Rate limiting (#1385)

* Initial rate limiting

* Move rate limiting to client API

* Update rate limits to hopefully be self-cleaning

* Use X-Forwarded-For, add comments

* Reduce rate limit threshold

* Tweak interval

* Configurable backoff

* Review comments, set cleanup interval to 30 seconds

* Allow generate-config to produce sane CI config

* Fix Complement dockerfile

clientapi/routing/rate_limiting.go

@@ -0,0 +1,99 @@
+package routing
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/util"
+)
+
+type rateLimits struct {
+	limits           map[string]chan struct{}
+	limitsMutex      sync.RWMutex
+	enabled          bool
+	requestThreshold int64
+	cooloffDuration  time.Duration
+}
+
+func newRateLimits(cfg *config.RateLimiting) *rateLimits {
+	l := &rateLimits{
+		limits:           make(map[string]chan struct{}),
+		enabled:          cfg.Enabled,
+		requestThreshold: cfg.Threshold,
+		cooloffDuration:  time.Duration(cfg.CooloffMS) * time.Millisecond,
+	}
+	if l.enabled {
+		go l.clean()
+	}
+	return l
+}
+
+func (l *rateLimits) clean() {
+	for {
+		// On a 30 second interval, we'll take an exclusive write
+		// lock of the entire map and see if any of the channels are
+		// empty. If they are then we will close and delete them,
+		// freeing up memory.
+		time.Sleep(time.Second * 30)
+		l.limitsMutex.Lock()
+		for k, c := range l.limits {
+			if len(c) == 0 {
+				close(c)
+				delete(l.limits, k)
+			}
+		}
+		l.limitsMutex.Unlock()
+	}
+}
+
+func (l *rateLimits) rateLimit(req *http.Request) *util.JSONResponse {
+	// If rate limiting is disabled then do nothing.
+	if !l.enabled {
+		return nil
+	}
+
+	// Lock the map long enough to check for rate limiting. We hold it
+	// for longer here than we really need to but it makes sure that we
+	// also don't conflict with the cleaner goroutine which might clean
+	// up a channel after we have retrieved it otherwise.
+	l.limitsMutex.RLock()
+	defer l.limitsMutex.RUnlock()
+
+	// First of all, work out if X-Forwarded-For was sent to us. If not
+	// then we'll just use the IP address of the caller.
+	caller := req.RemoteAddr
+	if forwardedFor := req.Header.Get("X-Forwarded-For"); forwardedFor != "" {
+		caller = forwardedFor
+	}
+
+	// Look up the caller's channel, if they have one. If they don't then
+	// let's create one.
+	rateLimit, ok := l.limits[caller]
+	if !ok {
+		l.limits[caller] = make(chan struct{}, l.requestThreshold)
+		rateLimit = l.limits[caller]
+	}
+
+	// Check if the user has got free resource slots for this request.
+	// If they don't then we'll return an error.
+	select {
+	case rateLimit <- struct{}{}:
+	default:
+		// We hit the rate limit. Tell the client to back off.
+		return &util.JSONResponse{
+			Code: http.StatusTooManyRequests,
+			JSON: jsonerror.LimitExceeded("You are sending too many requests too quickly!", l.cooloffDuration.Milliseconds()),
+		}
+	}
+
+	// After the time interval, drain a resource from the rate limiting
+	// channel. This will free up space in the channel for new requests.
+	go func() {
+		<-time.After(l.cooloffDuration)
+		<-rateLimit
+	}()
+	return nil
+}