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

@@ -5,45 +5,81 @@
 package mail
 
 import (
+	"crypto/rand"
+	"errors"
 	"strings"
 	"testing"
 )
 
 // TestRandomStringSecure tests the randomStringSecure method
 func TestRandomStringSecure(t *testing.T) {
-	tt := []struct {
-		testName     string
-		length       int
-		mustNotMatch string
-	}{
-		{"20 chars", 20, "'"},
-		{"100 chars", 100, "'"},
-		{"1000 chars", 1000, "'"},
-	}
+	t.Run("randomStringSecure with varying length", func(t *testing.T) {
+		tt := []struct {
+			testName     string
+			length       int
+			mustNotMatch string
+		}{
+			{"20 chars", 20, "'"},
+			{"100 chars", 100, "'"},
+			{"1000 chars", 1000, "'"},
+		}
 
-	for _, tc := range tt {
-		t.Run(tc.testName, func(t *testing.T) {
-			rs, err := randomStringSecure(tc.length)
-			if err != nil {
-				t.Errorf("random string generation failed: %s", err)
-			}
-			if strings.Contains(rs, tc.mustNotMatch) {
-				t.Errorf("random string contains unexpected character. got: %s, not-expected: %s",
-					rs, tc.mustNotMatch)
-			}
-			if len(rs) != tc.length {
-				t.Errorf("random string length does not match. expected: %d, got: %d", tc.length, len(rs))
-			}
-		})
-	}
+		for _, tc := range tt {
+			t.Run(tc.testName, func(t *testing.T) {
+				rs, err := randomStringSecure(tc.length)
+				if err != nil {
+					t.Errorf("random string generation failed: %s", err)
+				}
+				if strings.Contains(rs, tc.mustNotMatch) {
+					t.Errorf("random string contains unexpected character. got: %s, not-expected: %s",
+						rs, tc.mustNotMatch)
+				}
+				if len(rs) != tc.length {
+					t.Errorf("random string length does not match. expected: %d, got: %d", tc.length, len(rs))
+				}
+			})
+		}
+	})
+	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")
+}