proposal: bytes: add Buffer.Init

[cwbhhjl]

This method adds the ability to replace a Buffer's buf.

Currently bytes.NewBuffer can be used to initialize Buffer with a custom buf, or we can copy a custom buf into Buffer via Write, but in performance-sensitive scenarios, I think it would be useful to avoid heap allocation or memcpy. So I want to add a method to replace a Buffer's buf.

a possible implementation:

func (b *Buffer) Init(buf []byte) {
	b.buf = buf
	b.off = 0
	b.lastRead = opInvalid
}

[seankhliao]

how is this different from Reset?

[cwbhhjl]

how is this different from Reset?

If we get the data and want to wrap it in Buffer, currently we can

1. bytes.NewBuffer(buf), may need to allocate on the heap
2. already have a buffer

   buffer.Reset()
   buffer.Grow(len(buf))
   buffer.Write(buf)

   need a copy

[ianlancetaylor]

Can you show an example where this comes up? Thanks.

(I think you could actually do this today via

    b.Reset(buf)
    b.Grow(len(buf))

but I'm not sure we want to guarantee that.)

[cwbhhjl]

@ianlancetaylor

Can you show an example where this comes up? Thanks.

I am using an encoding library similar to protobuf, the encoder is something like

func Encode(Marshaler m) []byte {
    bzSize := m.Size()
    var buf = bytes.NewBuffer(make([]byte, 0, bzSize))
    m.MarshalTo(buf)
    return buf.Bytes()
}

buf will escape to the heap because of the interface, If sync.Pool is used, a copy is required at the end.

func Encode(Marshaler m) (bz []byte) {
    bzSize := m.Size()
    var buf = pool.Get().(*bytes.Buffer)
    buf.Reset()
    buf.Grow(bzSize)
    m.MarshalTo(buf)
    bz = make([]byte, bzSize)
    copy(bz, buf.Bytes())
    pool.Put(buf)
    return
}

It is used in performance sensitive scenarios, so I want to reduce allocation or copy

[DeedleFake]

Perhaps SetBytes() would be a better name?

[cwbhhjl]

Perhaps SetBytes() would be a better name?

@DeedleFake good idea, Buffer already has a Bytes method, I agree that SetBytes is a better name

[rsc]

This doesn't seem like a terribly common operation, and the proposed b.Init(buf) can already be written as *b = bytes.NewBuffer(buf).

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely decline.
— rsc for the proposal review group

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group