Initial QSfera import

Server/vendor/github.com/open-policy-agent/opa/v1/util/channel.go

@@ -0,0 +1,32 @@
+package util
+
+import (
+	"github.com/open-policy-agent/opa/v1/metrics"
+)
+
+// This prevents getting blocked forever writing to a full buffer, in case another routine fills the last space.
+// Retrying maxEventRetry times to drop the oldest event. Dropping the incoming event if there still isn't room.
+const maxEventRetry = 1000
+
+// PushFIFO pushes data into a buffered channel without blocking when full, making room by dropping the oldest data.
+// An optional metric can be recorded when data is dropped.
+func PushFIFO[T any](buffer chan T, data T, metrics metrics.Metrics, metricName string) {
+
+	for range maxEventRetry {
+		// non-blocking send to the buffer, to prevent blocking if buffer is full so room can be made.
+		select {
+		case buffer <- data:
+			return
+		default:
+		}
+
+		// non-blocking drop from the buffer to make room for incoming event
+		select {
+		case <-buffer:
+			if metrics != nil && metricName != "" {
+				metrics.Counter(metricName).Incr()
+			}
+		default:
+		}
+	}
+}