copying / cloning an io.Reader

[Johann Höchtl]

A function returns an  io.Reader. When I pass that reader to eg. json.UnMarshal, the reader get's consumed. How can I clone that reader so to re-use it?

[Brad Fitzpatrick]

Assume an io.Reader is hooked up to a satellite dish receiving a message (maybe a 1TB message) from an alien civilization who only transmits their message once every thousand years.

There's no "rewinding" that.

If you want a copy, you need to copy it as you read it, perhaps to disk or memory, depending on its size.

io.TeeReader + bytes.Buffer is a common approach.

On Fri, Jan 17, 2014 at 10:20 AM, Johann Höchtl <johann....@gmail.com> wrote:

A function returns an  io.Reader. When I pass that reader to eg. json.UnMarshal, the reader get's consumed. How can I clone that reader so to re-use it?

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[Johann Höchtl]

On 2014-01-17 19:24, Brad Fitzpatrick wrote:
> Assume an io.Reader is hooked up to a satellite dish receiving a message
> (maybe a 1TB message) from an alien civilization who only transmits
> their message once every thousand years.
>
> There's no "rewinding" that.
>
> If you want a copy, you need to copy it as you read it, perhaps to disk
> or memory, depending on its size.
>
> io.TeeReader + bytes.Buffer is a common approach.
>
>

Yes, seems to be black-out day, this works of course

buf, _ := ioutil.ReadAll(mdjsonreader)
minimaljsonbuffer := bytes.NewBuffer(buf)
mdjson := bytes.NewBuffer(buf)
>

[Kevin Gillette]

If the function is completely deterministic, then it is usually better to just call the function again to get a fresh io.Reader that'll produce identical content, rather than buffering it.

[jeremy....@gmail.com]

Why not use Multiwriter and Copy ?