[dev.boringcrypto] crypto/hmac: add test for Write/Sum after Sum

This is documented to work (in hash.Hash's definition)
and existing code assumes it works. Add a test.

Change-Id: I63546f3b2d66222683a4f268a4eaff835fd836fe
Reviewed-on: https://go-review.googlesource.com/63911
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/hmac/hmac_test.go

@@ -5,6 +5,7 @@
 package hmac
 
 import (
+	"bytes"
 	"crypto/md5"
 	"crypto/sha1"
 	"crypto/sha256"
@@ -594,6 +595,42 @@ func TestEqual(t *testing.T) {
 	}
 }
 
+func TestWriteAfterSum(t *testing.T) {
+	h := New(sha1.New, nil)
+	h.Write([]byte("hello"))
+	sumHello := h.Sum(nil)
+
+	h = New(sha1.New, nil)
+	h.Write([]byte("hello world"))
+	sumHelloWorld := h.Sum(nil)
+
+	// Test that Sum has no effect on future Sum or Write operations.
+	// This is a bit unusual as far as usage, but it's allowed
+	// by the definition of Go hash.Hash, and some clients expect it to work.
+	h = New(sha1.New, nil)
+	h.Write([]byte("hello"))
+	if sum := h.Sum(nil); !bytes.Equal(sum, sumHello) {
+		t.Fatalf("1st Sum after hello = %x, want %x", sum, sumHello)
+	}
+	if sum := h.Sum(nil); !bytes.Equal(sum, sumHello) {
+		t.Fatalf("2nd Sum after hello = %x, want %x", sum, sumHello)
+	}
+
+	h.Write([]byte(" world"))
+	if sum := h.Sum(nil); !bytes.Equal(sum, sumHelloWorld) {
+		t.Fatalf("1st Sum after hello world = %x, want %x", sum, sumHelloWorld)
+	}
+	if sum := h.Sum(nil); !bytes.Equal(sum, sumHelloWorld) {
+		t.Fatalf("2nd Sum after hello world = %x, want %x", sum, sumHelloWorld)
+	}
+
+	h.Reset()
+	h.Write([]byte("hello"))
+	if sum := h.Sum(nil); !bytes.Equal(sum, sumHello) {
+		t.Fatalf("Sum after Reset + hello = %x, want %x", sum, sumHello)
+	}
+}
+
 func BenchmarkHMACSHA256_1K(b *testing.B) {
 	key := make([]byte, 32)
 	buf := make([]byte, 1024)