migrate/database/driver.go

// Package database provides the Database interface.
// All database drivers must implement this interface, register themselves,
// optionally provide a `WithInstance` function and pass the tests
// in package database/testing.
package database

import (
	"fmt"
	"io"
	nurl "net/url"
	"sync"
)

var (
	ErrLocked = fmt.Errorf("can't acquire lock")
)

const NilVersion int = -1

var driversMu sync.RWMutex
var drivers = make(map[string]Driver)

// Driver is the interface every database driver must implement.
//
// How to implement a database driver?
//   1. Implement this interface.
//   2. Optionally, add a function named `WithInstance`.
//      This function should accept an existing DB instance and a Config{} struct
//      and return a driver instance.
//   3. Add a test that calls database/testing.go:Test()
//   4. Add own tests for Open(), WithInstance() (when provided) and Close().
//      All other functions are tested by tests in database/testing.
//      Saves you some time and makes sure all database drivers behave the same way.
//   5. Call Register in init().
//
// Guidelines:
//   * Don't try to correct user input. Don't assume things.
//     When in doubt, return an error and explain the situation to the user.
//   * All configuration input must come from the URL string in func Open()
//     or the Config{} struct in WithInstance. Don't os.Getenv().
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. Run the migration and store
	// the version. migration can be nil. In that case, just store the version.
	// When version -1 is given, the state should be as if no migration had been run.
	Run(version int, migration io.Reader) error

	// Version returns the currently active version.
	// When no migration has been run yet, it must return -1.
	// If the returned version is < -1 it will panic (in the test).
	Version() (int, error)

	// Drop deletes everyting in the database.
	Drop() error
}

// Open returns a new driver instance.
func Open(url string) (Driver, error) {
	u, err := nurl.Parse(url)
	if err != nil {
		return nil, err
	}

	if u.Scheme == "" {
		return nil, fmt.Errorf("database driver: invalid URL scheme")
	}

	driversMu.RLock()
	d, ok := drivers[u.Scheme]
	driversMu.RUnlock()
	if !ok {
		return nil, fmt.Errorf("database driver: unknown driver %v (forgotton import?)", u.Scheme)
	}

	return d.Open(url)
}

// Register globally registers a driver.
func Register(name string, driver Driver) {
	driversMu.Lock()
	defer driversMu.Unlock()
	if driver == nil {
		panic("Register driver is nil")
	}
	if _, dup := drivers[name]; dup {
		panic("Register called twice for driver " + name)
	}
	drivers[name] = driver
}