servewaf: each decision heuristic is its own module (closes #195)

Each check servewaf runs on a request is a type in its own file behind
one interface, heuristic, and is registered in one of four ordered lists
in heuristics.go. Order and behaviour are unchanged.

Every refusal or ban now logs heuristic, the name of the heuristic that
made it, and heuristic_figures, the figures it acted on. Both are in
docs/request-log.md, and SPEC.md names each heuristic at its step.

Model: opus-5-5

internal/servewaf/bans.go

@@ -1,7 +1,10 @@
 package servewaf
 
+// The heuristic active_ban, and the code that makes bans, which several
+// heuristics share. active_ban reads SWWAF_MODE; the bans are the ban ledger's.
+
 import (
-	"net/http"
+	"context"
 	"net/netip"
 	"time"
 
@@ -18,11 +21,15 @@ func (rq *request) banResponse(action string) *refusal {
 	return &refusal{status: rq.h.config.BanResponse, action: action}
 }
 
-// banned reports whether a ban on a netblock the client is in covers the
-// request at now, and notes for the log line when that ban ends. A
-// request that makes the ban permanent, or in observe mode would have,
-// raises the alert for it.
-func (rq *request) banned(now time.Time) bool {
+// activeBan refuses a client a ban covers: a ban on a netblock the client is
+// in, in force at now. It notes for the log line when that ban ends. A request
+// that makes the ban permanent, or in observe mode would have, raises the alert
+// for it. Its figures are the ban's netblock and when it ends.
+type activeBan struct{}
+
+func (activeBan) name() string { return "active_ban" }
+
+func (activeBan) check(_ context.Context, rq *request, now time.Time) *verdict {
 	check := rq.h.ledger.Check
 	if rq.h.config.Observe {
 		check = rq.h.ledger.Find // the ban refuses nothing, and stays as it is
@@ -38,85 +45,22 @@ func (rq *request) banned(now time.Time) bool {
 		rq.alertBan(ban)
 	}
 
-	return banned
-}
-
-// limitBroken counts the request for the rate limits at now, notes the
-// client's counts for the log line, and reports whether the request takes
-// the client over a rate limit, as its limit percentage lowers it, which
-// breaks it.
-func (rq *request) limitBroken(now time.Time) bool {
-	counts, hit, over := rq.h.limiter.Count(rq.h.clientGroup(rq.client), now,
-		rq.limitPercent.percent)
-	rq.line.Counts = counts
-
-	if over {
-		rq.banForLimit(now, hit, rq.h.config.BanResponse)
+	if !banned {
+		return nil
 	}
 
-	return over
-}
-
-// countBytes counts the request's bytes, as countedBytes gives them, for
-// the byte limits, once its report has come, and notes the client's byte totals
-// for the log line; its requests stay there as the rate limits counted them.
-// Only an allowed request has them counted, and only one the rate limits
-// counted; in observe mode, not one that enforce mode would have refused. Bytes
-// that take the client over a byte limit, as its limit percentage for the byte
-// limits lowers it, break it; the response was passed on whole.
-func (rq *request) countBytes() {
-	if !rq.counted || rq.line.WouldAction != "" {
-		return
-	}
-
-	now := rq.h.now()
-
-	counts, hit, over := rq.h.limiter.CountBytes(rq.h.clientGroup(rq.client), now,
-		rq.countedBytes(), rq.bytesPercent.percent)
-	rq.line.Counts.MinuteBytes = counts.MinuteBytes
-	rq.line.Counts.HourBytes = counts.HourBytes
-	rq.line.Counts.DayBytes = counts.DayBytes
-
-	if over {
-		rq.banForLimit(now, hit, rq.out.status)
+	return &verdict{
+		action: requestlog.ActionBanned,
+		figures: map[string]any{
+			"netblock": ban.Netblock.String(), "ban_expires": rq.line.BanExpires,
+		},
 	}
 }
 
-// countRefusal counts the request for the error burst once it has been
-// answered, if servewaf refused it after a rule file match, a trap path or a
-// Core Rule Set match, and in observe mode if enforce mode would have: more than
-// SWWAF_ERROR_BURST_THRESHOLD such refusals of the client within a minute
-// break a limit. A client in SWWAF_ALLOW_NETS, which the checks skip, is
-// not counted, and nothing is while the threshold is off.
-func (rq *request) countRefusal() {
-	cfg := rq.h.config
-	if cfg.ErrorBurstThreshold == 0 {
-		return
-	}
-
-	if !rq.attack && !rq.ruleBlocked && !rq.wafBlocked {
-		return
-	}
-
-	now := rq.h.now()
-
-	hit, over := rq.h.limiter.CountRefusal(rq.h.clientGroup(rq.client), now,
-		cfg.ErrorBurstThreshold)
-	if !over {
-		return
-	}
-
-	// What the client was sent, or in observe mode would have been.
-	status := rq.out.status
-
-	switch rq.line.WouldAction {
-	case requestlog.ActionRuleBlocked, requestlog.ActionWAFBlocked:
-		status = http.StatusForbidden
-	case requestlog.ActionBanned:
-		status = cfg.BanResponse
-	}
-
-	rq.banForLimit(now, hit, status)
+// limitFigures are the figures of a heuristic that broke hit, a limit: its
+// window, the count and the limit.
+func limitFigures(hit ratelimit.Hit) map[string]any {
+	return map[string]any{"window": hit.Window, "count": hit.Count, figureLimit: hit.Limit}
 }
 
 // countedBytes returns the request's bytes, once it has ended, as the