func (errorReader) Read([]byte) (int, error) {
	return 0, os.ErrInvalid
}
Read on an errorReader already returns an error.
package mongocompare
import (
	"context"
	"crypto/sha1"
	"encoding/base64"
	"fmt"
	"github.com/mongodb/mongo-tools-common/db"
	"github.com/mongodb/mongo-tools-common/intents"
	"github.com/mongodb/mongo-tools-common/log"
	"github.com/mongodb/mongo-tools-common/util"
	"io"
	"os"
	"path/filepath"
	"strings"
)
type NilPos struct{}
func (NilPos) Pos() int64 {
	return -1
}
type writeFlusher interface {
	Flush() error
	io.Writer
}
type writeFlushCloser struct {
	writeFlusher
}
type errorReader struct{}