fix(token): use dedicated .lock file to guard entire refresh critical section

The previous implementation had two correctness issues:

1. flock on .tmp was ineffective for cross-process exclusion.
   After fs::rename(), the .tmp inode disappears.  A second process
   opening .tmp gets a brand-new inode, so both processes hold flocks
   on different inodes simultaneously — no mutual exclusion occurs.

2. The critical section was too narrow.  The in-process tokio::Mutex
   only serializes tasks within the same process.  Two separate mdrs
   processes could both read the cache, both decide a refresh was
   needed, and both call the token-refresh endpoint before either had
   written the new token back — risking double-refresh and potential
   failures on servers that use refresh-token rotation.

Fix: introduce a dedicated `cache/{remote}.lock` file as the cross-
process advisory lock target.  The lock file is never renamed, so its
inode remains stable for the entire critical section.  The flock now
wraps the complete read-check-refresh-write cycle in
load_cache_with_token_refresh(), and the redundant flock on .tmp in
refresh_and_persist() is removed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/commands/shared.rs

@@ -232,32 +232,56 @@ pub fn load_cache(remote: &str) -> Result<Cache, Box<dyn std::error::Error>> {
 /// Locking strategy:
 /// - Per-remote `tokio::sync::Mutex` serializes concurrent async tasks within
 ///   the same process.
-/// - `flock(LOCK_EX)` on the cache file serializes across separate processes
-///   on the same host.
+/// - `flock(LOCK_EX)` on a dedicated `cache/{remote}.lock` file serializes
+///   the entire read-check-refresh-write cycle across separate processes on
+///   the same host.  The lock file is stable (never renamed), so the flock
+///   reliably protects the same inode for the lifetime of the critical section.
 pub async fn load_cache_with_token_refresh(
     remote: &str,
 ) -> Result<Cache, Box<dyn std::error::Error>> {
-    // Acquire the in-process async mutex for this remote
+    // Acquire the in-process async mutex for this remote first.
     let lock = get_remote_lock(remote);
     let _guard = lock.lock().await;
 
-    // Re-read the cache inside the lock (another task may have already refreshed)
-    let mut cache = load_cache(remote)?;
+    // Acquire an exclusive cross-process file lock.  This ensures that no
+    // other process can race through the read-check-refresh-write cycle at
+    // the same time as us.  The lock file is separate from the cache file so
+    // that it never disappears (unlike a .tmp file that gets renamed away).
+    let lock_path = cache_file_path(remote).with_extension("lock");
+    use fs2::FileExt;
+    let lock_file = fs::OpenOptions::new()
+        .write(true)
+        .create(true)
+        .open(&lock_path)?;
+    lock_file.lock_exclusive()?;
 
-    if crate::token::is_expired(&cache.token.refresh) {
-        return Err(format!(
-            "Session for `{}` has expired. Please run `mdrs login {}` again.",
-            remote, remote
-        )
-        .into());
+    // Re-read the cache inside the lock: another process may have already
+    // refreshed the token since we last checked.
+    let result: Result<Cache, Box<dyn std::error::Error>> = async {
+        let mut cache = load_cache(remote)?;
+
+        if crate::token::is_expired(&cache.token.refresh) {
+            return Err(format!(
+                "Session for `{}` has expired. Please run `mdrs login {}` again.",
+                remote, remote
+            )
+            .into());
+        }
+
+        if crate::token::is_refresh_required(&cache.token.access, &cache.token.refresh) {
+            let new_access = refresh_and_persist(remote, &cache).await?;
+            cache.token.access = new_access;
+        }
+
+        Ok(cache)
     }
+    .await;
 
-    if crate::token::is_refresh_required(&cache.token.access, &cache.token.refresh) {
-        let new_access = refresh_and_persist(remote, &cache).await?;
-        cache.token.access = new_access;
-    }
+    // Release the cross-process file lock.  The OS will also release it when
+    // lock_file is dropped, but we unlock explicitly for clarity.
+    lock_file.unlock()?;
 
-    Ok(cache)
+    result
 }
 
 /// Call the token-refresh endpoint and write the new access token back to the
@@ -293,21 +317,18 @@ async fn refresh_and_persist(
     obj["token"]["access"] = serde_json::Value::String(new_access.clone());
     obj["digest"] = serde_json::Value::String(new_digest);
 
-    // Write atomically: write to .tmp then rename, while holding an exclusive
-    // flock on the .tmp file for cross-process safety.
+    // Write atomically: write to .tmp then rename.  The caller already holds
+    // the exclusive .lock file, so no additional flock is needed here.
     let tmp_path = cache_path.with_extension("tmp");
     {
-        use fs2::FileExt;
         use std::io::Write;
         let mut tmp_file = fs::OpenOptions::new()
             .write(true)
             .create(true)
             .truncate(true)
             .open(&tmp_path)?;
-        tmp_file.lock_exclusive()?;
         tmp_file.write_all(serde_json::to_string(&obj)?.as_bytes())?;
         tmp_file.flush()?;
-        tmp_file.unlock()?;
     }
     #[cfg(unix)]
     {