[#231] node: Fix race condition in TTL cache

Use key locker to lock by key.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

cmd/frostfs-node/cache.go

@@ -24,25 +24,72 @@ 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
+type locker struct {
+	mtx     *sync.Mutex
+	waiters int // not protected by mtx, must used outer mutex to update concurrently
+}
+
+type keyLocker[K comparable] struct {
+	lockers    map[K]*locker
+	lockersMtx *sync.Mutex
+}
+
+func newKeyLocker[K comparable]() *keyLocker[K] {
+	return &keyLocker[K]{
+		lockers:    make(map[K]*locker),
+		lockersMtx: &sync.Mutex{},
+	}
+}
+
+func (l *keyLocker[K]) LockKey(key K) {
+	l.lockersMtx.Lock()
+
+	if locker, found := l.lockers[key]; found {
+		locker.waiters++
+		l.lockersMtx.Unlock()
+
+		locker.mtx.Lock()
+		return
+	}
+
+	locker := &locker{
+		mtx:     &sync.Mutex{},
+		waiters: 1,
+	}
+	locker.mtx.Lock()
+
+	l.lockers[key] = locker
+	l.lockersMtx.Unlock()
+}
+
+func (l *keyLocker[K]) UnlockKey(key K) {
+	l.lockersMtx.Lock()
+	defer l.lockersMtx.Unlock()
+
+	locker, found := l.lockers[key]
+	if !found {
+		return
+	}
+
+	if locker.waiters == 1 {
+		delete(l.lockers, key)
+	}
+	locker.waiters--
+
+	locker.mtx.Unlock()
 }
 
 // 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
+	ttl time.Duration
 
 	sz int
 
 	cache *lru.Cache[K, *valueWithTime[V]]
 
 	netRdr netValueReader[K, V]
+
+	keyLocker *keyLocker[K]
 }
 
 // complicates netValueReader with TTL caching mechanism.
@@ -51,72 +98,48 @@ func newNetworkTTLCache[K comparable, V any](sz int, ttl time.Duration, netRdr n
 	fatalOnErr(err)
 
 	return &ttlNetCache[K, V]{
-		ttl:     ttl,
-		sz:      sz,
-		cache:   cache,
-		netRdr:  netRdr,
-		progMap: make(map[K]*valueInProgress[V]),
+		ttl:       ttl,
+		sz:        sz,
+		cache:     cache,
+		netRdr:    netRdr,
+		keyLocker: newKeyLocker[K](),
 	}
 }
 
-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) {
-	valWithTime, ok := c.cache.Peek(key)
-	if ok {
-		if time.Since(valWithTime.t) < c.ttl {
-			return valWithTime.v, valWithTime.e
-		}
-
-		c.cache.Remove(key)
+	val, ok := c.cache.Peek(key)
+	if ok && time.Since(val.t) < c.ttl {
+		return val.v, val.e
 	}
 
-	c.m.RLock()
-	valInProg, ok := c.progMap[key]
-	c.m.RUnlock()
+	c.keyLocker.LockKey(key)
+	defer c.keyLocker.UnlockKey(key)
 
-	if ok {
-		return waitForUpdate(valInProg)
+	val, ok = c.cache.Peek(key)
+	if ok && time.Since(val.t) < c.ttl {
+		return val.v, val.e
 	}
 
-	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()
+	c.cache.Add(key, &valueWithTime[V]{
+		v: v,
+		t: time.Now(),
+		e: err,
+	})
 
 	return v, err
 }
 
 func (c *ttlNetCache[K, V]) set(k K, v V, e error) {
+	c.keyLocker.LockKey(k)
+	defer c.keyLocker.UnlockKey(k)
+
 	c.cache.Add(k, &valueWithTime[V]{
 		v: v,
 		t: time.Now(),
@@ -125,6 +148,9 @@ func (c *ttlNetCache[K, V]) set(k K, v V, e error) {
 }
 
 func (c *ttlNetCache[K, V]) remove(key K) {
+	c.keyLocker.LockKey(key)
+	defer c.keyLocker.UnlockKey(key)
+
 	c.cache.Remove(key)
 }
 

cmd/frostfs-node/cache_test.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sync/errgroup"
+)
+
+func TestKeyLocker(t *testing.T) {
+	taken := false
+	eg, _ := errgroup.WithContext(context.Background())
+	keyLocker := newKeyLocker[int]()
+	for i := 0; i < 100; i++ {
+		eg.Go(func() error {
+			keyLocker.LockKey(0)
+			defer keyLocker.UnlockKey(0)
+
+			require.False(t, taken)
+			taken = true
+			require.True(t, taken)
+			time.Sleep(10 * time.Millisecond)
+			taken = false
+			require.False(t, taken)
+
+			return nil
+		})
+	}
+	require.NoError(t, eg.Wait())
+}