Add a type argument to the ABI accessor function signature. (swiftlang#939)

We access the test content record section directly instead of using some
(non-existent) type-safe Swift API. On some platforms (Darwin in
particular), it is possible for two copies of the testing library to be
loaded at runtime, and for them to have incompatible definitions of
types like `Test`. That means one library's `@Test` macro could produce
a test content record that is then read by the other library's discovery
logic, resulting in a stack smash or worse.

We can resolve this issue by adding a `type` argument to the accessor
function we define for test content records; the body of the accessor
can then compare the value of this argument against the expected Swift
type of its result and, if they don't match, bail early.

The new argument is defined as a pointer _to_ a Swift type rather than
as a Swift type directly because `@convention(c)` functions cannot
directly reference Swift types. The value of the new argument can be
safely ignored if the type of the test content record's value is and
always has been `@frozen`.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Documentation/ABI/TestContent.md

@@ -41,10 +41,16 @@ Regardless of platform, all test content records created and discoverable by the
 testing library have the following layout:
 
 ```swift
+typealias Accessor = @convention(c) (
+  _ outValue: UnsafeMutableRawPointer,
+  _ type: UnsafeRawPointer,
+  _ hint: UnsafeRawPointer?
+) -> CBool
+
 typealias TestContentRecord = (
   kind: UInt32,
   reserved1: UInt32,
-  accessor: (@convention(c) (_ outValue: UnsafeMutableRawPointer, _ hint: UnsafeRawPointer?) -> CBool)?,
+  accessor: Accessor?,
   context: UInt,
   reserved2: UInt
 )
@@ -54,10 +60,16 @@ This type has natural size, stride, and alignment. Its fields are native-endian.
 If needed, this type can be represented in C as a structure:
 
 ```c
+typedef bool (* SWTAccessor)(
+  void *outValue,
+  const void *type,
+  const void *_Nullable hint
+);
+
 struct SWTTestContentRecord {
   uint32_t kind;
   uint32_t reserved1;
-  bool (* _Nullable accessor)(void *outValue, const void *_Null_unspecified hint);
+  SWTAccessor _Nullable accessor;
   uintptr_t context;
   uintptr_t reserved2;
 };
@@ -105,42 +117,59 @@ If `accessor` is `nil`, the test content record is ignored. The testing library
 may, in the future, define record kinds that do not provide an accessor function
 (that is, they represent pure compile-time information only.)
 
-The second argument to this function, `hint`, is an optional input that can be
+The second argument to this function, `type`, is a pointer to the type[^mightNotBeSwift]
+(not a bitcast Swift type) of the value expected to be written to `outValue`.
+This argument helps to prevent memory corruption if two copies of Swift Testing
+or a third-party library are inadvertently loaded into the same process. If the
+value at `type` does not match the test content record's expected type, the
+accessor function must return `false` and must not modify `outValue`.
+
+<!-- TODO: discuss this argument's value in Embedded Swift (no metatypes) -->
+
+[^mightNotBeSwift]: Although this document primarily deals with Swift, the test
+  content record section is generally language-agnostic. The use of languages
+  other than Swift is beyond the scope of this document. With that in mind, it
+  is _technically_ feasible for a test content accessor to be written in (for
+  example) C++, expect the `type` argument to point to a C++ value of type
+  `std::type_info`, and write a C++ class instance to `outValue`.
+
+The third argument to this function, `hint`, is an optional input that can be
 passed to help the accessor function determine if its corresponding test content
 record matches what the caller is looking for. If the caller passes `nil` as the
 `hint` argument, the accessor behaves as if it matched (that is, no additional
 filtering is performed.)
 
-The concrete Swift type of the value written to `outValue` and the value pointed
-to by `hint` depend on the kind of record:
+The concrete Swift type of the value written to `outValue`, the type pointed to
+by `type`, and the value pointed to by `hint` depend on the kind of record:
 
 - For test or suite declarations (kind `0x74657374`), the accessor produces an
-  asynchronous Swift function that returns an instance of `Test`:
+  asynchronous Swift function[^notAccessorSignature] that returns an instance of
+  `Testing.Test`:
 
   ```swift
   @Sendable () async -> Test
   ```
 
-  This signature is not the signature of `accessor`, but of the Swift function
-  reference it writes to `outValue`. This level of indirection is necessary
-  because loading a test or suite declaration is an asynchronous operation, but
-  C functions cannot be `async`.
+  [^notAccessorSignature]: This signature is not the signature of `accessor`,
+    but of the Swift function reference it writes to `outValue`. This level of
+    indirection is necessary because loading a test or suite declaration is an
+    asynchronous operation, but C functions cannot be `async`.
 
   Test content records of this kind do not specify a type for `hint`. Always
   pass `nil`.
 
 - For exit test declarations (kind `0x65786974`), the accessor produces a
-  structure describing the exit test (of type `__ExitTest`.)
+  structure describing the exit test (of type `Testing.__ExitTest`.)
 
-  Test content records of this kind accept a `hint` of type `__ExitTest.ID`.
+  Test content records of this kind accept a `hint` of type `Testing.__ExitTest.ID`.
   They only produce a result if they represent an exit test declared with the
   same ID (or if `hint` is `nil`.)
 
