feat(xy): exclusive pidfile guard

crates/xy/src/pidfile.rs

@@ -0,0 +1,46 @@
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::{Path, PathBuf};
+
+#[derive(Debug)]
+pub struct PidFile {
+    path: PathBuf,
+    _file: File,
+}
+
+impl PidFile {
+    pub fn acquire(path: &Path) -> std::io::Result<Self> {
+        let mut f = OpenOptions::new()
+            .write(true).create_new(true).mode(0o600).open(path)?;
+        writeln!(f, "{}", std::process::id())?;
+        Ok(Self { path: path.to_path_buf(), _file: f })
+    }
+}
+
+impl Drop for PidFile {
+    fn drop(&mut self) { let _ = std::fs::remove_file(&self.path); }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[test]
+    fn acquire_then_second_fails() {
+        let dir = tempdir().unwrap();
+        let p = dir.path().join("x.pid");
+        let _g = PidFile::acquire(&p).unwrap();
+        let err = PidFile::acquire(&p).unwrap_err();
+        assert_eq!(err.kind(), std::io::ErrorKind::AlreadyExists);
+    }
+
+    #[test]
+    fn drop_removes_file() {
+        let dir = tempdir().unwrap();
+        let p = dir.path().join("x.pid");
+        { let _g = PidFile::acquire(&p).unwrap(); assert!(p.exists()); }
+        assert!(!p.exists());
+    }
+}