Per-client request rate limits over a minute, an hour and a day (closes #43)

Each client, one IPv4 address or one IPv6 /64, is counted in two buckets
per window, the earlier weighted by how much of it the window covers; at
most 20,000 clients are kept, least recently seen dropped first. A
request over SWWAF_RATE_LIMIT_PER_MINUTE, _HOUR or _DAY (1000, 10000,
50000, or off) gets 429 before reaching the app. Refused requests count,
413s included. A clock set back over a second behind a bucket's start
restarts that window. The log line gains limit_hit and the action
rate_limited.

Deviation from SPEC.md, per the issue: the 20,000 bound and /64 are fixed.
Judgement call: golang-lru/v2 holds the table; httprate does not count refused requests.
Deviation: go.mod and go.sum hand-written; no make target tidies them.

Model: opus-5-5

README.md

@@ -412,8 +412,9 @@ refusal comes with `SWWAF_ALLOW_NETS` in milestone 3 or later.
   the checks, passes the request to the app and the answer back with the
   standard library's `httputil.ReverseProxy` within the timeouts and size
   limits, and writes the request's log line. Its `check` method is where a
-  request is refused before anything reaches the app: for a rate limit, and,
-  with the rest of milestone 2, for the country lists.
+  request is refused before anything reaches the app: for a rate limit, for an
+  announced body over the size limit, and, with the rest of milestone 2, for the
+  country lists.
 - `internal/ratelimit`: counts each client's requests and tells when one takes
   it over a rate limit.
 - `internal/requestlog`: the lines on stdout: the request log line and the

internal/proxy/ratelimits_test.go

@@ -31,7 +31,7 @@ func TestRateLimitRefusesWith429BeforeTheApp(t *testing.T) {
 		{client, http.StatusTooManyRequests},
 		{"203.0.113.10", http.StatusOK},
 		{"2001:db8::1", http.StatusOK},
-		{"2001:db8::ffff:2", http.StatusTooManyRequests},
+		{"2001:db8::8000:0:0:1", http.StatusTooManyRequests},
 		{"2001:db8:0:1::1", http.StatusOK},
 	}
 

internal/ratelimit/ratelimit.go

@@ -101,9 +101,16 @@ type buckets struct {
 // under way, and those in the bucket before it weighted by how much of
 // that bucket the window still covers.
 //
-// Concurrent requests can be counted out of order, so now can be before
-// the bucket under way began; such a request is counted in that bucket.
+// Concurrent requests can be counted out of order, so now can be a moment
+// before the bucket under way began; such a request is counted in that
+// bucket. A request dated more than a second before it means the clock
+// was set back, and the buckets start afresh: otherwise the bucket before
+// would keep its full weight until the clock caught up.
 func (b *buckets) add(now time.Time, length time.Duration) float64 {
+	if now.Before(b.start.Add(-time.Second)) {
+		*b = buckets{}
+	}
+
 	start := now.Truncate(length)
 	if start.After(b.start) {
 		if start.Equal(b.start.Add(length)) {

internal/ratelimit/ratelimit_test.go

@@ -87,6 +87,48 @@ func TestRefusedRequestsCount(t *testing.T) {
 	wantCount(t, limiter, within, later, "")
 }
 
+func TestRequestCountedLateGoesInTheBucketUnderWay(t *testing.T) {
+	t.Parallel()
+
+	limiter := ratelimit.New(ratelimit.Limits{PerMinute: limit})
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	for range limit {
+		wantCount(t, limiter, client, start, "")
+	}
+
+	// A concurrent request dated a moment before the bucket under way, but
+	// counted after it began, is counted in it: 3 + 1 is over the limit.
+	wantCount(t, limiter, client, start.Add(-time.Millisecond), minute)
+}
+
+func TestClockSetBackStartsTheBucketsAfresh(t *testing.T) {
+	t.Parallel()
+
+	limiter := ratelimit.New(ratelimit.Limits{PerHour: limit})
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	for range limit {
+		wantCount(t, limiter, client, start, "")
+	}
+
+	// Half an hour into the next bucket: 3 / 2 + 1 is within the limit.
+	wantCount(t, limiter, client, start.Add(time.Hour+time.Hour/2), "")
+
+	// The clock is set back an hour. Counted in the bucket under way, the
+	// next request would find the bucket before it at full weight, 3 + 2,
+	// over the limit until the clock caught up. The buckets start afresh
+	// instead, and the client is refused only past the limit again.
+	setBack := start.Add(time.Hour / 2)
+	for range limit {
+		wantCount(t, limiter, client, setBack, "")
+	}
+
+	wantCount(t, limiter, client, setBack, hour)
+}
+
 func TestKeepsAtMost20000ClientsDroppingTheLeastRecentlySeen(t *testing.T) {
 	t.Parallel()