Cannot use (type []byte) as type io.Reader

Question

I don't understand the error, this is my main.go that I execute in the machine "A":

package main

import (
    "fmt"
    "net"
    "os"
    "github.com/mistifyio/go-zfs"
)

func main() {
    // Listen for incoming connections.
    l, err := net.Listen("tcp", "192.168.99.5:9977")
    if err != nil ...
    // Close the listener when the application closes.
    defer l.Close()
    fmt.Println("Listening on " + CONN_HOST + ":" + CONN_PORT)
    for {
        // Listen for an incoming connection.
        conn, err := l.Accept()
        if err != nil ...

        //Handle connections in a new goroutine.
        go handleRequest(conn)
    }
}

// Handles incoming requests.
func handleRequest(conn net.Conn) {
    // Make a buffer to hold incoming data.
    buff := make([]byte, 1024)
    // Read the incoming connection into the buffer.
    _, err := conn.Read(buff)
    if err != nil {
        fmt.Printf("Error reading: %s.\n", err.Error())
    }
    // ReceiveSnapshot
    ds, err := zfs.ReceiveSnapshot(buff, "tank/replication")
    if err != nil {
        fmt.Printf("Error receiving: %s.\n", err.Error())
    }
    fmt.Printf("%s... done!\n", ds)
    // Send a response back to person contacting us.
    conn.Write([]byte("Received!"))
    // Close the connection when you're done with it.
    conn.Close()
}

Now, I show you the function ReceiveSnapshot from github.com/mistifyio/go-zfs/zfs.go:

type command struct {
    Command string
    Stdin   io.Reader
    Stdout  io.Writer
}

func ReceiveSnapshot(input io.Reader, name string) (*Dataset, error) {
    c := command{Command: "zfs", Stdin: input}
    _, err := c.Run("receive", name)
    if err != nil {
        return nil, err
    }
    return GetDataset(name)
}

I have seen in golang pkg the doc of io.Reader:

type Reader interface {
        Read(p []byte) (n int, err error)
}

Why do I receive the error...

• cannot use buff (type []byte) as type io.Reader in argument to zfs.ReceiveSnapshot: []byte does not implement io.Reader (missing Read method)

...when I make go install ?

Thermomotor

What is unclear about the error message? A []byte does not implement the Read method of an io.Reader, therefore it cannot be used as an io.Reader. Please work through the Methods and Interfaces chapter of the Tour of Go (starting here: tour.golang.org/methods/1) – Lifesaver

I know that the message is clear but if I ask this it's because I don't understand (and I have read the tour...) the difference between buff := make([]byte) and the Read method Read (p []byte) – Thermomotor

Answer 1

I think you are missing a step in your logic when you think that []byte would be equivalent to Reader just because the Reader's Read method receives a []byte as a parameter.

Let me try to clarify:

Your ReceiveSnapshot function expects a Reader as a parameter:

ReceiveSnapshot( input io.Reader ...

In order for a type to fulfil the Reader interface, that type should itself implement this function:

Read(p []byte) (n int, err error)

Note that the type should implement that function in order to be a Reader.

[]byte does not implement a Read function. It is just a coincidence that the argument to Read happens to be a []byte.

In order for this to work, you need to send ReceiveSnapshot a proper Reader.

Luckily for you, having a []byte and wanting to Read for it is a common situation so the API provides an easy way to do this:

https://golang.org/pkg/bytes/#NewReader

You just need to send bytes.NewReader(buff) to your ReceiveSnapshot function instead of just buff.

Selfregard

Thank you so much! @Selfregard – Thermomotor

I keep coming back to this super helpful answer, thanks so much @eugenjoy. – Chrysotile

Answer 2

Short answer: Wrap your buffer in a Reader type by using bytes.NewReader

Alternatively, you could use bytes.NewBuffer with similar effect.

If the source is a string, you can use strings.NewReader.

The list of Readers goes on and on: https://golang.org/search?q=Read#Global

---

Explanation of the deeper question

The deeper question being: Why doesn't an array support the io.Reader interface directly?

io.Reader supports the concept of reading from a general data stream for which the total size is not necessarily known in advance. In order to support this, Read is called repeatedly until all input data is exhausted. In many languages, similar read functions must be called at least twice, where the final call returns a flag indicating end-of-file.

By returning two values (one of which is type error), Go makes it possible for reading of arrays to complete in one-shot, but only if the destination buffer is large enough to consume all available data -- which isn't always known in advance.

The io.Reader interface specifies the signature and behavior of the Read() function:

func (T) Read(b []byte) (n int, err error)

Read populates the given byte slice with data and returns the number of bytes populated and an error value. It returns an io.EOF error when the stream ends.

So due to the way the io.Reader interface works, a simple byte buffer is not capable of implementing it. A wrapper structure is required in order to remember the state between subsequent calls to Read().

For the sake of interest, here's an example showing how that can be implemented...

type MyReader struct {
    src []byte
    pos int
}

func (r *MyReader) Read(dst []byte) (n int, err error) {
    n = copy(dst, r.src[r.pos:])
    r.pos += n
    if r.pos == len(r.src) {
        return n, io.EOF
    }
    return
}

func NewMyReader(b []byte) *MyReader { return &MyReader{b, 0} }

Notice, also, that the []byte parameter to Read() is the destination buffer, not the source.

Galvano