cryptobyte: add (*Builder).Unwrite and (*Builder).SetError

Unwrite allows programs to rollback builders more reliably and
efficiently than by copying a Builder (which might waste an allocation
and depends on internal behavior). This is useful for example to remove
a length-prefixed field if it ends up being empty.

SetError allows simple Builder extensions to set errors without making
MarshalingValue wrappers.

Based on the experience of CL 144115.

Change-Id: I9a785b81b51b15af49418b5bdb71c4ef222ccc46
Reviewed-on: https://go-review.googlesource.com/c/145317
Reviewed-by: Adam Langley <agl@golang.org>

cryptobyte/builder.go

@@ -50,6 +50,12 @@ func NewFixedBuilder(buffer []byte) *Builder {
 	}
 }
 
+// SetError sets the value to be returned as the error from Bytes. Writes
+// performed after calling SetError are ignored.
+func (b *Builder) SetError(err error) {
+	b.err = err
+}
+
 // Bytes returns the bytes written by the builder or an error if one has
 // occurred during building.
 func (b *Builder) Bytes() ([]byte, error) {
@@ -94,7 +100,7 @@ func (b *Builder) AddBytes(v []byte) {
 	b.add(v...)
 }
 
-// BuilderContinuation is continuation-passing interface for building
+// BuilderContinuation is a continuation-passing interface for building
 // length-prefixed byte sequences. Builder methods for length-prefixed
 // sequences (AddUint8LengthPrefixed etc) will invoke the BuilderContinuation
 // supplied to them. The child builder passed to the continuation can be used
@@ -290,6 +296,26 @@ func (b *Builder) add(bytes ...byte) {
 	b.result = append(b.result, bytes...)
 }
 
+// Unwrite rolls back n bytes written directly to the Builder. An attempt by a
+// child builder passed to a continuation to unwrite bytes from its parent will
+// panic.
+func (b *Builder) Unwrite(n int) {
+	if b.err != nil {
+		return
+	}
+	if b.child != nil {
+		panic("attempted unwrite while child is pending")
+	}
+	length := len(b.result) - b.pendingLenLen - b.offset
+	if length < 0 {
+		panic("cryptobyte: internal error")
+	}
+	if n > length {
+		panic("cryptobyte: attempted to unwrite more than was written")
+	}
+	b.result = b.result[:len(b.result)-n]
+}
+
 // A MarshalingValue marshals itself into a Builder.
 type MarshalingValue interface {
 	// Marshal is called by Builder.AddValue. It receives a pointer to a builder