chore(arg0): advisory-lock janitor for codex tmp paths (#10039)

## Description

### What changed
- Switch the arg0 helper root from `~/.codex/tmp/path` to
`~/.codex/tmp/path2`
- Add `Arg0PathEntryGuard` to keep both the `TempDir` and an exclusive
`.lock` file alive for the process lifetime
- Add a startup janitor that scans `path2` and deletes only directories
whose lock can be acquired

### Tests
- `cargo clippy -p codex-arg0`
- `cargo clippy -p codex-core`
- `cargo test -p codex-arg0`
- `cargo test -p codex-core`

codex-rs/arg0/src/lib.rs

@@ -1,3 +1,4 @@
+use std::fs::File;
 use std::future::Future;
 use std::path::Path;
 use std::path::PathBuf;
@@ -10,8 +11,24 @@ use tempfile::TempDir;
 const LINUX_SANDBOX_ARG0: &str = "codex-linux-sandbox";
 const APPLY_PATCH_ARG0: &str = "apply_patch";
 const MISSPELLED_APPLY_PATCH_ARG0: &str = "applypatch";
+const LOCK_FILENAME: &str = ".lock";
 
-pub fn arg0_dispatch() -> Option<TempDir> {
+/// Keeps the per-session PATH entry alive and locked for the process lifetime.
+pub struct Arg0PathEntryGuard {
+    _temp_dir: TempDir,
+    _lock_file: File,
+}
+
+impl Arg0PathEntryGuard {
+    fn new(temp_dir: TempDir, lock_file: File) -> Self {
+        Self {
+            _temp_dir: temp_dir,
+            _lock_file: lock_file,
+        }
+    }
+}
+
+pub fn arg0_dispatch() -> Option<Arg0PathEntryGuard> {
     // Determine if we were invoked via the special alias.
     let mut args = std::env::args_os();
     let argv0 = args.next().unwrap_or_default();
@@ -149,7 +166,7 @@ where
 ///
 /// IMPORTANT: This function modifies the PATH environment variable, so it MUST
 /// be called before multiple threads are spawned.
-pub fn prepend_path_entry_for_codex_aliases() -> std::io::Result<TempDir> {
+pub fn prepend_path_entry_for_codex_aliases() -> std::io::Result<Arg0PathEntryGuard> {
     let codex_home = codex_core::config::find_codex_home()?;
     #[cfg(not(debug_assertions))]
     {
@@ -167,7 +184,7 @@ pub fn prepend_path_entry_for_codex_aliases() -> std::io::Result<TempDir> {
 
     std::fs::create_dir_all(&codex_home)?;
     // Use a CODEX_HOME-scoped temp root to avoid cluttering the top-level directory.
-    let temp_root = codex_home.join("tmp").join("path");
+    let temp_root = codex_home.join("tmp").join("arg0");
     std::fs::create_dir_all(&temp_root)?;
     #[cfg(unix)]
     {
@@ -177,11 +194,25 @@ pub fn prepend_path_entry_for_codex_aliases() -> std::io::Result<TempDir> {
         std::fs::set_permissions(&temp_root, std::fs::Permissions::from_mode(0o700))?;
     }
 
+    // Best-effort cleanup of stale per-session dirs. Ignore failures so startup proceeds.
+    if let Err(err) = janitor_cleanup(&temp_root) {
+        eprintln!("WARNING: failed to clean up stale arg0 temp dirs: {err}");
+    }
+
     let temp_dir = tempfile::Builder::new()
         .prefix("codex-arg0")
         .tempdir_in(&temp_root)?;
     let path = temp_dir.path();
 
+    let lock_path = path.join(LOCK_FILENAME);
+    let lock_file = File::options()
+        .read(true)
+        .write(true)
+        .create(true)
+        .truncate(false)
+        .open(&lock_path)?;
+    lock_file.try_lock()?;
+
     for filename in &[
         APPLY_PATCH_ARG0,
         MISSPELLED_APPLY_PATCH_ARG0,
@@ -231,5 +262,107 @@ pub fn prepend_path_entry_for_codex_aliases() -> std::io::Result<TempDir> {
         std::env::set_var("PATH", updated_path_env_var);
     }
 
