const NilVersion int = -1

var (
	ErrLocked    = fmt.Errorf("can't acquire lock")
	ErrNotLocked = fmt.Errorf("can't unlock, as not currently locked")
)

type Driver interface {
	// Open returns a new driver instance configured with parameters
	// coming from the URL string. Migrate will call this function
	// only once per instance.
	Open(url string) (Driver, error)

	// Close closes the underlying database instance managed by the driver.
	// Migrate will call this function only once per instance.
	Close() error

	// Lock should acquire a database lock so that only one migration process
	// can run at a time. Migrate will call this function before Run is called.
	// If the implementation can't provide this functionality, return nil.
	// Return database.ErrLocked if database is already locked.
	Lock() error

	// Unlock should release the lock. Migrate will call this function after
	// all migrations have been run.
	Unlock() error

	// Run applies a migration to the database. migration is guaranteed to be not nil.
	Run(migration io.Reader) error

	// SetVersion saves version and dirty state.
	// Migrate will call this function before and after each call to Run.
	// version must be >= -1. -1 means NilVersion.
	SetVersion(version int, dirty bool) error

	// Version returns the currently active version and if the database is dirty.
	// When no migration has been applied, it must return version -1.
	// Dirty means, a previous migration failed and user interaction is required.
	Version() (version int, dirty bool, err error)

	// Drop deletes everything in the database.
	// Note that this is a breaking action, a new call to Open() is necessary to
	// ensure subsequent calls work as expected.
	Drop() error
}

type Error struct {
	// Optional: the line number
	Line uint

	// Query is a query excerpt
	Query []byte

	// Err is a useful/helping error message for humans
	Err string

	// OrigErr is the underlying error
	OrigErr error
}