TUN-528: Move cloudflared into a separate repo

vendor/zombiezen.com/go/capnproto2/readlimit.go

@@ -0,0 +1,38 @@
+package capnp
+
+import "sync/atomic"
+
+// A ReadLimiter tracks the number of bytes read from a message in order
+// to avoid amplification attacks as detailed in
+// https://capnproto.org/encoding.html#amplification-attack.
+// It is safe to use from multiple goroutines.
+type ReadLimiter struct {
+	limit uint64
+}
+
+// canRead reports whether the amount of bytes can be stored safely.
+func (rl *ReadLimiter) canRead(sz Size) bool {
+	for {
+		curr := atomic.LoadUint64(&rl.limit)
+		ok := curr >= uint64(sz)
+		var new uint64
+		if ok {
+			new = curr - uint64(sz)
+		} else {
+			new = 0
+		}
+		if atomic.CompareAndSwapUint64(&rl.limit, curr, new) {
+			return ok
+		}
+	}
+}
+
+// Reset sets the number of bytes allowed to be read.
+func (rl *ReadLimiter) Reset(limit uint64) {
+	atomic.StoreUint64(&rl.limit, limit)
+}
+
+// Unread increases the limit by sz.
+func (rl *ReadLimiter) Unread(sz Size) {
+	atomic.AddUint64(&rl.limit, uint64(sz))
+}