fsck: detect duplicate keys (different UUIDs, same key)

replay_nodes now tracks all UUIDs per key using a temporary multimap.
Warns on duplicates so they can be manually resolved.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/store/persist.rs

@@ -91,12 +91,16 @@ impl Store {
         Ok(store)
     }
 
-    /// Replay node log, keeping latest version per UUID
+    /// Replay node log, keeping latest version per UUID.
+    /// Tracks all UUIDs seen per key to detect duplicates.
     fn replay_nodes(&mut self, path: &Path) -> Result<(), String> {
         let file = fs::File::open(path)
             .map_err(|e| format!("open {}: {}", path.display(), e))?;
         let mut reader = BufReader::new(file);
 
+        // Track all non-deleted UUIDs per key to detect duplicates
+        let mut key_uuids: HashMap<String, Vec<[u8; 16]>> = HashMap::new();
+
         while let Ok(msg) = serialize::read_message(&mut reader, message::ReaderOptions::new()) {
             let log = msg.get_root::<memory_capnp::node_log::Reader>()
                 .map_err(|e| format!("read node log: {}", e))?;
@@ -110,13 +114,28 @@ impl Store {
                     if node.deleted {
                         self.nodes.remove(&node.key);
                         self.uuid_to_key.remove(&node.uuid);
+                        if let Some(uuids) = key_uuids.get_mut(&node.key) {
+                            uuids.retain(|u| *u != node.uuid);
+                        }
                     } else {
                         self.uuid_to_key.insert(node.uuid, node.key.clone());
-                        self.nodes.insert(node.key.clone(), node);
+                        self.nodes.insert(node.key.clone(), node.clone());
+                        let uuids = key_uuids.entry(node.key).or_default();
+                        if !uuids.contains(&node.uuid) {
+                            uuids.push(node.uuid);
                         }
                     }
                 }
             }
+        }
+
+        // Report duplicate keys
+        for (key, uuids) in &key_uuids {
+            if uuids.len() > 1 {
+                eprintln!("WARNING: key '{}' has {} UUIDs (duplicate nodes)", key, uuids.len());
+            }
+        }
+
         Ok(())
     }