feat(lib/policy/expressions): add system load average to bot expression inputs

This lets Anubis dynamically react to system load in order to
increase and decrease the required level of scrutiny. High load? More
scrutiny required. Low load? Less scrutiny required.

lib/policy/celchecker.go

@@ -73,6 +73,12 @@ func (cr *CELRequest) ResolveName(name string) (any, bool) {
 		return expressions.URLValues{Values: cr.URL.Query()}, true
 	case "headers":
 		return expressions.HTTPHeaders{Header: cr.Header}, true
+	case "load_1m":
+		return expressions.Load1(), true
+	case "load_5m":
+		return expressions.Load5(), true
+	case "load_15m":
+		return expressions.Load15(), true
 	default:
 		return nil, false
 	}

lib/policy/expressions/environment.go

@@ -23,6 +23,9 @@ func BotEnvironment() (*cel.Env, error) {
 		cel.Variable("path", cel.StringType),
 		cel.Variable("query", cel.MapType(cel.StringType, cel.StringType)),
 		cel.Variable("headers", cel.MapType(cel.StringType, cel.StringType)),
+		cel.Variable("load_1m", cel.DoubleType),
+		cel.Variable("load_5m", cel.DoubleType),
+		cel.Variable("load_15m", cel.DoubleType),
 	)
 }
 

lib/policy/expressions/loadavg.go

@@ -0,0 +1,69 @@
+package expressions
+
+import (
+	"context"
+	"log/slog"
+	"sync"
+	"time"
+
+	"github.com/shirou/gopsutil/v4/load"
+)
+
+type loadAvg struct {
+	lock sync.RWMutex
+	data *load.AvgStat
+}
+
+func (l *loadAvg) updateThread(ctx context.Context) {
+	ticker := time.NewTicker(15 * time.Second)
+	defer ticker.Stop()
+
+	l.update()
+
+	for {
+		select {
+		case <-ticker.C:
+			l.update()
+		case <-ctx.Done():
+			return
+		}
+	}
+}
+
+func (l *loadAvg) update() {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	var err error
+	l.data, err = load.Avg()
+	if err != nil {
+		slog.Debug("can't get load average", "err", err)
+	}
+}
+
+var (
+	globalLoadAvg *loadAvg
+)
+
+func init() {
+	globalLoadAvg = &loadAvg{}
+	go globalLoadAvg.updateThread(context.Background())
+}
+
+func Load1() float64 {
+	globalLoadAvg.lock.RLock()
+	defer globalLoadAvg.lock.RUnlock()
+	return globalLoadAvg.data.Load1
+}
+
+func Load5() float64 {
+	globalLoadAvg.lock.RLock()
+	defer globalLoadAvg.lock.RUnlock()
+	return globalLoadAvg.data.Load5
+}
+
+func Load15() float64 {
+	globalLoadAvg.lock.RLock()
+	defer globalLoadAvg.lock.RUnlock()
+	return globalLoadAvg.data.Load15
+}