cryptobyte: don't ignore bytes added to BuilderContinuations of fixed-size Builders

Builders created with NewFixedBuilder were broken when used with
BuilderContinuations. The length of the bytes written to the
continuation would get added correctly to the parent, but the actual
content would be discarded.

For example, the BytesOrPanic() in TestFixedBuilderLengthPrefixed would
return [00 08] instead of [00 08 ff ff ff ff ff ff ff ff].

Change-Id: I80837a9bf3562751addcb827274649d9f52fc79a
Reviewed-on: https://go-review.googlesource.com/c/148882
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

cryptobyte/builder.go

@@ -274,9 +274,11 @@ func (b *Builder) flushChild() {
 		return
 	}
 
-	if !b.fixedSize {
-		b.result = child.result // In case child reallocated result.
+	if b.fixedSize && &b.result[0] != &child.result[0] {
+		panic("cryptobyte: BuilderContinuation reallocated a fixed-size buffer")
 	}
+
+	b.result = child.result
 }
 
 func (b *Builder) add(bytes ...byte) {
@@ -284,7 +286,7 @@ func (b *Builder) add(bytes ...byte) {
 		return
 	}
 	if b.child != nil {
-		panic("attempted write while child is pending")
+		panic("cryptobyte: attempted write while child is pending")
 	}
 	if len(b.result)+len(bytes) < len(bytes) {
 		b.err = errors.New("cryptobyte: length overflow")
@@ -304,7 +306,7 @@ func (b *Builder) Unwrite(n int) {
 		return
 	}
 	if b.child != nil {
-		panic("attempted unwrite while child is pending")
+		panic("cryptobyte: attempted unwrite while child is pending")
 	}
 	length := len(b.result) - b.pendingLenLen - b.offset
 	if length < 0 {

cryptobyte/cryptobyte_test.go

@@ -412,6 +412,33 @@ func TestUnwrite(t *testing.T) {
 	})
 }
 
+func TestFixedBuilderLengthPrefixed(t *testing.T) {
+	bufCap := 10
+	inner := bytes.Repeat([]byte{0xff}, bufCap-2)
+	buf := make([]byte, 0, bufCap)
+	b := NewFixedBuilder(buf)
+	b.AddUint16LengthPrefixed(func(b *Builder) {
+		b.AddBytes(inner)
+	})
+	if got := b.BytesOrPanic(); len(got) != bufCap {
+		t.Errorf("Expected output lenght to be %d, got %d", bufCap, len(got))
+	}
+}
+
+func TestFixedBuilderPanicReallocate(t *testing.T) {
+	defer func() {
+		recover()
+	}()
+
+	b := NewFixedBuilder(make([]byte, 0, 10))
+	b1 := NewFixedBuilder(make([]byte, 0, 10))
+	b.AddUint16LengthPrefixed(func(b *Builder) {
+		*b = *b1
+	})
+
+	t.Error("Builder did not panic")
+}
+
 // ASN.1
 
 func TestASN1Int64(t *testing.T) {