switch to math/rand/v2 in most places

this allows removing some ugly instantiations of an rng based on the current
time.

Intn is now IntN for our concurrency-safe prng wrapper to match the randv2 api.

v2 exists since go1.22, which we already require.

dmarc/dmarc.go

@@ -15,7 +15,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
-	mathrand "math/rand"
+	mathrand2 "math/rand/v2"
 	"time"
 
 	"github.com/mjl-/mox/dkim"
@@ -257,7 +257,7 @@ func Verify(ctx context.Context, elog *slog.Logger, resolver dns.Resolver, msgFr
 
 	// Record can request sampling of messages to apply policy.
 	// See ../rfc/7489:1432
-	useResult = !applyRandomPercentage || record.Percentage == 100 || mathrand.Intn(100) < record.Percentage
+	useResult = !applyRandomPercentage || record.Percentage == 100 || mathrand2.IntN(100) < record.Percentage
 
 	// We treat "quarantine" and "reject" the same. Thus, we also don't "downgrade"
 	// from reject to quarantine if this message was sampled out.

dmarcdb/eval.go

@@ -252,7 +252,7 @@ var jitterRand = mox.NewPseudoRand()
 // Jitter so we don't cause load at exactly whole hours, other processes may
 // already be doing that.
 var jitteredTimeUntil = func(t time.Time) time.Duration {
-	return time.Until(t.Add(time.Duration(30+jitterRand.Intn(60)) * time.Second))
+	return time.Until(t.Add(time.Duration(30+jitterRand.IntN(60)) * time.Second))
 }
 
 // Start launches a goroutine that wakes up at each whole hour (plus jitter) and

junk.go

@@ -214,12 +214,12 @@ messages are shuffled, with optional random seed.`
 	hamFiles := listDir(hamDir)
 	spamFiles := listDir(spamDir)
 
-	var rand *mathrand.Rand
+	var seed int64
 	if a.seed {
-		rand = mathrand.New(mathrand.NewSource(time.Now().UnixMilli()))
-	} else {
-		rand = mathrand.New(mathrand.NewSource(0))
+		seed = time.Now().UnixMilli()
 	}
+	// Still at math/rand (v1 instead of v2) for potential comparison to earlier test results.
+	rand := mathrand.New(mathrand.NewSource(seed))
 
 	shuffle := func(l []string) {
 		count := len(l)

mox-/rand.go

@@ -4,19 +4,23 @@ import (
 	cryptorand "crypto/rand"
 	"encoding/binary"
 	"fmt"
-	mathrand "math/rand"
+	mathrand2 "math/rand/v2"
 	"sync"
 )
 
 type rand struct {
-	rand *mathrand.Rand
+	rand *mathrand2.Rand
 	sync.Mutex
 }
 
 // NewPseudoRand returns a new PRNG seeded with random bytes from crypto/rand. Its
 // functions can be called concurrently.
 func NewPseudoRand() *rand {
-	return &rand{rand: mathrand.New(mathrand.NewSource(CryptoRandInt()))}
+	var seed [32]byte
+	if _, err := cryptorand.Read(seed[:]); err != nil {
+		panic(err)
+	}
+	return &rand{rand: mathrand2.New(mathrand2.NewChaCha8(seed))}
 }
 
 func (r *rand) Float64() float64 {
@@ -25,16 +29,10 @@ func (r *rand) Float64() float64 {
 	return r.rand.Float64()
 }
 
-func (r *rand) Intn(n int) int {
+func (r *rand) IntN(n int) int {
 	r.Lock()
 	defer r.Unlock()
-	return r.rand.Intn(n)
-}
-
-func (r *rand) Read(buf []byte) (int, error) {
-	r.Lock()
-	defer r.Unlock()
-	return r.rand.Read(buf)
+	return r.rand.IntN(n)
 }
 
 // CryptoRandInt returns a cryptographically random number.

mtastsdb/refresh.go

@@ -5,7 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
-	mathrand "math/rand"
+	mathrand2 "math/rand/v2"
 	"runtime/debug"
 	"time"
 
