bytes: add test to ensure shallow copy of NewBuffer does not allocate

At present, there is no API to reset the underlying []byte
of an existing Buffer struct, except to shallow copy
the entire Buffer struct.

Updates #67004

Change-Id: I08998f7a95ae5bde0897d86247d47f23cd784583
Reviewed-on: https://go-review.googlesource.com/c/go/+/581297
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Joedian Reid <joedian@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/bytes/buffer_test.go b/src/bytes/buffer_test.go
index 845e5e2209..322e7367c7 100644
--- a/src/bytes/buffer_test.go
+++ b/src/bytes/buffer_test.go
@@ -94,6 +94,21 @@ func TestNewBuffer(t *testing.T) {
 	check(t, "NewBuffer", buf, testString)
 }
 
+var buf Buffer
+
+// Calling NewBuffer and immediately shallow copying the Buffer struct
+// should not result in any allocations.
+// This can be used to reset the underlying []byte of an existing Buffer.
+func TestNewBufferShallow(t *testing.T) {
+	n := testing.AllocsPerRun(1000, func() {
+		buf = *NewBuffer(testBytes)
+	})
+	if n > 0 {
+		t.Errorf("allocations occurred while shallow copying")
+	}
+	check(t, "NewBuffer", &buf, testString)
+}
+
 func TestNewBufferString(t *testing.T) {
 	buf := NewBufferString(testString)
 	check(t, "NewBufferString", buf, testString)