Don't use posix_spawn_file_actions_addchdir_np on macOS.

library/std/src/sys/unix/process/process_unix.rs

@@ -314,10 +314,20 @@ impl Command {
             ) -> libc::c_int
         }
         let addchdir = match self.get_cwd() {
-            Some(cwd) => match posix_spawn_file_actions_addchdir_np.get() {
-                Some(f) => Some((f, cwd)),
-                None => return Ok(None),
-            },
+            Some(cwd) => {
+                if cfg!(target_os = "macos") {
+                    // There is a bug in macOS where a relative executable
+                    // path like "../myprogram" will cause `posix_spawn` to
+                    // successfully launch the program, but erroneously return
+                    // ENOENT when used with posix_spawn_file_actions_addchdir_np
+                    // which was introduced in macOS 10.15.
+                    return Ok(None);
+                }
+                match posix_spawn_file_actions_addchdir_np.get() {
+                    Some(f) => Some((f, cwd)),
+                    None => return Ok(None),
+                }
+            }
             None => None,
         };
 

src/test/ui/command/command-current-dir.rs

@@ -0,0 +1,49 @@
+// run-pass
+// ignore-emscripten no processes
+// ignore-sgx no processes
+
+use std::env;
+use std::fs;
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    // Checks the behavior of current_dir when used with a relative exe path.
+    let me = env::current_exe().unwrap();
+    if matches!(env::args().skip(1).next().as_deref(), Some("current-dir")) {
+        let cwd = env::current_dir().unwrap();
+        assert_eq!(cwd.file_name().unwrap(), "bar");
+        std::process::exit(0);
+    }
+    let exe = me.file_name().unwrap();
+    let cwd = me.parent().unwrap();
+    eprintln!("cwd={:?}", cwd);
+    // Change directory to where the exectuable is located, since this test
+    // fundamentally needs to use relative paths. In some cases (like
+    // remote-test-server), the current_dir can be somewhere else, so make
+    // sure it is something we can use. We assume we can write to this
+    // directory.
+    env::set_current_dir(&cwd).unwrap();
+    let foo = cwd.join("foo");
+    let bar = cwd.join("bar");
+    fs::create_dir_all(&foo).unwrap();
+    fs::create_dir_all(&bar).unwrap();
+    fs::copy(&me, foo.join(exe)).unwrap();
+
+    // Unfortunately this is inconsistent based on the platform, see
+    // https://github.com/rust-lang/rust/issues/37868. On Windows,
+    // it is relative *before* changing the directory, and on Unix
+    // it is *after* changing the directory.
+    let relative_exe = if cfg!(windows) {
+        Path::new("foo").join(exe)
+    } else {
+        Path::new("../foo").join(exe)
+    };
+
+    let status = Command::new(relative_exe)
+        .arg("current-dir")
+        .current_dir("bar")
+        .status()
+        .unwrap();
+    assert!(status.success());
+}