private func snap<T>(_ value: T, name: String? = nil, indent: Int = 0) -> String
 
   let lines =
     ["\(indentation)\(bullet) \(name.map { "\($0): " } ?? "")\(description)\n"]
-    + children.map { snap($1, name: $0, indent: indent + 2) }
+    + children.map { snap($1, name: $0, indent: indent + 2, visitedValues: visitedValues) }
 
   return lines.joined()
 }
 
-private func sort(_ children: Mirror.Children) -> Mirror.Children {
+private func sort(_ children: Mirror.Children, visitedValues: Set<ObjectIdentifier>) -> Mirror.Children {
   return .init(
     children
-      .map({ (child: $0, snap: snap($0)) })
+      .map({ (child: $0, snap: snap($0, visitedValues: visitedValues)) })
       .sorted(by: { $0.snap < $1.snap })
       .map({ $0.child })
   )

Tests/SnapshotTestingTests/SnapshotTestingTests.swift

@@ -14,7 +14,7 @@ import XCTest
   import SwiftUI
 #endif
 #if canImport(WebKit)
-  import WebKit
+  @preconcurrency import WebKit
 #endif
 #if canImport(UIKit)
   import UIKit.UIView
@@ -36,6 +36,26 @@ final class SnapshotTestingTests: XCTestCase {
     assertSnapshot(of: user, as: .dump)
   }
 
+  func testRecursion() {
+    withSnapshotTesting {
+      class Father {
+        var child: Child?
+        init(_ child: Child? = nil) { self.child = child }
+      }
+      class Child {
+        let father: Father
+        init(_ father: Father) {
+          self.father = father
+          father.child = self
+        }
+      }
+      let father = Father()
+      let child = Child(father)
+      assertSnapshot(of: father, as: .dump)
+      assertSnapshot(of: child, as: .dump)
+    }
+  }
+
   @available(macOS 10.13, tvOS 11.0, *)
   func testAnyAsJson() throws {
     struct User: Encodable { let id: Int, name: String, bio: String }

Tests/SnapshotTestingTests/__Snapshots__/SnapshotTestingTests/testRecursion.1.txt

@@ -0,0 +1,4 @@
+▿ Father
+  ▿ child: Optional<Child>
+    ▿ some: Child
+      ▿ father (circular reference detected)

Tests/SnapshotTestingTests/__Snapshots__/SnapshotTestingTests/testRecursion.2.txt

@@ -0,0 +1,4 @@
+▿ Child
+  ▿ father: Father
+    ▿ child: Optional<Child>
+      ▿ some (circular reference detected)