[musl] use posix_spawn if a directory change was requested

library/std/src/process/tests.rs

@@ -96,9 +96,23 @@ fn stdout_works() {
 #[test]
 #[cfg_attr(any(windows, target_os = "vxworks"), ignore)]
 fn set_current_dir_works() {
+    // On many Unix platforms this will use the posix_spawn path.
     let mut cmd = shell_cmd();
     cmd.arg("-c").arg("pwd").current_dir("/").stdout(Stdio::piped());
     assert_eq!(run_output(cmd), "/\n");
+
+    // Also test the fork/exec path by setting a pre_exec function.
+    #[cfg(unix)]
+    {
+        use crate::os::unix::process::CommandExt;
+
+        let mut cmd = shell_cmd();
+        cmd.arg("-c").arg("pwd").current_dir("/").stdout(Stdio::piped());
+        unsafe {
+            cmd.pre_exec(|| Ok(()));
+        }
+        assert_eq!(run_output(cmd), "/\n");
+    }
 }
 
 #[test]

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

@@ -448,7 +448,6 @@ impl Command {
         use core::sync::atomic::{AtomicU8, Ordering};
 
         use crate::mem::MaybeUninit;
-        use crate::sys::weak::weak;
         use crate::sys::{self, cvt_nz, on_broken_pipe_flag_used};
 
         if self.get_gid().is_some()
@@ -462,6 +461,8 @@ impl Command {
 
         cfg_if::cfg_if! {
             if #[cfg(target_os = "linux")] {
+                use crate::sys::weak::weak;
+
                 weak! {
                     fn pidfd_spawnp(
                         *mut libc::c_int,
@@ -575,16 +576,44 @@ impl Command {
             }
         }
 
-        // Solaris, glibc 2.29+, and musl 1.24+ can set a new working directory,
-        // and maybe others will gain this non-POSIX function too. We'll check
-        // for this weak symbol as soon as it's needed, so we can return early
-        // otherwise to do a manual chdir before exec.
-        weak! {
-            fn posix_spawn_file_actions_addchdir_np(
-                *mut libc::posix_spawn_file_actions_t,
-                *const libc::c_char
-            ) -> libc::c_int
+        type PosixSpawnAddChdirFn = unsafe extern "C" fn(
+            *mut libc::posix_spawn_file_actions_t,
+            *const libc::c_char,
+        ) -> libc::c_int;
+
+        /// Get the function pointer for adding a chdir action to a
+        /// `posix_spawn_file_actions_t`, if available, assuming a dynamic libc.
+        ///
+        /// Some platforms can set a new working directory for a spawned process in the
+        /// `posix_spawn` path. This function looks up the function pointer for adding
+        /// such an action to a `posix_spawn_file_actions_t` struct.
+        #[cfg(not(all(target_os = "linux", target_env = "musl")))]
+        fn get_posix_spawn_addchdir() -> Option<PosixSpawnAddChdirFn> {
+            use crate::sys::weak::weak;
+
+            weak! {
+                fn posix_spawn_file_actions_addchdir_np(
+                    *mut libc::posix_spawn_file_actions_t,
+                    *const libc::c_char
+                ) -> libc::c_int
+            }
+
+            posix_spawn_file_actions_addchdir_np.get()
+        }
+
+        /// Get the function pointer for adding a chdir action to a
+        /// `posix_spawn_file_actions_t`, if available, on platforms where the function
+        /// is known to exist.
+        ///
+        /// Weak symbol lookup doesn't work with statically linked libcs, so in cases
+        /// where static linking is possible we need to either check for the presence
+        /// of the symbol at compile time or know about it upfront.
+        #[cfg(all(target_os = "linux", target_env = "musl"))]
+        fn get_posix_spawn_addchdir() -> Option<PosixSpawnAddChdirFn> {
+            // Our minimum required musl supports this function, so we can just use it.
+            Some(libc::posix_spawn_file_actions_addchdir_np)
         }
+
         let addchdir = match self.get_cwd() {
             Some(cwd) => {
                 if cfg!(target_vendor = "apple") {
@@ -597,7 +626,10 @@ impl Command {
                         return Ok(None);
                     }
                 }
-                match posix_spawn_file_actions_addchdir_np.get() {
+                // Check for the availability of the posix_spawn addchdir
+                // function now. If it isn't available, bail and use the
+                // fork/exec path.
+                match get_posix_spawn_addchdir() {
                     Some(f) => Some((f, cwd)),
                     None => return Ok(None),
                 }