@@ -71,12 +71,11 @@ func refresh1(ctx context.Context, log mlog.Log, resolver dns.Resolver, sleep fu
 	}
 
 	// Randomize list.
-	rand := mathrand.New(mathrand.NewSource(time.Now().UnixNano()))
 	for i := range prs {
 		if i == 0 {
 			continue
 		}
-		j := rand.Intn(i + 1)
+		j := mathrand2.IntN(i + 1)
 		prs[i], prs[j] = prs[j], prs[i]
 	}
 
@@ -87,7 +86,7 @@ func refresh1(ctx context.Context, log mlog.Log, resolver dns.Resolver, sleep fu
 		go refreshDomain(ctx, log, DB, resolver, pr)
 		if i < len(prs)-1 {
 			interval := 3 * int64(time.Hour) / int64(len(prs)-1)
-			extra := time.Duration(rand.Int63n(interval) - interval/2)
+			extra := time.Duration(mathrand2.Int64N(interval) - interval/2)
 			next := start.Add(time.Duration(int64(i+1)*interval) + extra)
 			d := next.Sub(timeNow())
 			if d > 0 {

queue/hook.go

@@ -1119,7 +1119,7 @@ func hookDeliver(log mlog.Log, h Hook) {
 	} else {
 		backoff = hookIntervals[len(hookIntervals)-1] * time.Duration(2)
 	}
-	backoff += time.Duration(jitter.Intn(200)-100) * backoff / 10000
+	backoff += time.Duration(jitter.IntN(200)-100) * backoff / 10000
 	h.Attempts++
 	now := time.Now()
 	h.NextAttempt = now.Add(backoff)

queue/queue.go

@@ -1370,7 +1370,7 @@ func deliver(log mlog.Log, resolver dns.Resolver, m0 Msg) {
 			return fmt.Errorf("get message to be delivered: %v", err)
 		}
 
-		backoff = time.Duration(7*60+30+jitter.Intn(10)-5) * time.Second
+		backoff = time.Duration(7*60+30+jitter.IntN(10)-5) * time.Second
 		for i := 0; i < m0.Attempts; i++ {
 			backoff *= time.Duration(2)
 		}

tlsrptsend/send.go

@@ -73,7 +73,7 @@ var jitterRand = mox.NewPseudoRand()
 // Jitter so we don't cause load at exactly midnight, other processes may
 // already be doing that.
 var jitteredTimeUntil = func(t time.Time) time.Duration {
-	return time.Until(t.Add(time.Duration(240+jitterRand.Intn(120)) * time.Second))
+	return time.Until(t.Add(time.Duration(240+jitterRand.IntN(120)) * time.Second))
 }
 
 // Start launches a goroutine that wakes up just after 00:00 UTC to send TLSRPT

webmail/eventwriter.go

@@ -7,7 +7,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
-	mathrand "math/rand"
+	mathrand2 "math/rand/v2"
 	"net/http"
 	"runtime/debug"
 	"sync"
@@ -71,9 +71,6 @@ func (ew *eventWriter) write(name string, v any) error {
 	return ew.out.Flush()
 }
 
-// For random wait between min and max delay.
-var waitGen = mathrand.New(mathrand.NewSource(time.Now().UnixNano()))
-
 // Schedule an event for writing to the connection. If events get a delay, this
 // function still returns immediately.
 func (ew *eventWriter) xsendEvent(ctx context.Context, log mlog.Log, name string, v any) {
@@ -136,7 +133,7 @@ func (ew *eventWriter) xsendEvent(ctx context.Context, log mlog.Log, name string
 	}
 	// If we have an events channel, we have a goroutine that write the events, delayed.
 	if ew.events != nil {
-		wait := ew.waitMin + time.Duration(waitGen.Intn(1000))*(ew.waitMax-ew.waitMin)/1000
+		wait := ew.waitMin + time.Duration(mathrand2.IntN(1000))*(ew.waitMax-ew.waitMin)/1000
 		when := time.Now().Add(wait)
 		ew.events <- struct {
 			name string