Reimplement #1265 using modifyMVar

Author: amesgen

ouroboros-consensus/changelog.d/js-cache-modifymvar.md

@@ -0,0 +1,3 @@
+### Patch
+
+- Replace `bracketOnError` with `modifyMVar` in the Cached Index implementation.

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Storage/ImmutableDB/Impl/Index/Cache.hs

@@ -574,26 +574,23 @@ getChunkInfo ::
 getChunkInfo cacheEnv chunk = do
     lastUsed <- LastUsed <$> getMonotonicTime
     -- Make sure we don't leave an empty MVar in case of an exception.
-    mbCacheHit <- bracketOnError (takeMVar cacheVar) (tryPutMVar cacheVar) $
+    mbCacheHit <- modifyMVar cacheVar $
       \cached@Cached { currentChunk, currentChunkInfo, nbPastChunks } -> if
         | chunk == currentChunk -> do
           -- Cache hit for the current chunk
-          putMVar cacheVar cached
           traceWith tracer $ TraceCurrentChunkHit chunk nbPastChunks
-          return $ Just $ Left currentChunkInfo
+          return (cached, Just $ Left currentChunkInfo)
         | Just (pastChunkInfo, cached') <- lookupPastChunkInfo chunk lastUsed cached -> do
           -- Cache hit for an chunk in the past
-          putMVar cacheVar cached'
           traceWith tracer $ TracePastChunkHit chunk nbPastChunks
-          return $ Just $ Right pastChunkInfo
+          return (cached', Just $ Right pastChunkInfo)
         | otherwise -> do
           -- Cache miss for an chunk in the past. We don't want to hold on to
           -- the 'cacheVar' MVar, blocking all other access to the cace, while
           -- we're reading things from disk, so put it back now and update the
           -- cache afterwards.
-          putMVar cacheVar cached
           traceWith tracer $ TracePastChunkMiss chunk nbPastChunks
-          return Nothing
+          return (cached, Nothing)
     case mbCacheHit of
       Just hit -> return hit
       Nothing  -> do