// An FS is an abstraction over commonly-used functions in the os and io
// packages.
type FS interface {
    Chmod(name string, mode fs.FileMode) error
    Chown(name string, uid, git int) error
    Chtimes(name string, atime, mtime time.Time) error
    Create(name string) (*os.File, error)
    Glob(pattern string) ([]string, error)
    Lchown(name string, uid, git int) error
    Link(oldname, newname string) error
    Lstat(name string) (fs.FileInfo, error)
    Mkdir(name string, perm fs.FileMode) error
    Open(name string) (fs.File, error)
    OpenFile(name string, flag int, perm fs.FileMode) (*os.File, error)
    PathSeparator() rune
    RawPath(name string) (string, error)
    ReadDir(dirname string) ([]fs.DirEntry, error)
    ReadFile(filename string) ([]byte, error)
    Readlink(name string) (string, error)
    Remove(name string) error
    RemoveAll(name string) error
    Rename(oldpath, newpath string) error
    Stat(name string) (fs.FileInfo, error)
    Symlink(oldname, newname string) error
    Truncate(name string, size int64) error
    WriteFile(filename string, data []byte, perm fs.FileMode) error
}

// writeConfigFile is the function we're going to test. It can make arbitrary
// changes to the filesystem through fileSystem.
func writeConfigFile(fileSystem vfs.FS) error {
    return fileSystem.WriteFile("/home/user/app.conf", []byte(`app config`), 0644)
}

// TestWriteConfigFile is our test function.
func TestWriteConfigFile(t *testing.T) {
    // Create and populate an temporary directory with a home directory.
    fileSystem, cleanup, err := vfst.NewTestFS(map[string]any{
        "/home/user/.bashrc": "# contents of user's .bashrc\n",
    })

    // Check that the directory was populated successfully.
    if err != nil {
        t.Fatalf("vfsTest.NewTestFS(_) == _, _, %v, want _, _, <nil>", err)
    }

    // Ensure that the temporary directory is removed.
    defer cleanup()

    // Call the function we want to test.
    if err := writeConfigFile(fileSystem); err != nil {
        t.Error(err)
    }

    // Check properties of the filesystem after our function has modified it.
    vfst.RunTests(t, fileSystem, "app_conf",
        vfst.TestPath("/home/user/app.conf",
            vfst.TestModeIsRegular(),
            vfst.TestModePerm(0644),
            vfst.TestContentsString("app config"),
        ),
    )
}