add support for tor (#369)

irc/connection_limits/tor.go

@@ -0,0 +1,55 @@
+// Copyright (c) 2019 Shivaram Lingamneni <slingamn@cs.stanford.edu>
+// released under the MIT license
+
+package connection_limits
+
+import (
+	"errors"
+	"sync"
+	"time"
+)
+
+var (
+	ErrLimitExceeded    = errors.New("too many concurrent connections")
+	ErrThrottleExceeded = errors.New("too many recent connection attempts")
+)
+
+// TorLimiter is a combined limiter and throttler for use on connections
+// proxied from a Tor hidden service (so we don't have meaningful IPs,
+// a notion of CIDR width, etc.)
+type TorLimiter struct {
+	sync.Mutex
+
+	numConnections int
+	maxConnections int
+	throttle       GenericThrottle
+}
+
+func (tl *TorLimiter) Configure(maxConnections int, duration time.Duration, maxConnectionsPerDuration int) {
+	tl.Lock()
+	defer tl.Unlock()
+	tl.maxConnections = maxConnections
+	tl.throttle.Duration = duration
+	tl.throttle.Limit = maxConnectionsPerDuration
+}
+
+func (tl *TorLimiter) AddClient() error {
+	tl.Lock()
+	defer tl.Unlock()
+
+	if tl.maxConnections != 0 && tl.maxConnections <= tl.numConnections {
+		return ErrLimitExceeded
+	}
+	throttled, _ := tl.throttle.Touch()
+	if throttled {
+		return ErrThrottleExceeded
+	}
+	tl.numConnections += 1
+	return nil
+}
+
+func (tl *TorLimiter) RemoveClient() {
+	tl.Lock()
+	tl.numConnections -= 1
+	tl.Unlock()
+}