bytes: fix Replace so it actually copies

The documentation for bytes.Replace says it copies
the slice but it won't necessarily copy them.  Since
the data is mutable, breaking the contract is an issue.

We either have to fix this by making the copy at all
times, as suggested in this CL, or we should change the
documentation and perhaps make better use of the fact
it's fine to mutate the slice in place otherwise.

R=golang-dev, bradfitz, adg, rsc
CC=golang-dev
https://golang.org/cl/5081043

2 files changed, 19 insertions, 8 deletions

diff --git a/src/pkg/bytes/bytes.go b/src/pkg/bytes/bytes.go
index 5119fce949..ea6bf5ec20 100644
--- a/src/pkg/bytes/bytes.go
+++ b/src/pkg/bytes/bytes.go
@@ -572,13 +572,18 @@ func Runes(s []byte) []int {
 // non-overlapping instances of old replaced by new.
 // If n < 0, there is no limit on the number of replacements.
 func Replace(s, old, new []byte, n int) []byte {
-	if n == 0 {
-		return s // avoid allocation
-	}
-	// Compute number of replacements.
-	if m := Count(s, old); m == 0 {
-		return s // avoid allocation
-	} else if n <= 0 || m < n {
+	m := 0
+	if n != 0 {
+		// Compute number of replacements.
+		m = Count(s, old)
+	}
+	if m == 0 {
+		// Nothing to do. Just copy.
+		t := make([]byte, len(s))
+		copy(t, s)
+		return t
+	}
+	if n < 0 || m < n {
 		n = m
 	}
 
diff --git a/src/pkg/bytes/bytes_test.go b/src/pkg/bytes/bytes_test.go
index 9444358a85..1679279d36 100644
--- a/src/pkg/bytes/bytes_test.go
+++ b/src/pkg/bytes/bytes_test.go
@@ -829,9 +829,15 @@ var ReplaceTests = []ReplaceTest{
 
 func TestReplace(t *testing.T) {
 	for _, tt := range ReplaceTests {
-		if s := string(Replace([]byte(tt.in), []byte(tt.old), []byte(tt.new), tt.n)); s != tt.out {
+		in := append([]byte(tt.in), []byte("<spare>")...)
+		in = in[:len(tt.in)]
+		out := Replace(in, []byte(tt.old), []byte(tt.new), tt.n)
+		if s := string(out); s != tt.out {
 			t.Errorf("Replace(%q, %q, %q, %d) = %q, want %q", tt.in, tt.old, tt.new, tt.n, s, tt.out)
 		}
+		if cap(in) == cap(out) && &in[:1][0] == &out[:1][0] {
+			t.Errorf("Replace(%q, %q, %q, %d) didn't copy", tt.in, tt.old, tt.new, tt.n)
+		}
 	}
 }