Add a thread-safe implementation of Process.currentDirectoryURL

posix_spawn_file_actions_addchdir_np is the official way to set the working directory of a spawned process and is supported on both macOS and glibc (Linux, etc.). This makes Process.currentDirectoryURL thread-safe, as the current approach will result in the working directory being nondeterministically assigned when spawning processes across multiple threads, using different working directories.

Sources/CoreFoundation/CFPlatform.c

@@ -2274,6 +2274,23 @@ CF_EXPORT int _CFPosixSpawnFileActionsAddClose(_CFPosixSpawnFileActionsRef file_
   return posix_spawn_file_actions_addclose((posix_spawn_file_actions_t *)file_actions, filedes);
 }
 
+CF_EXPORT int _CFPosixSpawnFileActionsChdir(_CFPosixSpawnFileActionsRef file_actions, const char *path) {
+  // Standardized as posix_spawn_file_actions_addchdir in POSIX.1-2024 (June 2024)
+  // _np variant available in:
+  //  - Solaris 11.3 (October 2015)
+  //  - Glibc 2.29 (February 2019)
+  //  - macOS 10.15 (October 2019)
+  //  - musl 1.1.24 (October 2019)
+  //  - FreeBSD 13.1 (May 2022)
+  //  - Android 14 (October 2023)
+  // standard variant available in:
+  //  - Solaris 11.4 (August 2018)
+  //  - NetBSD 10.0 (March 2024)
+  // missing in:
+  //  - OpenBSD
+  return posix_spawn_file_actions_addchdir_np((posix_spawn_file_actions_t *)file_actions, path);
+}
+
 CF_EXPORT int _CFPosixSpawn(pid_t *_CF_RESTRICT pid, const char *_CF_RESTRICT path, _CFPosixSpawnFileActionsRef file_actions, _CFPosixSpawnAttrRef _Nullable _CF_RESTRICT attrp, char *_Nullable const argv[_Nullable _CF_RESTRICT], char *_Nullable const envp[_Nullable _CF_RESTRICT]) {
   return posix_spawn(pid, path, (posix_spawn_file_actions_t *)file_actions, (posix_spawnattr_t *)attrp, argv, envp);
 }

Sources/CoreFoundation/include/ForSwiftFoundationOnly.h

@@ -725,6 +725,7 @@ CF_EXPORT int _CFPosixSpawnFileActionsDestroy(_CFPosixSpawnFileActionsRef file_a
 CF_EXPORT void _CFPosixSpawnFileActionsDealloc(_CFPosixSpawnFileActionsRef file_actions);
 CF_EXPORT int _CFPosixSpawnFileActionsAddDup2(_CFPosixSpawnFileActionsRef file_actions, int filedes, int newfiledes);
 CF_EXPORT int _CFPosixSpawnFileActionsAddClose(_CFPosixSpawnFileActionsRef file_actions, int filedes);
+CF_EXPORT int _CFPosixSpawnFileActionsChdir(_CFPosixSpawnFileActionsRef file_actions, const char *path);
 #ifdef __cplusplus
 CF_EXPORT int _CFPosixSpawn(pid_t *_CF_RESTRICT pid, const char *_CF_RESTRICT path, _CFPosixSpawnFileActionsRef file_actions, _CFPosixSpawnAttrRef _Nullable _CF_RESTRICT attrp, char *const argv[], char *const envp[]);
 #else

Sources/Foundation/Process.swift

@@ -925,6 +925,9 @@ open class Process: NSObject, @unchecked Sendable {
         for fd in addclose.filter({ $0 >= 0 }) {
             try _throwIfPosixError(_CFPosixSpawnFileActionsAddClose(fileActions, fd))
         }
+        if let dir = currentDirectoryURL?.path {
+            try _throwIfPosixError(_CFPosixSpawnFileActionsChdir(fileActions, dir))
+        }
 
 #if canImport(Darwin) || os(Android) || os(OpenBSD)
         var spawnAttrs: posix_spawnattr_t? = nil
@@ -947,17 +950,6 @@ open class Process: NSObject, @unchecked Sendable {
         }
 #endif
 
-        let fileManager = FileManager()
-        let previousDirectoryPath = fileManager.currentDirectoryPath
-        if let dir = currentDirectoryURL?.path, !fileManager.changeCurrentDirectoryPath(dir) {
-            throw _NSErrorWithErrno(errno, reading: true, url: currentDirectoryURL)
-        }
-
-        defer {
-            // Reset the previous working directory path.
-            _ = fileManager.changeCurrentDirectoryPath(previousDirectoryPath)
-        }
-
         // Launch
         var pid = pid_t()