Specify the module to find the ABI entry point function in (in our own tests). (#801)

The order in which Xcode loads the Apple-vendored copy of Swift Testing
vs. a developer-supplied XCTest bundle is unspecified. Our tests assume
that the "current" copy of Swift Testing (that is, the copy they linked
against) is the first one loaded, which is not always true when running
tests in Xcode. The modified tests end up finding the wrong entry point
functions when called within Xcode if the load order has changed.

This PR explicitly specifies that we are looking for the entry point
functions in the module the tests are linked against. The change is
applied across all platforms that support dynamic linking for the sake
of consistency as any of them might have linked a copy of Swift Testing
from the toolchain _and_ the package itself.

Resolves rdar://139140302.

### 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

Tests/TestingTests/ABIEntryPointTests.swift

@@ -53,11 +53,13 @@ struct ABIEntryPointTests {
     recordHandler: @escaping @Sendable (_ recordJSON: UnsafeRawBufferPointer) -> Void = { _ in }
   ) async throws -> CInt {
     // Get the ABI entry point by dynamically looking it up at runtime.
-    let copyABIEntryPoint_v0 = try #require(
-      symbol(named: "swt_copyABIEntryPoint_v0").map {
-        unsafeBitCast($0, to: (@convention(c) () -> UnsafeMutableRawPointer).self)
-      }
-    )
+    let copyABIEntryPoint_v0 = try withTestingLibraryImageAddress { testingLibrary in
+      try #require(
+        symbol(in: testingLibrary, named: "swt_copyABIEntryPoint_v0").map {
+          unsafeBitCast($0, to: (@convention(c) () -> UnsafeMutableRawPointer).self)
+        }
+      )
+    }
     let abiEntryPoint = copyABIEntryPoint_v0().assumingMemoryBound(to: ABIEntryPoint_v0.self)
     defer {
       abiEntryPoint.deinitialize(count: 1)
@@ -133,11 +135,13 @@ struct ABIEntryPointTests {
     // NOTE: The standard Linux linker does not allow exporting symbols from
     // executables, so dlsym() does not let us find this function on that
     // platform when built as an executable rather than a dynamic library.
-    let abiv0_getEntryPoint = try #require(
-      symbol(named: "swt_abiv0_getEntryPoint").map {
-        unsafeBitCast($0, to: (@convention(c) () -> UnsafeRawPointer).self)
-      }
-    )
+    let abiv0_getEntryPoint = try withTestingLibraryImageAddress { testingLibrary in
+      try #require(
+        symbol(in: testingLibrary, named: "swt_abiv0_getEntryPoint").map {
+          unsafeBitCast($0, to: (@convention(c) () -> UnsafeRawPointer).self)
+        }
+      )
+    }
 #endif
     let abiEntryPoint = unsafeBitCast(abiv0_getEntryPoint(), to: ABIv0.EntryPoint.self)
 
@@ -163,4 +167,36 @@ struct ABIEntryPointTests {
   }
 #endif
 }
+
+#if !SWT_NO_DYNAMIC_LINKING
+private func withTestingLibraryImageAddress<R>(_ body: (ImageAddress?) throws -> R) throws -> R {
+  let addressInTestingLibrary = unsafeBitCast(ABIv0.entryPoint, to: UnsafeRawPointer.self)
+
+  var testingLibraryAddress: ImageAddress?
+#if SWT_TARGET_OS_APPLE
+  var info = Dl_info()
+  try #require(0 != dladdr(addressInTestingLibrary, &info))
+
+  testingLibraryAddress = dlopen(info.dli_fname, RTLD_NOLOAD)
+  try #require(testingLibraryAddress != nil)
+  defer {
+    dlclose(testingLibraryAddress)
+  }
+#elseif os(Linux) || os(FreeBSD) || os(Android)
+  // When using glibc, dladdr() is only available if __USE_GNU is specified.
+#elseif os(Windows)
+  let flags = DWORD(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS)
+  try addressInTestingLibrary.withMemoryRebound(to: wchar_t.self, capacity: MemoryLayout<UnsafeRawPointer>.stride / MemoryLayout<wchar_t>.stride) { addressInTestingLibrary in
+    try #require(GetModuleHandleExW(flags, addressInTestingLibrary, &testingLibraryAddress))
+  }
+  defer {
+    FreeLibrary(testingLibraryAddress)
+  }
+#else
+#warning("Platform-specific implementation missing: cannot find the testing library image the test suite is linked against")
+#endif
+
+  return try body(testingLibraryAddress)
+}
+#endif
 #endif