-    Ok(temp_dir)
+    Ok(Arg0PathEntryGuard::new(temp_dir, lock_file))
+}
+
+fn janitor_cleanup(temp_root: &Path) -> std::io::Result<()> {
+    let entries = match std::fs::read_dir(temp_root) {
+        Ok(entries) => entries,
+        Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(()),
+        Err(err) => return Err(err),
+    };
+
+    for entry in entries.flatten() {
+        let path = entry.path();
+        if !path.is_dir() {
+            continue;
+        }
+
+        // Skip the directory if locking fails or the lock is currently held.
+        let Some(_lock_file) = try_lock_dir(&path)? else {
+            continue;
+        };
+
+        match std::fs::remove_dir_all(&path) {
+            Ok(()) => {}
+            // Expected TOCTOU race: directory can disappear after read_dir/lock checks.
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => continue,
+            Err(err) => return Err(err),
+        }
+    }
+
+    Ok(())
+}
+
+fn try_lock_dir(dir: &Path) -> std::io::Result<Option<File>> {
+    let lock_path = dir.join(LOCK_FILENAME);
+    let lock_file = match File::options().read(true).write(true).open(&lock_path) {
+        Ok(file) => file,
+        Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(None),
+        Err(err) => return Err(err),
+    };
+
+    match lock_file.try_lock() {
+        Ok(()) => Ok(Some(lock_file)),
+        Err(std::fs::TryLockError::WouldBlock) => Ok(None),
+        Err(err) => Err(err.into()),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::LOCK_FILENAME;
+    use super::janitor_cleanup;
+    use std::fs;
+    use std::fs::File;
+    use std::path::Path;
+
+    fn create_lock(dir: &Path) -> std::io::Result<File> {
+        let lock_path = dir.join(LOCK_FILENAME);
+        File::options()
+            .read(true)
+            .write(true)
+            .create(true)
+            .truncate(false)
+            .open(lock_path)
+    }
+
+    #[test]
+    fn janitor_skips_dirs_without_lock_file() -> std::io::Result<()> {
+        let root = tempfile::tempdir()?;
+        let dir = root.path().join("no-lock");
+        fs::create_dir(&dir)?;
+
+        janitor_cleanup(root.path())?;
+
+        assert!(dir.exists());
+        Ok(())
+    }
+
+    #[test]
+    fn janitor_skips_dirs_with_held_lock() -> std::io::Result<()> {
+        let root = tempfile::tempdir()?;
+        let dir = root.path().join("locked");
+        fs::create_dir(&dir)?;
+        let lock_file = create_lock(&dir)?;
+        lock_file.try_lock()?;
+
+        janitor_cleanup(root.path())?;
+
+        assert!(dir.exists());
+        Ok(())
+    }
+
+    #[test]
+    fn janitor_removes_dirs_with_unlocked_lock() -> std::io::Result<()> {
+        let root = tempfile::tempdir()?;
+        let dir = root.path().join("stale");
+        fs::create_dir(&dir)?;
+        create_lock(&dir)?;
+
+        janitor_cleanup(root.path())?;
+
+        assert!(!dir.exists());
+        Ok(())
+    }
 }

codex-rs/core/tests/suite/mod.rs

@@ -1,16 +1,57 @@
 // Aggregates all former standalone integration tests as modules.
+use std::ffi::OsString;
+
+use codex_arg0::Arg0PathEntryGuard;
 use codex_arg0::arg0_dispatch;
 use ctor::ctor;
 use tempfile::TempDir;
 
+struct TestCodexAliasesGuard {
+    _codex_home: TempDir,
+    _arg0: Arg0PathEntryGuard,
+    _previous_codex_home: Option<OsString>,
+}
+
+const CODEX_HOME_ENV_VAR: &str = "CODEX_HOME";
+
 // This code runs before any other tests are run.
 // It allows the test binary to behave like codex and dispatch to apply_patch and codex-linux-sandbox
 // based on the arg0.
 // NOTE: this doesn't work on ARM
 #[ctor]
-pub static CODEX_ALIASES_TEMP_DIR: TempDir = unsafe {
+pub static CODEX_ALIASES_TEMP_DIR: TestCodexAliasesGuard = unsafe {
     #[allow(clippy::unwrap_used)]
-    arg0_dispatch().unwrap()
+    let codex_home = tempfile::Builder::new()
+        .prefix("codex-core-tests")
+        .tempdir()
+        .unwrap();
+    let previous_codex_home = std::env::var_os(CODEX_HOME_ENV_VAR);
+    // arg0_dispatch() creates helper links under CODEX_HOME/tmp. Point it at a
+    // test-owned temp dir so startup never mutates the developer's real ~/.codex.
+    //
+    // Safety: #[ctor] runs before tests start, so no test threads exist yet.
+    unsafe {
+        std::env::set_var(CODEX_HOME_ENV_VAR, codex_home.path());
+    }
+
+    #[allow(clippy::unwrap_used)]
+    let arg0 = arg0_dispatch().unwrap();
+    // Restore the process environment immediately so later tests observe the
+    // same CODEX_HOME state they started with.
+    match previous_codex_home.as_ref() {
+        Some(value) => unsafe {
+            std::env::set_var(CODEX_HOME_ENV_VAR, value);
+        },
+        None => unsafe {
+            std::env::remove_var(CODEX_HOME_ENV_VAR);
+        },
+    }
+
+    TestCodexAliasesGuard {
+        _codex_home: codex_home,
+        _arg0: arg0,
+        _previous_codex_home: previous_codex_home,
+    }
 };
 
 #[cfg(not(target_os = "windows"))]