package main

import (
    "bytes"
    "errors"
    "fmt"
    "io/ioutil"
    "os"
    "strings"
    "github.com/stretchr/testify/assert"
    "testing"
)

type slurper func(filename string) ([]byte, error)

// findRoot finds the device your rootfs is mounted on
func findRoot(slurper slurper) (string, error) {
    const mountsFname = "/proc/mounts"

    buf, err := slurper(mountsFname)
    if err != nil {
        return "", err
    }

    for _, line := range bytes.Split(buf, []byte("\n")) {
        fields := strings.Fields(string(line))

        if fields[1] == "/" {
            return fields[0], nil
        }

    }

    return "", errors.New("Root partition not found in " + mountsFname)

}

func main() {
    root, err := findRoot(ioutil.ReadFile)
    if err != nil {
        fmt.Fprintf(os.Stderr, "Error: %v\n", err)
        os.Exit(2)
    }

    fmt.Println("Root:", root)
}

func mockSlurper(mockFS map[string]string) slurper {
    return func(filename string) ([]byte, error) {
        content, ok := mockFS[filename]

        if !ok {
            return nil, fmt.Errorf("%v not found", filename)
        }

        return []byte(content), nil
    }
}

func TestFindRoot(t *testing.T) {
    slurper := mockSlurper(map[string]string{
        "/proc/mounts": `/dev/mapper/fedora-thin--root / ext4 rw,relatime,discard 0 0
/dev/mapper/fedora-boot /boot ext4 rw,relatime,discard 0 0
/dev/mapper/crypthome /home ext4 rw,relatime,discard,stripe=16 0 0
/dev/mapper/fedora-thin--root /var/lib/docker/containers ext4 rw,relatime,discard 0 0
/dev/mapper/fedora-thin--root /var/lib/docker/devicemapper ext4 rw,relatime,discard 0 0
`})

    root, err := findRoot(slurper)
    assert.NoError(t, err)
    assert.Equal(t, root, "/dev/mapper/fedora-thin--root")
}

func TestFindRootNegative(t *testing.T) {
    slurper := mockSlurper(map[string]string{
        "/jank/file": `jank content of a jank file
`})

    content, err := findRoot(slurper)
    assert.Error(t, err)
    assert.Equal(t, content, "")
}