Add CopyOf utility method

Add a utility method that returns a copy of a slice of strings.
This is primarily useful when appending to a string slice to avoid
accidentally reusing the backing array.

Test: util_test.go
Change-Id: I7801fc7ca19e27ddc9f1b1b452788b723c7f619c

android/util.go

@@ -20,6 +20,11 @@ import (
 	"strings"
 )
 
+// CopyOf returns a new slice that has the same contents as s.
+func CopyOf(s []string) []string {
+	return append([]string(nil), s...)
+}
+
 func JoinWithPrefix(strs []string, prefix string) string {
 	if len(strs) == 0 {
 		return ""

android/util_test.go

@@ -15,6 +15,7 @@
 package android
 
 import (
+	"fmt"
 	"reflect"
 	"testing"
 )
@@ -359,3 +360,47 @@ func TestRemoveFromList(t *testing.T) {
 		})
 	}
 }
+
+func ExampleCopyOf() {
+	a := []string{"1", "2", "3"}
+	b := CopyOf(a)
+	a[0] = "-1"
+	fmt.Printf("a = %q\n", a)
+	fmt.Printf("b = %q\n", b)
+
+	// Output:
+	// a = ["-1" "2" "3"]
+	// b = ["1" "2" "3"]
+}
+
+func ExampleCopyOf_append() {
+	a := make([]string, 1, 2)
+	a[0] = "foo"
+
+	fmt.Println("Without CopyOf:")
+	b := append(a, "bar")
+	c := append(a, "baz")
+	fmt.Printf("a = %q\n", a)
+	fmt.Printf("b = %q\n", b)
+	fmt.Printf("c = %q\n", c)
+
+	a = make([]string, 1, 2)
+	a[0] = "foo"
+
+	fmt.Println("With CopyOf:")
+	b = append(CopyOf(a), "bar")
+	c = append(CopyOf(a), "baz")
+	fmt.Printf("a = %q\n", a)
+	fmt.Printf("b = %q\n", b)
+	fmt.Printf("c = %q\n", c)
+
+	// Output:
+	// Without CopyOf:
+	// a = ["foo"]
+	// b = ["foo" "baz"]
+	// c = ["foo" "baz"]
+	// With CopyOf:
+	// a = ["foo"]
+	// b = ["foo" "bar"]
+	// c = ["foo" "baz"]
+}

dexpreopt/dexpreopt.go

@@ -578,9 +578,7 @@ func replace(l []string, from, to string) {
 	}
 }
 
-func copyOf(l []string) []string {
+var copyOf = android.CopyOf
-	return append([]string(nil), l...)
-}
 
 func anyHavePrefix(l []string, prefix string) bool {
 	for _, x := range l {