feat: add stable file locking using std::fs APIs (#2894)

## Summary

This PR implements advisory file locking for the message history using
Rust 1.89+ stabilized std::fs::File locking APIs, eliminating the need
for external dependencies.

## Key Changes

- **Stable API Usage**: Uses std::fs::File::try_lock() and
try_lock_shared() APIs stabilized in Rust 1.89
- **Cross-Platform Compatibility**: 
  - Unix systems use try_lock_shared() for advisory read locks
  - Windows systems use try_lock() due to different lock semantics
- **Retry Logic**: Maintains existing retry behavior for concurrent
access scenarios
- **No External Dependencies**: Removes need for external file locking
crates

## Technical Details

The implementation provides advisory file locking to prevent corruption
when multiple Codex processes attempt to write to the message history
file simultaneously. The locking is platform-aware to handle differences
in Windows vs Unix file locking behavior.

## Testing

- ✅ Builds successfully on all platforms
- ✅ Existing message history tests pass
- ✅ File locking retry logic verified

Related to discussion in #2773 about using stabilized Rust APIs instead
of external dependencies.

---------

Co-authored-by: Michael Bolin <[email protected]>

Author: pchuri

codex-rs/core/src/message_history.rs

@@ -105,47 +105,34 @@ pub(crate) async fn append_entry(text: &str, session_id: &Uuid, config: &Config)
     // Ensure permissions.
     ensure_owner_only_permissions(&history_file).await?;
 
-    // Lock file.
-    acquire_exclusive_lock_with_retry(&history_file).await?;
-
-    // We use sync I/O with spawn_blocking() because we are using a
-    // [`std::fs::File`] instead of a [`tokio::fs::File`] to leverage an
-    // advisory file locking API that is not available in the async API.
+    // Perform a blocking write under an advisory write lock using std::fs.
     tokio::task::spawn_blocking(move || -> Result<()> {
-        history_file.write_all(line.as_bytes())?;
-        history_file.flush()?;
-        Ok(())
+        // Retry a few times to avoid indefinite blocking when contended.
+        for _ in 0..MAX_RETRIES {
+            match history_file.try_lock() {
+                Ok(()) => {
+                    // While holding the exclusive lock, write the full line.
+                    history_file.write_all(line.as_bytes())?;
+                    history_file.flush()?;
+                    return Ok(());
+                }
+                Err(std::fs::TryLockError::WouldBlock) => {
+                    std::thread::sleep(RETRY_SLEEP);
+                }
+                Err(e) => return Err(e.into()),
+            }
+        }
+
+        Err(std::io::Error::new(
+            std::io::ErrorKind::WouldBlock,
+            "could not acquire exclusive lock on history file after multiple attempts",
+        ))
     })
     .await??;
 
     Ok(())
 }
 
-/// Attempt to acquire an exclusive advisory lock on `file`, retrying up to 10
-/// times if the lock is currently held by another process. This prevents a
-/// potential indefinite wait while still giving other writers some time to
-/// finish their operation.
-async fn acquire_exclusive_lock_with_retry(file: &File) -> Result<()> {
-    use tokio::time::sleep;
-
-    for _ in 0..MAX_RETRIES {
-        match file.try_lock() {
-            Ok(()) => return Ok(()),
-            Err(e) => match e {
-                std::fs::TryLockError::WouldBlock => {
-                    sleep(RETRY_SLEEP).await;
-                }
-                other => return Err(other.into()),
-            },
-        }
-    }
-
-    Err(std::io::Error::new(
-        std::io::ErrorKind::WouldBlock,
-        "could not acquire exclusive lock on history file after multiple attempts",
-    ))
-}
-
 /// Asynchronously fetch the history file's *identifier* (inode on Unix) and
 /// the current number of entries by counting newline characters.
 pub(crate) async fn history_metadata(config: &Config) -> (u64, usize) {
@@ -221,29 +208,42 @@ pub(crate) fn lookup(log_id: u64, offset: usize, config: &Config) -> Option<Hist
         return None;
     }
 
-    // Open & lock file for reading.
-    if let Err(e) = acquire_shared_lock_with_retry(&file) {
-        tracing::warn!(error = %e, "failed to acquire shared lock on history file");
-        return None;
-    }
-
-    let reader = BufReader::new(&file);
-    for (idx, line_res) in reader.lines().enumerate() {
-        let line = match line_res {
-            Ok(l) => l,
-            Err(e) => {
-                tracing::warn!(error = %e, "failed to read line from history file");
+    // 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();
+
+        match lock_result {
+            Ok(()) => {
+                let reader = BufReader::new(&file);
+                for (idx, line_res) in reader.lines().enumerate() {
+                    let line = match line_res {
+                        Ok(l) => l,
+                        Err(e) => {
+                            tracing::warn!(error = %e, "failed to read line from history file");
+                            return None;
+                        }
+                    };
+
+                    if idx == offset {
+                        match serde_json::from_str::<HistoryEntry>(&line) {
+                            Ok(entry) => return Some(entry),
+                            Err(e) => {
+                                tracing::warn!(error = %e, "failed to parse history entry");
+                                return None;
+                            }
+                        }
+                    }
+                }
+                // Not found at requested offset.
                 return None;
             }
-        };
-
-        if idx == offset {
-            match serde_json::from_str::<HistoryEntry>(&line) {
-                Ok(entry) => return Some(entry),
-                Err(e) => {
-                    tracing::warn!(error = %e, "failed to parse history entry");
-                    return None;
-                }
+            Err(std::fs::TryLockError::WouldBlock) => {
+                std::thread::sleep(RETRY_SLEEP);
+            }
+            Err(e) => {
+                tracing::warn!(error = %e, "failed to acquire shared lock on history file");
+                return None;
             }
         }
     }
@@ -258,26 +258,6 @@ pub(crate) fn lookup(log_id: u64, offset: usize, config: &Config) -> Option<Hist
     None
 }
 
-#[cfg(unix)]
-fn acquire_shared_lock_with_retry(file: &File) -> Result<()> {
-    for _ in 0..MAX_RETRIES {
-        match file.try_lock_shared() {
-            Ok(()) => return Ok(()),
-            Err(e) => match e {
-                std::fs::TryLockError::WouldBlock => {
-                    std::thread::sleep(RETRY_SLEEP);
-                }
-                other => return Err(other.into()),
-            },
-        }
-    }
-
-    Err(std::io::Error::new(
-        std::io::ErrorKind::WouldBlock,
-        "could not acquire shared lock on history file after multiple attempts",
-    ))
-}
-
 /// On Unix systems ensure the file permissions are `0o600` (rw-------). If the
 /// permissions cannot be changed the error is propagated to the caller.
 #[cfg(unix)]