crypto/rand: make Read use io.ReadFull

Fixes #6084

R=golang-dev, rsc, dave
CC=golang-dev
https://golang.org/cl/13523044

src/pkg/crypto/rand/example_test.go

@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"crypto/rand"
 	"fmt"
-	"io"
 )
 
 // This example reads 10 cryptographically secure pseudorandom numbers from
@@ -16,7 +15,7 @@ import (
 func ExampleRead() {
 	c := 10
 	b := make([]byte, c)
-	_, err := io.ReadFull(rand.Reader, b)
+	_, err := rand.Read(b)
 	if err != nil {
 		fmt.Println("error:", err)
 		return

src/pkg/crypto/rand/rand.go

@@ -14,5 +14,8 @@ import "io"
 // On Windows systems, Reader uses the CryptGenRandom API.
 var Reader io.Reader
 
-// Read is a helper function that calls Reader.Read.
-func Read(b []byte) (n int, err error) { return Reader.Read(b) }
+// Read is a helper function that calls Reader.Read using io.ReadFull.
+// On return, n == len(b) if and only if err == nil.
+func Read(b []byte) (n int, err error) {
+	return io.ReadFull(Reader, b)
+}