Set default for BackfillQueueSize if negative

Signed-off-by: notfelineit <[email protected]>

go/vt/vtadmin/cache/cache.go

@@ -54,6 +54,9 @@ const (
 	// backfill requests to still process, if a config is passed with a
 	// non-positive BackfillRequestTTL.
 	DefaultBackfillRequestTTL = time.Millisecond * 100
+	// DefaultBackfillQueueSize is the default value used for the size of the
+	// backfill queue, if a config is passed with a non-positive BackfillQueueSize.
+	DefaultBackfillQueueSize = 1
 )
 
 // Config is the configuration for a cache.
@@ -125,6 +128,11 @@ func New[Key Keyer, Value any](fillFunc func(ctx context.Context, req Key) (Valu
 		cfg.BackfillRequestTTL = DefaultBackfillRequestTTL
 	}
 
+	if cfg.BackfillQueueSize <= 0 {
+		log.Warningf("BackfillQueueSize (%v) must be positive, defaulting to %v", cfg.BackfillQueueSize, DefaultBackfillQueueSize)
+		cfg.BackfillQueueSize = DefaultBackfillQueueSize
+	}
+
 	c := &Cache[Key, Value]{
 		cache:     cache.New(cfg.DefaultExpiration, cfg.CleanupInterval),
 		lastFill:  map[string]time.Time{},