Race conditions in io.Pipe?

Question

I have a function which returns the Reader end of an io.Pipe and kicks off a go-routine which writes data to the Writer end of it, and then closes the pipe.

func GetPipeReader() io.ReadCloser {
    r, w := io.Pipe()
    go func() {
        _, err := io.CopyN(w, SomeReaderOfSize(N), N)
        w.CloseWithError(err)
    }()
    return r
}

func main() {
    var buf bytes.Buffer
    io.Copy(&buf, GetPipeReader())
    println("got", buf.Len(), "bytes")
}

https://play.golang.org/p/OAijIwmtRr

This seems to always work in my testing, in that I get all the data I wrote. But the API docs are a bit worrying to me:

func Pipe() (*PipeReader, *PipeWriter)

Pipe creates a synchronous in-memory pipe. [...] Reads on one end are matched with writes on the other, [...] there is no internal buffering.

func (w *PipeWriter) CloseWithError(err error) error

CloseWithError closes the writer; subsequent reads from the read half of the pipe will return no bytes and the error err, or EOF if err is nil.

What I want to know is, what are the possible race conditions here? Is is plausible that my go-routine will write a bunch of data and then close the pipe before I can read it all?

Do I need to use a channel for some signalling on when to close? What can go wrong, basically.

The docs mention that "It is safe to call Read and Write in parallel with each other or with Close. Close will complete once pending I/O is done", so I don't see where could a race occur. — Ainar-G
Stop worrying and run the code under the race detector :-). Note that reads and writes are matched: The data you write is been read the moment you write it. Just reads after close drop data. Just make sure that reads actually consume all you want to write. — Volker
The race detector?! Amazing, I didn't realise there was such a thing. Taking a look now. I'll play with it later. — Dave

Filipe Gonçalves Filipe Gonçalves · Accepted Answer · 2015-12-03T13:07:12

No, there are no race conditions. As the documentation mentions, reads on one end are matched with writes on the other. So, when CloseWithError() is reached, it means every Write has successfully completed and been matched with a corresponding Read - so the other end must have read everything there was to read.

Race conditions in io.Pipe?

1 Answers