Use math/rand global random source and deprecate PRNG global variable

rehttp.go

@@ -64,6 +64,11 @@ import (
 // PRNG is the *math.Rand value to use to add jitter to the backoff
 // algorithm used in ExpJitterDelay. By default it uses a *rand.Rand
 // initialized with a source based on the current time in nanoseconds.
+//
+// Deprecated: math/rand sources can panic if used concurrently without
+// synchronization. PRNG is no longer used by this package and its use
+// outside this package is discouraged.
+// https://github.com/PuerkitoBio/rehttp/issues/12
 var PRNG = rand.New(rand.NewSource(time.Now().UnixNano()))
 
 // terribly named interface to detect errors that support Temporary.
@@ -264,7 +269,24 @@ func ExpJitterDelay(base, max time.Duration) DelayFn {
 		exp := math.Pow(2, float64(attempt.Index))
 		top := float64(base) * exp
 		return time.Duration(
-			PRNG.Int63n(int64(math.Min(float64(max), top))),
+			rand.Int63n(int64(math.Min(float64(max), top))),
+		)
+	}
+}
+
+// ExpJitterDelayWithRand returns a DelayFn that returns a delay
+// between 0 and base * 2^attempt capped at max (an exponential
+// backoff delay with jitter). The provided generator will be used
+// in place of rand.Int63n.
+//
+// See the full jitter algorithm in:
+// http://www.awsarchitectureblog.com/2015/03/backoff.html
+func ExpJitterDelayWithRand(base, max time.Duration, generator func(n int64) int64) DelayFn {
+	return func(attempt Attempt) time.Duration {
+		exp := math.Pow(2, float64(attempt.Index))
+		top := float64(base) * exp
+		return time.Duration(
+			generator(int64(math.Min(float64(max), top))),
 		)
 	}
 }

rehttp_delayfn_test.go

@@ -27,3 +27,14 @@ func TestExpJitterDelay(t *testing.T) {
 		assert.True(t, delay <= actual, "%d: %s > %s", i, delay, actual)
 	}
 }
+
+func TestExpJitterDelayWithRand(t *testing.T) {
+	fn := ExpJitterDelayWithRand(time.Second, 5*time.Second, func(n int64) int64 { return 999_999_999 % n })
+	for i := 0; i < 10; i++ {
+		delay := fn(Attempt{Index: i})
+		top := math.Pow(2, float64(i)) * float64(time.Second)
+		actual := time.Duration(math.Min(float64(5*time.Second), top))
+		assert.True(t, delay > 0, "%d: %s <= 0", i, delay)
+		assert.True(t, delay <= actual, "%d: %s > %s", i, delay, actual)
+	}
+}

rng_pre120.go

@@ -0,0 +1,15 @@
+//go:build !go1.20
+// +build !go1.20
+
+package rehttp
+
+import (
+	"math/rand"
+	"time"
+)
+
+// Only seed the global random source on versions prior to 1.20 since
+// it's pre-seeded starting in 1.20
+func init() {
+	rand.Seed(time.Now().UnixNano())
+}