random: speed up String function for generating larger blocks

cmd/test/makefiles/makefiles.go

@@ -224,7 +224,7 @@ func (r *chargenReader) Read(p []byte) (n int, err error) {
 func fileName() (name string) {
 	for {
 		length := randSource.Intn(maxFileNameLength-minFileNameLength) + minFileNameLength
-		name = random.StringFn(length, randSource.Intn)
+		name = random.StringFn(length, randSource)
 		if _, found := fileNames[name]; !found {
 			break
 		}

lib/random/random.go

@@ -4,28 +4,35 @@ package random
 import (
 	cryptorand "crypto/rand"
 	"encoding/base64"
-	"encoding/binary"
 	"fmt"
-	mathrand "math/rand"
+	"io"
 )
 
 // StringFn create a random string for test purposes using the random
 // number generator function passed in.
 //
 // Do not use these for passwords.
-func StringFn(n int, randIntn func(n int) int) string {
+func StringFn(n int, randReader io.Reader) string {
 	const (
 		vowel     = "aeiou"
 		consonant = "bcdfghjklmnpqrstvwxyz"
 		digit     = "0123456789"
 	)
-	pattern := []string{consonant, vowel, consonant, vowel, consonant, vowel, consonant, digit}
-	out := make([]byte, n)
-	p := 0
+	var (
+		pattern = []string{consonant, vowel, consonant, vowel, consonant, vowel, consonant, digit}
+		out     = make([]byte, n)
+		p       = 0
+	)
+	_, err := io.ReadFull(randReader, out)
+	if err != nil {
+		panic(fmt.Sprintf("internal error: failed to read from random reader: %v", err))
+	}
 	for i := range out {
 		source := pattern[p]
 		p = (p + 1) % len(pattern)
-		out[i] = source[randIntn(len(source))]
+		// this generation method means the distribution is slightly biased. However these
+		// strings are not for passwords so this is deemed OK.
+		out[i] = source[out[i]%byte(len(source))]
 	}
 	return string(out)
 }
@@ -34,7 +41,7 @@ func StringFn(n int, randIntn func(n int) int) string {
 //
 // Do not use these for passwords.
 func String(n int) string {
-	return StringFn(n, mathrand.Intn)
+	return StringFn(n, cryptorand.Reader)
 }
 
 // Password creates a crypto strong password which is just about