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

internal/proxy/client.go

@@ -53,8 +53,9 @@ const ipv6GroupPrefix = 64
 
 // clientGroup is the client a request is counted toward: its IPv4
 // address, or the /64 its IPv6 address is in, since one abuser usually
-// holds a whole /64.
+// holds a whole /64. An IPv4 address in IPv6 form counts as IPv4.
 func clientGroup(addr netip.Addr) netip.Prefix {
+	addr = addr.Unmap()
 	if addr.Is6() {
 		return netip.PrefixFrom(addr, ipv6GroupPrefix).Masked()
 	}

internal/proxy/ratelimits_test.go

@@ -21,18 +21,23 @@ func TestRateLimitRefusesWith429BeforeTheApp(t *testing.T) {
 		rateLimitPerMinute: "1",
 	})
 
+	const otherClient = "203.0.113.10"
+
 	// With a limit of one request a minute, a client's second request is
-	// refused. A client is one IPv4 address, or one IPv6 /64.
+	// refused. A client is one IPv4 address, or one IPv6 /64; an IPv4
+	// address in IPv6 form is that IPv4 address.
 	requests := []struct {
-		client string
+		client string // as X-Forwarded-For names it
+		logged string // as the log line's client_ip names it
 		want   int
 	}{
-		{client, http.StatusOK},
-		{client, http.StatusTooManyRequests},
-		{"203.0.113.10", http.StatusOK},
-		{"2001:db8::1", http.StatusOK},
-		{"2001:db8::8000:0:0:1", http.StatusTooManyRequests},
-		{"2001:db8:0:1::1", http.StatusOK},
+		{client, client, http.StatusOK},
+		{client, client, http.StatusTooManyRequests},
+		{otherClient, otherClient, http.StatusOK},
+		{"::ffff:" + otherClient, otherClient, http.StatusTooManyRequests},
+		{"2001:db8::1", "2001:db8::1", http.StatusOK},
+		{"2001:db8::8000:0:0:1", "2001:db8::8000:0:0:1", http.StatusTooManyRequests},
+		{"2001:db8:0:1::1", "2001:db8:0:1::1", http.StatusOK},
 	}
 
 	for i, sent := range requests {
@@ -41,8 +46,8 @@ func TestRateLimitRefusesWith429BeforeTheApp(t *testing.T) {
 		wantStatus(t, do(t, req), sent.want)
 
 		line := out.requestLines(t, i+1)[i]
-		if line.ClientIP != sent.client {
-			t.Errorf("log line has client_ip %q, want %q", line.ClientIP, sent.client)
+		if line.ClientIP != sent.logged {
+			t.Errorf("log line has client_ip %q, want %q", line.ClientIP, sent.logged)
 		}
 
 		if sent.want == http.StatusOK {