fix(utils): update existing keys in-place in FifoCache::push

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## v0.15.0 (TBD)
 
+- Fixed `FifoCache::push` creating a ghost eviction entry when called with a key that already exists in the cache. The duplicate queue entry inflated the eviction queue length beyond the number of live map entries, consumed an eviction slot without a corresponding map value, and caused a still-live entry to be prematurely dropped when the ghost surfaced as the oldest key. Existing keys are now updated in-place without modifying the eviction queue.
+
 - Added `ca-certificates` to the node Docker runtime image so outbound `https` connections work in containerized deployments ([#1661](https://github.com/0xMiden/node/issues/1661)).
 - Reworked `SyncNotes` store queries to fetch multiple matching blocks within one database transaction while preserving the response payload cap ([#2027](https://github.com/0xMiden/node/pull/2027)).
 - Added composite index `idx_transactions_account_block_txid` on `transactions(account_id, block_num, transaction_id)` to speed up `select_transactions_records` queries used by `SyncTransactions` ([#1965](https://github.com/0xMiden/node/issues/1965)).

crates/utils/src/fifo_cache.rs

@@ -39,8 +39,20 @@ where
     }
 
     /// Inserts a key-value pair, evicting the oldest entry if the cache is at capacity.
+    /// Inserts or updates `key` with `value`.
+    ///
+    /// If `key` already exists, its value is updated in-place without touching the eviction
+    /// queue. Re-enqueueing an existing key would create a ghost entry: the queue would
+    /// grow beyond the number of live map entries, consume an eviction slot for a key that
+    /// may no longer exist, and prematurely evict a valid entry when the ghost surfaces as
+    /// the oldest key.
     pub fn push(&self, key: K, value: V) {
         let mut inner = self.0.lock().expect("fifo cache lock poisoned");
+        // Key already in cache: update value in-place, leave eviction queue unchanged.
+        if inner.map.contains_key(&key) {
+            inner.map.insert(key, value);
+            return;
+        }
         if inner.eviction.len() >= inner.capacity.get() {
             if let Some(oldest) = inner.eviction.pop_front() {
                 inner.map.remove(&oldest);