Reset the working directory of child processes we spawn on Windows.

Sources/Testing/ExitTests/SpawnProcess.swift

@@ -284,25 +284,47 @@ func spawnExecutable(
       let commandLine = _escapeCommandLine(CollectionOfOne(executablePath) + arguments)
       let environ = environment.map { "\($0.key)=\($0.value)" }.joined(separator: "\0") + "\0\0"
 
+      // CreateProcessW() may modify the command line argument, so we must make
+      // a mutable copy of it. (environ is also passed as a mutable raw pointer,
+      // but it is not documented as actually being mutated.)
+      let commandLineCopy = commandLine.withCString(encodedAs: UTF16.self) { _wcsdup($0) }
+      defer {
+        free(commandLineCopy)
+      }
+
+      // On Windows, a process holds a reference to its current working
+      // directory, which prevents other processes from deleting it. This causes
+      // code to fail if it tries to set the working directory to a temporary
+      // path. SEE: https://github.com/swiftlang/swift-testing/issues/1209
+      //
+      // This problem manifests for us when we spawn a child process without
+      // setting its working directory, which causes it to default to that of
+      // the parent process. To avoid this problem, we set the working directory
+      // of the new process to the root directory of the boot volume (which is
+      // unlikely to be deleted, one hopes).
+      //
+      // SEE: https://devblogs.microsoft.com/oldnewthing/20101109-00/?p=12323
+      let workingDirectoryPath = rootDirectoryPath
+
       var flags = DWORD(CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT | EXTENDED_STARTUPINFO_PRESENT)
 #if DEBUG
       // Start the process suspended so we can attach a debugger if needed.
       flags |= DWORD(CREATE_SUSPENDED)
 #endif
 
-      return try commandLine.withCString(encodedAs: UTF16.self) { commandLine in
-        try environ.withCString(encodedAs: UTF16.self) { environ in
+      return try environ.withCString(encodedAs: UTF16.self) { environ in
+        try workingDirectoryPath.withCString(encodedAs: UTF16.self) { workingDirectoryPath in
           var processInfo = PROCESS_INFORMATION()
 
           guard CreateProcessW(
             nil,
-            .init(mutating: commandLine),
+            commandLineCopy,
             nil,
             nil,
             true, // bInheritHandles
             flags,
             .init(mutating: environ),
-            nil,
+            workingDirectoryPath,
             startupInfo.pointer(to: \.StartupInfo)!,
             &processInfo
           ) else {

Sources/Testing/Support/FileHandle.swift

@@ -719,4 +719,35 @@ func setFD_CLOEXEC(_ flag: Bool, onFileDescriptor fd: CInt) throws {
   }
 }
 #endif
+
+/// The path to the root directory of the boot volume.
+///
+/// On Windows, this string is usually of the form `"C:\"`. On UNIX-like
+/// platforms, it is always equal to `"/"`.
+let rootDirectoryPath: String = {
+#if os(Windows)
+  var result: String?
+
+  // The boot volume is, except in some legacy scenarios, the volume that
+  // contains the system Windows directory. For an explanation of the difference
+  // between the Windows directory and the _system_ Windows directory, see
+  // https://devblogs.microsoft.com/oldnewthing/20140723-00/?p=423 .
+  let count = GetSystemWindowsDirectoryW(nil, 0)
+  if count > 0 {
+    withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: Int(count) + 1) { buffer in
+      _ = GetSystemWindowsDirectoryW(buffer.baseAddress!, UINT(buffer.count))
+      let rStrip = PathCchStripToRoot(buffer.baseAddress!, buffer.count)
+      if rStrip == S_OK || rStrip == S_FALSE {
+        result = String.decodeCString(buffer.baseAddress!, as: UTF16.self)?.result
+      }
+    }
+  }
+
+  // If we weren't able to get a path, fall back to "C:\" on the assumption that
+  // it's the common case and most likely correct.
+  return result ?? #"C:\"#
+#else
+  return "/"
+#endif
+}()
 #endif

Tests/TestingTests/Support/FileHandleTests.swift

@@ -201,6 +201,17 @@ struct FileHandleTests {
 #endif
   }
 #endif
+
+  @Test("Root directory path is correct")
+  func rootDirectoryPathIsCorrect() throws {
+#if os(Windows)
+    if let systemDrive = Environment.variable(named: "SYSTEMDRIVE") {
+      #expect(rootDirectoryPath.starts(with: systemDrive))
+    }
+#else
+    #expect(rootDirectoryPath == "/")
+#endif
+  }
 }
 
 // MARK: - Fixtures