lib/blockcache: eliminate possible race when Cache.Put is called for the same entry from multiple goroutines

The race could result in incorrect cache size tracking, which, in turn, could result in too frequent cache cleaning

lib/blockcache/blockcache.go

@@ -109,20 +109,21 @@ func (c *Cache) cleaner() {
 }
 
 func (c *Cache) cleanByTimeout() {
-	currentTime := fasttime.UnixTimestamp()
+	// Delete items accessed more than five minutes ago.
+	// This time should be enough for repeated queries.
+	lastAccessTime := fasttime.UnixTimestamp()-5*60
 	c.mu.Lock()
+	defer c.mu.Unlock()
+
 	for _, pes := range c.m {
 		for offset, e := range pes {
-			// Delete items accessed more than five minutes ago.
+			if lastAccessTime > atomic.LoadUint64(&e.lastAccessTime) {
-			// This time should be enough for repeated queries.
-			if currentTime-atomic.LoadUint64(&e.lastAccessTime) > 5*60 {
 				c.updateSizeBytes(-e.block.SizeBytes())
 				delete(pes, offset)
 				// do not delete the entry from c.perKeyMisses, since it is removed by Cache.cleaner later.
 			}
 		}
 	}
-	c.mu.Unlock()
 }
 
 // GetBlock returns a block for the given key k from c.
@@ -167,14 +168,19 @@ func (c *Cache) PutBlock(k Key, b Block) {
 
 	// Store b in the cache.
 	c.mu.Lock()
-	e := &cacheEntry{
+	defer c.mu.Unlock()
-		lastAccessTime: fasttime.UnixTimestamp(),
+
-		block:          b,
-	}
 	pes := c.m[k.Part]
 	if pes == nil {
 		pes = make(map[uint64]*cacheEntry)
 		c.m[k.Part] = pes
+	} else if pes[k.Offset] != nil {
+		// The block has been already registered by concurrent goroutine.
+		return
+	}
+	e := &cacheEntry{
+		lastAccessTime: fasttime.UnixTimestamp(),
+		block:          b,
 	}
 	pes[k.Offset] = e
 	c.updateSizeBytes(e.block.SizeBytes())
@@ -187,13 +193,11 @@ func (c *Cache) PutBlock(k Key, b Block) {
 				delete(pes, offset)
 				// do not delete the entry from c.perKeyMisses, since it is removed by Cache.cleaner later.
 				if c.SizeBytes() < maxSizeBytes {
-					goto end
+					return
 				}
 			}
 		}
 	}
-end:
-	c.mu.Unlock()
 }
 
 // Len returns the number of blocks in the cache c.