[#1248] placement: Decouple ContainerNodes() cache from the placement builder.

Also, write tests.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/object_manager/placement/cache.go

@@ -0,0 +1,69 @@
+package placement
+
+import (
+	"crypto/sha256"
+	"fmt"
+	"sync"
+
+	cid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/container/id"
+	netmapSDK "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/netmap"
+	"github.com/hashicorp/golang-lru/v2/simplelru"
+)
+
+// ContainerNodesCache caches results of ContainerNodes() invocation between epochs.
+type ContainerNodesCache struct {
+	// mtx protects lastEpoch and containerCache fields.
+	mtx sync.Mutex
+	// lastEpoch contains network map epoch for all values in the container cache.
+	lastEpoch uint64
+	// containerCache caches container nodes by ID. It is used to skip `GetContainerNodes` invocation if
+	// neither netmap nor container has changed.
+	containerCache simplelru.LRUCache[cid.ID, [][]netmapSDK.NodeInfo]
+}
+
+// defaultContainerCacheSize is the default size for the container cache.
+const defaultContainerCacheSize = 10
+
+// NewContainerNodesCache creates new cache which saves the result of the ContainerNodes() invocations.
+// If size is <= 0, defaultContainerCacheSize (10) is used.
+func NewContainerNodesCache(size int) *ContainerNodesCache {
+	if size <= 0 {
+		size = defaultContainerCacheSize
+	}
+
+	cache, _ := simplelru.NewLRU[cid.ID, [][]netmapSDK.NodeInfo](size, nil) // no error
+	return &ContainerNodesCache{
+		containerCache: cache,
+	}
+}
+
+// ContainerNodes returns the result of nm.ContainerNodes(), possibly from the cache.
+func (c *ContainerNodesCache) ContainerNodes(nm *netmapSDK.NetMap, cnr cid.ID, p netmapSDK.PlacementPolicy) ([][]netmapSDK.NodeInfo, error) {
+	c.mtx.Lock()
+	if nm.Epoch() == c.lastEpoch {
+		raw, ok := c.containerCache.Get(cnr)
+		c.mtx.Unlock()
+		if ok {
+			return raw, nil
+		}
+	} else {
+		c.lastEpoch = nm.Epoch()
+		c.containerCache.Purge()
+		c.mtx.Unlock()
+	}
+
+	binCnr := make([]byte, sha256.Size)
+	cnr.Encode(binCnr)
+
+	cn, err := nm.ContainerNodes(p, binCnr)
+	if err != nil {
+		return nil, fmt.Errorf("could not get container nodes: %w", err)
+	}
+
+	c.mtx.Lock()
+	if c.lastEpoch == nm.Epoch() {
+		c.containerCache.Add(cnr, cn)
+	}
+	c.mtx.Unlock()
+	return cn, nil
+}