Refactor and expand random string tests

Refactored the test for `randomStringSecure` to better organize test cases using subtests. Added new test cases to check failures with a broken rand.Reader, improving test coverage and robustness.

random_test.go

@@ -5,12 +5,15 @@
 package mail
 
 import (
+	"crypto/rand"
+	"errors"
 	"strings"
 	"testing"
 )
 
 // TestRandomStringSecure tests the randomStringSecure method
 func TestRandomStringSecure(t *testing.T) {
+	t.Run("randomStringSecure with varying length", func(t *testing.T) {
 		tt := []struct {
 			testName     string
 			length       int
@@ -36,14 +39,47 @@ func TestRandomStringSecure(t *testing.T) {
 				}
 			})
 		}
+	})
+	t.Run("randomStringSecure fails on broken rand Reader (first read)", func(t *testing.T) {
+		defaultRandReader := rand.Reader
+		t.Cleanup(func() { rand.Reader = defaultRandReader })
+		rand.Reader = &randReader{failon: 1}
+		if _, err := randomStringSecure(22); err == nil {
+			t.Fatalf("expected failure on broken rand Reader")
+		}
+	})
+	t.Run("randomStringSecure fails on broken rand Reader (second read)", func(t *testing.T) {
+		defaultRandReader := rand.Reader
+		t.Cleanup(func() { rand.Reader = defaultRandReader })
+		rand.Reader = &randReader{failon: 0}
+		if _, err := randomStringSecure(22); err == nil {
+			t.Fatalf("expected failure on broken rand Reader")
+		}
+	})
 }
 
 func BenchmarkGenerator_RandomStringSecure(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
-		_, err := randomStringSecure(22)
+		_, err := randomStringSecure(10)
 		if err != nil {
 			b.Errorf("RandomStringFromCharRange() failed: %s", err)
 		}
 	}
 }
+
+// randReader is type that satisfies the io.Reader interface. It can fail on a specific read
+// operations and is therefore useful to test consecutive reads with errors
+type randReader struct {
+	failon uint8
+	call   uint8
+}
+
+// Read implements the io.Reader interface for the randReader type
+func (r *randReader) Read(p []byte) (int, error) {
+	if r.call == r.failon {
+		r.call++
+		return len(p), nil
+	}
+	return 0, errors.New("broken reader")
+}