chunked: fix reuse of the layers cache

the global singleton was never updated, causing the cache to be always
recreated for each layer.

It is not possible to keep the layersCache mutex for the entire load()
since it calls into some store APIs causing a deadlock since
findDigestInternal() is already called while some store locks are
held.

Another benefit is that now only one goroutine can run load()
preventing multiple calls to load() to happen in parallel doing the
same work.

Closes: #2023

Signed-off-by: Giuseppe Scrivano <[email protected]>

pkg/chunked/cache_linux.go

@@ -82,6 +82,7 @@ func (c *layer) release() {
 		if err := unix.Munmap(c.mmapBuffer); err != nil {
 			logrus.Warnf("Error Munmap: layer %q: %v", c.id, err)
 		}
+		c.mmapBuffer = nil
 	}
 }
 
@@ -110,7 +111,7 @@ func getLayersCacheRef(store storage.Store) *layersCache {
 		cache.refs++
 		return cache
 	}
-	cache := &layersCache{
+	cache = &layersCache{
 		store: store,
 		refs:  1,
 	}
@@ -779,14 +780,14 @@ func (c *layersCache) findDigestInternal(digest string) (string, string, int64,
 		return "", "", -1, nil
 	}
 
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
+
 	binaryDigest, err := makeBinaryDigest(digest)
 	if err != nil {
 		return "", "", 0, err
 	}
 
-	c.mutex.RLock()
-	defer c.mutex.RUnlock()
-
 	for _, layer := range c.layers {
 		if !layer.cacheFile.bloomFilter.maybeContains(binaryDigest) {
 			continue