Fix panic in shouldRetry type assertion and remove deprecated rand.Seed

- Use comma-ok idiom in shouldRetry to handle non-*Err error types safely
- Remove deprecated rand.Seed call that was re-seeding on every backoff
- Add test coverage for shouldRetry with various error types

The shouldRetry function previously used a direct type assertion
that would panic if passed an error not of type *Err. While this
function is internal and typically receives *Err values, the type
assertion should be safe to handle unexpected inputs.

The rand.Seed call has been deprecated since Go 1.20 as the global
random number generator is now automatically seeded. Additionally,
calling Seed on every retry attempt was wasteful.

Author: Nepomuk5665

retries/retries.go

@@ -77,7 +77,6 @@ func backoff(attempt int) time.Duration {
 		wait = maxWait
 	}
 	// add some random jitter
-	rand.Seed(time.Now().UnixNano())
 	jitter := rand.Intn(int(maxJitter)-int(minJitter)+1) + int(minJitter)
 	wait += time.Duration(jitter)
 	return wait
@@ -233,8 +232,8 @@ func shouldRetry(err error) bool {
 	if err == nil {
 		return false
 	}
-	e := err.(*Err)
-	if e == nil {
+	e, ok := err.(*Err)
+	if !ok || e == nil {
 		return false
 	}
 	return !e.Halt

retries/retries_test.go

@@ -127,6 +127,23 @@ func TestRetriesNegativeTimeout(t *testing.T) {
 	assert.Equal(t, r.config.Timeout(), time.Duration(-1))
 }
 
+func TestShouldRetryWithNonErrType(t *testing.T) {
+	// Test that shouldRetry handles non-*Err types gracefully without panicking
+	regularError := errors.New("a regular error")
+	assert.False(t, shouldRetry(regularError))
+
+	// Test with nil
+	assert.False(t, shouldRetry(nil))
+
+	// Test with *Err that should not retry
+	haltErr := Halt(errors.New("halt error"))
+	assert.False(t, shouldRetry(haltErr))
+
+	// Test with *Err that should retry
+	continueErr := Continue(errors.New("continue error"))
+	assert.True(t, shouldRetry(continueErr))
+}
+
 func ExampleNew() {
 	// Default retries for 20 minutes on any error
 	New[int]()