MemMapFS: uses shared file position

[vetinari]

EDITed: fixed typo
The following code demonstrates this behavior, it should print

    OS=<datadata>
    FS=<datadata>

but the FS line reads FS=<testdatadata>

package main

import (
    "os"
    "fmt"
    "io/ioutil"
    "github.com/spf13/afero"
)

func main() {
    fileName := "./afero-demo2.txt"
    fh1, err := os.Create(fileName)
    if err != nil {
        panic("os.Create failed: "+err.Error())
    }
    _, err = fh1.Write([]byte("test"))
    if err != nil {
        panic("fh.Write failed: "+err.Error())
    }
    fh1.Seek(0, os.SEEK_SET)

    fh2, err := os.OpenFile(fileName, os.O_RDWR, 0777)
    if err != nil {
        panic("os.OpenFile failed: "+err.Error())
    }
    fh2.Seek(0, os.SEEK_END)
    fh2.Write([]byte("data"))
    fh2.Close()

    fh1.Write([]byte("data"))
    fh1.Close()
    // the file now should contain "datadata"
    data, _ := ioutil.ReadFile(fileName)
    fmt.Printf("OS=<%s>\n", data)

    fs := &afero.MemMapFs{}
    afh1, err := fs.Create(fileName)
    if err != nil {
        panic("fs.Create failed: "+err.Error())
    }
    _, err = afh1.Write([]byte("test"))
    if err != nil {
        panic("fh.Write failed: "+err.Error())
    }
    afh1.Seek(0, os.SEEK_SET)

    afh2, err := fs.OpenFile(fileName, os.O_RDWR, 0777)
    if err != nil {
        panic("fs.OpenFile failed: "+err.Error())
    }
    afh2.Seek(0, os.SEEK_END)
    afh2.Write([]byte("data"))
    afh2.Close()

    afh1.Write([]byte("data"))
    afh1.Close()
    // the file now should contain "datadata"
    data, _ = afero.ReadFile(fs, fileName)
    fmt.Printf("FS=<%s>\n", data)
}