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.

CoreFoundation/Base.subproj/CFPlatform.c

@@ -2251,6 +2251,13 @@ 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) {
+    if (__builtin_available(macOS 10.15, *)) {
+        return posix_spawn_file_actions_addchdir_np((posix_spawn_file_actions_t *)file_actions, path);
+    }
+    return ENOTSUP; // unreachable
+}
+
 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);
 }

CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h

@@ -736,6 +736,7 @@ 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

@@ -923,6 +923,9 @@ open class Process: NSObject {
         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
@@ -945,17 +948,6 @@ open class Process: NSObject {
         }
 #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()
         guard _CFPosixSpawn(&pid, launchPath, fileActions, &spawnAttrs, argv, envp) == 0 else {