[#1248] node: Do not update cache twice

Do not request morph values on morph cache misses concurrently.

Signed-off-by: Pavel Karpy <p.karpy@yadro.com>

CHANGELOG.md

@@ -70,6 +70,7 @@ Changelog for FrostFS Node
 - Non-alphabet nodes do not try to handle alphabet events (#181)
 - Failing SN and IR transactions because of incorrect scopes (#2230, #2263)
 - Global scope used for some transactions (#2230, #2263)
+- Concurrent morph cache misses (#30)
 
 ### Removed
 ### Updated

cmd/frostfs-node/cache.go

@@ -24,8 +24,18 @@ type valueWithTime[V any] struct {
 	e error
 }
 
+// valueInProgress is a struct that contains
+// values that are being fetched/updated.
+type valueInProgress[V any] struct {
+	m sync.RWMutex
+	v V
+	e error
+}
+
 // entity that provides TTL cache interface.
 type ttlNetCache[K comparable, V any] struct {
+	m       sync.RWMutex              // protects progMap
+	progMap map[K]*valueInProgress[V] // contains fetch-in-progress keys
 	ttl     time.Duration
 
 	sz int
@@ -45,28 +55,64 @@ func newNetworkTTLCache[K comparable, V any](sz int, ttl time.Duration, netRdr n
 		sz:      sz,
 		cache:   cache,
 		netRdr:  netRdr,
+		progMap: make(map[K]*valueInProgress[V]),
 	}
 }
 
+func waitForUpdate[V any](vip *valueInProgress[V]) (V, error) {
+	vip.m.RLock()
+	defer vip.m.RUnlock()
+
+	return vip.v, vip.e
+}
+
 // reads value by the key.
 //
 // updates the value from the network on cache miss or by TTL.
 //
 // returned value should not be modified.
 func (c *ttlNetCache[K, V]) get(key K) (V, error) {
-	val, ok := c.cache.Peek(key)
+	valWithTime, ok := c.cache.Peek(key)
 	if ok {
-		if time.Since(val.t) < c.ttl {
+		if time.Since(valWithTime.t) < c.ttl {
-			return val.v, val.e
+			return valWithTime.v, valWithTime.e
 		}
 
 		c.cache.Remove(key)
 	}
 
-	v, err := c.netRdr(key)
+	c.m.RLock()
+	valInProg, ok := c.progMap[key]
+	c.m.RUnlock()
 
+	if ok {
+		return waitForUpdate(valInProg)
+	}
+
+	c.m.Lock()
+	valInProg, ok = c.progMap[key]
+	if ok {
+		c.m.Unlock()
+		return waitForUpdate(valInProg)
+	}
+
+	valInProg = &valueInProgress[V]{}
+	valInProg.m.Lock()
+	c.progMap[key] = valInProg
+
+	c.m.Unlock()
+
+	v, err := c.netRdr(key)
 	c.set(key, v, err)
 
+	valInProg.v = v
+	valInProg.e = err
+	valInProg.m.Unlock()
+
+	c.m.Lock()
+	delete(c.progMap, key)
+	c.m.Unlock()
+
 	return v, err
 }