在golang中实现io.ReadWriteSeeker

Question

Is there an implementation of io.ReadWriteSeeker to use in Golang?

Since, bytes.Buffer does not implement Seek method, I need to find such an implementation to use as a buffer written by zipwriter and to be read with seeking.

In addition I wont go with Reader(buff.Bytes()) to covert with memory copy, because I can not afford double memory size for buffered data.

In addition, when using os.File as the option, if I wont call f.Sync, it will never touch file system, right? Thanks.

My simplified codes:

func process() {
  buff := new(bytes.Buffer)
  zipWriter := zip.NewWriter(buff)
  // here to add data into zipWriter in sequence
  zipWriter.Close()
  upload(buff) // upload(io.ReadSeeker)
}

Answer 1

For example, using the same underlying array for (uBuf and zBuf) buffers,

package main

import (
    "archive/zip"
    "bytes"
    "io"
)

func upload(io.ReadSeeker) {}

func process() {
    zBuf := new(bytes.Buffer)
    zipWriter := zip.NewWriter(zBuf)
    // add data into zipWriter in sequence
    zipWriter.Close()
    uBuf, zBuf := zBuf.Bytes(), nil
    // upload(io.ReadSeeker)
    upload(bytes.NewReader(uBuf))
}

func main() {}

Playground: https://play.golang.org/p/8TKmnL_vRY9

---

Package bytes

import "bytes" 

func (*Buffer) Bytes

func (b *Buffer) Bytes() []byte

Bytes returns a slice of length b.Len() holding the unread portion of the buffer. The slice is valid for use only until the next buffer modification (that is, only until the next call to a method like Read, Write, Reset, or Truncate). The slice aliases the buffer content at least until the next buffer modification, so immediate changes to the slice will affect the result of future reads.

The tuple assignment statement

    uBuf, zBuf := zBuf.Bytes(), nil

gets the slice descriptor for the zipped bytes (zBuf.Bytes()) and assigns it to the slice descriptor uBuf. A slice descriptor is a struct with a pointer to the underlying array, the slice length, and the slice capacity. For example,

type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}

Then, for safety, we assign nil to zBuf to ensure that no further changes can be made to its underlying array, which is now used by uBuf.