Author: Danny Milosavljevic <dannym@friendly-machines.com>
Date: 2026-01-25
License: ASL2.0
Subject: Use libc::flock instead of unstable std File::try_lock()/try_lock_shared().

The file_lock feature is tracked at <https://github.com/rust-lang/rust/issues/130994>.
and is not yet stable in old Rust versions like Rust 1.85.

The file_lock feature was stabilized after Rust 1.88, but we only have 1.88.

diff -u a/codex-rs/core/src/message_history.rs b/codex-rs/core/src/message_history.rs
--- a/codex-rs/core/src/message_history.rs
+++ b/codex-rs/core/src/message_history.rs
@@ -41,6 +41,8 @@
 use std::os::unix::fs::OpenOptionsExt;
 #[cfg(unix)]
 use std::os::unix::fs::PermissionsExt;
+#[cfg(unix)]
+use std::os::unix::io::AsRawFd;

 /// Filename that stores the message history inside `~/.codex`.
 const HISTORY_FILENAME: &str = "history.jsonl";
@@ -51,6 +53,28 @@
 const MAX_RETRIES: usize = 10;
 const RETRY_SLEEP: Duration = Duration::from_millis(100);

+// FIXME: Remove these helpers when Rust provides stable file locking API
+// The file_lock feature is tracked at <https://github.com/rust-lang/rust/issues/130994>.
+#[cfg(unix)]
+fn try_lock_exclusive(file: &File) -> std::io::Result<()> {
+    let ret = unsafe { libc::flock(file.as_raw_fd(), libc::LOCK_EX | libc::LOCK_NB) };
+    if ret == 0 {
+        Ok(())
+    } else {
+        Err(std::io::Error::last_os_error())
+    }
+}
+
+#[cfg(unix)]
+fn try_lock_shared(file: &File) -> std::io::Result<()> {
+    let ret = unsafe { libc::flock(file.as_raw_fd(), libc::LOCK_SH | libc::LOCK_NB) };
+    if ret == 0 {
+        Ok(())
+    } else {
+        Err(std::io::Error::last_os_error())
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
 pub struct HistoryEntry {
     pub session_id: String,
@@ -126,7 +150,7 @@
     tokio::task::spawn_blocking(move || -> Result<()> {
         // Retry a few times to avoid indefinite blocking when contended.
         for _ in 0..MAX_RETRIES {
-            match history_file.try_lock() {
+            match try_lock_exclusive(&history_file) {
                 Ok(()) => {
                     // While holding the exclusive lock, write the full line.
                     // We do not open the file with `append(true)` on Windows, so ensure the
@@ -137,10 +161,10 @@
                     enforce_history_limit(&mut history_file, history_max_bytes)?;
                     return Ok(());
                 }
-                Err(std::fs::TryLockError::WouldBlock) => {
+                Err(ref e) if e.kind() == std::io::ErrorKind::WouldBlock => {
                     std::thread::sleep(RETRY_SLEEP);
                 }
-                Err(e) => return Err(e.into()),
+                Err(e) => return Err(e),
             }
         }

@@ -341,7 +365,7 @@
     // Open & lock file for reading using a shared lock.
     // Retry a few times to avoid indefinite blocking.
     for _ in 0..MAX_RETRIES {
-        let lock_result = file.try_lock_shared();
+        let lock_result = try_lock_shared(&file);

         match lock_result {
             Ok(()) => {
@@ -368,7 +392,7 @@
                 // Not found at requested offset.
                 return None;
             }
-            Err(std::fs::TryLockError::WouldBlock) => {
+            Err(ref e) if e.kind() == std::io::ErrorKind::WouldBlock => {
                 std::thread::sleep(RETRY_SLEEP);
             }
             Err(e) => {