 > [!WARNING]
 > Calling code should use [`withUnsafeTemporaryAllocation(of:capacity:_:)`](https://developer.apple.com/documentation/swift/withunsafetemporaryallocation(of:capacity:_:))
-> and [`withUnsafePointer(to:_:)`](https://developer.apple.com/documentation/swift/withunsafepointer(to:_:)-35wrn),
-> respectively, to ensure the pointers passed to `accessor` are large enough and
-> are well-aligned. If they are not large enough to contain values of the
+> and/or [`withUnsafePointer(to:_:)`](https://developer.apple.com/documentation/swift/withunsafepointer(to:_:)-35wrn)
+> to ensure the pointers passed to `accessor` are large enough and are
+> well-aligned. If they are not large enough to contain values of the
 > appropriate types (per above), or if `hint` points to uninitialized or
 > incorrectly-typed memory, the result is undefined.
 

Sources/Testing/Discovery.swift

@@ -10,6 +10,26 @@
 
 private import _TestingInternals
 
+/// The type of the accessor function used to access a test content record.
+///
+/// - Parameters:
+///   - outValue: A pointer to uninitialized memory large enough to contain the
+///     corresponding test content record's value.
+///   - type: A pointer to the expected type of `outValue`. Use `load(as:)` to
+///     get the Swift type, not `unsafeBitCast(_:to:)`.
+///   - hint: An optional pointer to a hint value.
+///
+/// - Returns: Whether or not `outValue` was initialized. The caller is
+///   responsible for deinitializing `outValue` if it was initialized.
+///
+/// - Warning: This type is used to implement the `@Test` macro. Do not use it
+///   directly.
+public typealias __TestContentRecordAccessor = @convention(c) (
+  _ outValue: UnsafeMutableRawPointer,
+  _ type: UnsafeRawPointer,
+  _ hint: UnsafeRawPointer?
+) -> CBool
+
 /// The content of a test content record.
 ///
 /// - Parameters:
@@ -24,7 +44,7 @@ private import _TestingInternals
 public typealias __TestContentRecord = (
   kind: UInt32,
   reserved1: UInt32,
-  accessor: (@convention(c) (_ outValue: UnsafeMutableRawPointer, _ hint: UnsafeRawPointer?) -> CBool)?,
+  accessor: __TestContentRecordAccessor?,
   context: UInt,
   reserved2: UInt
 )
@@ -54,6 +74,20 @@ protocol TestContent: ~Copyable {
   /// By default, this type equals `Never`, indicating that this type of test
   /// content does not support hinting during discovery.
   associatedtype TestContentAccessorHint: Sendable = Never
+
+  /// The type to pass (by address) as the accessor function's `type` argument.
+  ///
+  /// The default value of this property is `Self.self`. A conforming type can
+  /// override the default implementation to substitute another type (e.g. if
+  /// the conforming type is not public but records are created during macro
+  /// expansion and can only reference public types.)
+  static var testContentAccessorTypeArgument: any ~Copyable.Type { get }
+}
+
+extension TestContent where Self: ~Copyable {
+  static var testContentAccessorTypeArgument: any ~Copyable.Type {
+    self
+  }
 }
 
 // MARK: - Individual test content records
@@ -108,18 +142,20 @@ struct TestContentRecord<T>: Sendable where T: TestContent & ~Copyable {
       return nil
     }
 
-    return withUnsafeTemporaryAllocation(of: T.self, capacity: 1) { buffer in
-      let initialized = if let hint {
-        withUnsafePointer(to: hint) { hint in
-          accessor(buffer.baseAddress!, hint)
+    return withUnsafePointer(to: T.testContentAccessorTypeArgument) { type in
+      withUnsafeTemporaryAllocation(of: T.self, capacity: 1) { buffer in
+        let initialized = if let hint {
+          withUnsafePointer(to: hint) { hint in
+            accessor(buffer.baseAddress!, type, hint)
+          }
+        } else {
+          accessor(buffer.baseAddress!, type, nil)
         }
-      } else {
-        accessor(buffer.baseAddress!, nil)
-      }
-      guard initialized else {
-        return nil
+        guard initialized else {
+          return nil
+        }
+        return buffer.baseAddress!.move()
       }
-      return buffer.baseAddress!.move()
     }
   }
 }

Sources/Testing/Test+Discovery.swift

@@ -28,8 +28,15 @@ extension Test {
       0x74657374
     }
 
+    static var testContentAccessorTypeArgument: any ~Copyable.Type {
+      Generator.self
+    }
+
+    /// The type of the actual (asynchronous) generator function.
+    typealias Generator = @Sendable () async -> Test
+
     /// The actual (asynchronous) accessor function.
-    case generator(@Sendable () async -> Test)
+    case generator(Generator)
   }
 
   /// All available ``Test`` instances in the process, according to the runtime.

Tests/TestingTests/MiscellaneousTests.swift

@@ -615,7 +615,10 @@ struct MiscellaneousTests {
     private static let record: __TestContentRecord = (
       0xABCD1234,
       0,
-      { outValue, hint in
+      { outValue, type, hint in
+        guard type.load(as: Any.Type.self) == DiscoverableTestContent.self else {
+          return false
+        }
         if let hint, hint.load(as: TestContentAccessorHint.self) != expectedHint {
           return false
         }