sql.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package sql provides a generic interface around SQL (or SQL-like)
// databases.
//
// The sql package must be used in conjunction with a database driver.
// See http://golang.org/s/sqldrivers for a list of drivers.
package sql

import (
	"container/list"
	"database/sql/driver"
	"errors"
	"fmt"
	"io"
	"runtime"
	"sync"
)

var drivers = make(map[string]driver.Driver)

// Register makes a database driver available by the provided name.
// If Register is called twice with the same name or if driver is nil,
// it panics.
func Register(name string, driver driver.Driver) {
	if driver == nil {
		panic("sql: Register driver is nil")
	}
	if _, dup := drivers[name]; dup {
		panic("sql: Register called twice for driver " + name)
	}
	drivers[name] = driver
}

// RawBytes is a byte slice that holds a reference to memory owned by
// the database itself. After a Scan into a RawBytes, the slice is only
// valid until the next call to Next, Scan, or Close.
type RawBytes []byte

// NullString represents a string that may be null.
// NullString implements the Scanner interface so
// it can be used as a scan destination:
//
//  var s NullString
//  err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
//  ...
//  if s.Valid {
//     // use s.String
//  } else {
//     // NULL value
//  }
//
type NullString struct {
	String string
	Valid  bool // Valid is true if String is not NULL
}

// Scan implements the Scanner interface.
func (ns *NullString) Scan(value interface{}) error {
	if value == nil {
		ns.String, ns.Valid = "", false
		return nil
	}
	ns.Valid = true
	return convertAssign(&ns.String, value)
}

// Value implements the driver Valuer interface.
func (ns NullString) Value() (driver.Value, error) {
	if !ns.Valid {
		return nil, nil
	}
	return ns.String, nil
}

// NullInt64 represents an int64 that may be null.
// NullInt64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullInt64 struct {
	Int64 int64
	Valid bool // Valid is true if Int64 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullInt64) Scan(value interface{}) error {
	if value == nil {
		n.Int64, n.Valid = 0, false
		return nil
	}
	n.Valid = true
	return convertAssign(&n.Int64, value)
}

// Value implements the driver Valuer interface.
func (n NullInt64) Value() (driver.Value, error) {
	if !n.Valid {
		return nil, nil
	}
	return n.Int64, nil
}

// NullFloat64 represents a float64 that may be null.
// NullFloat64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullFloat64 struct {
	Float64 float64
	Valid   bool // Valid is true if Float64 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullFloat64) Scan(value interface{}) error {
	if value == nil {
		n.Float64, n.Valid = 0, false
		return nil
	}
	n.Valid = true
	return convertAssign(&n.Float64, value)
}

// Value implements the driver Valuer interface.
func (n NullFloat64) Value() (driver.Value, error) {
	if !n.Valid {
		return nil, nil
	}
	return n.Float64, nil
}

// NullBool represents a bool that may be null.
// NullBool implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullBool struct {
	Bool  bool
	Valid bool // Valid is true if Bool is not NULL
}

// Scan implements the Scanner interface.
func (n *NullBool) Scan(value interface{}) error {
	if value == nil {
		n.Bool, n.Valid = false, false
		return nil
	}
	n.Valid = true
	return convertAssign(&n.Bool, value)
}

// Value implements the driver Valuer interface.
func (n NullBool) Value() (driver.Value, error) {
	if !n.Valid {
		return nil, nil
	}
	return n.Bool, nil
}

// Scanner is an interface used by Scan.
type Scanner interface {
	// Scan assigns a value from a database driver.
	//
	// The src value will be of one of the following restricted
	// set of types:
	//
	//    int64
	//    float64
	//    bool
	//    []byte
	//    string
	//    time.Time
	//    nil - for NULL values
	//
	// An error should be returned if the value can not be stored
	// without loss of information.
	Scan(src interface{}) error
}

// ErrNoRows is returned by Scan when QueryRow doesn't return a
// row. In such a case, QueryRow returns a placeholder *Row value that
// defers this error until a Scan.
var ErrNoRows = errors.New("sql: no rows in result set")

// DB is a database handle. It's safe for concurrent use by multiple
// goroutines.
//
// The sql package creates and frees connections automatically; it
// also maintains a free pool of idle connections. If the database has
// a concept of per-connection state, such state can only be reliably
// observed within a transaction. Once DB.Begin is called, the
// returned Tx is bound to a single connection. Once Commit or
// Rollback is called on the transaction, that transaction's
// connection is returned to DB's idle connection pool. The pool size
// can be controlled with SetMaxIdleConns.
type DB struct {
	driver driver.Driver
	dsn    string

	mu           sync.Mutex // protects following fields
	freeConn     *list.List // of *driverConn
	connRequests *list.List // of connRequest
	numOpen      int
	pendingOpens int
	// Used to sygnal the need for new connections
	// a goroutine running connectionOpener() reads on this chan and
	// maybeOpenNewConnections sends on the chan (one send per needed connection)
	// It is closed during db.Close(). The close tells the connectionOpener
	// goroutine to exit.
	openerCh chan struct{}
	closed   bool
	dep      map[finalCloser]depSet
	lastPut  map[*driverConn]string // stacktrace of last conn's put; debug only
	maxIdle  int                    // zero means defaultMaxIdleConns; negative means 0
	maxOpen  int                    // <= 0 means unlimited
}

// driverConn wraps a driver.Conn with a mutex, to
// be held during all calls into the Conn. (including any calls onto
// interfaces returned via that Conn, such as calls on Tx, Stmt,
// Result, Rows)
type driverConn struct {
	db *DB

	sync.Mutex  // guards following
	ci          driver.Conn
	closed      bool
	finalClosed bool // ci.Close has been called
	openStmt    map[driver.Stmt]bool

	// guarded by db.mu
	inUse      bool
	onPut      []func() // code (with db.mu held) run when conn is next returned
	dbmuClosed bool     // same as closed, but guarded by db.mu, for connIfFree
	// This is the Element returned by db.freeConn.PushFront(conn).
	// It's used by connIfFree to remove the conn from the freeConn list.
	listElem *list.Element
}

func (dc *driverConn) releaseConn(err error) {
	dc.db.putConn(dc, err)
}

func (dc *driverConn) removeOpenStmt(si driver.Stmt) {
	dc.Lock()
	defer dc.Unlock()
	delete(dc.openStmt, si)
}

func (dc *driverConn) prepareLocked(query string) (driver.Stmt, error) {
	si, err := dc.ci.Prepare(query)
	if err == nil {
		// Track each driverConn's open statements, so we can close them
		// before closing the conn.
		//
		// TODO(bradfitz): let drivers opt out of caring about
		// stmt closes if the conn is about to close anyway? For now
		// do the safe thing, in case stmts need to be closed.
		//
		// TODO(bradfitz): after Go 1.1, closing driver.Stmts
		// should be moved to driverStmt, using unique
		// *driverStmts everywhere (including from
		// *Stmt.connStmt, instead of returning a
		// driver.Stmt), using driverStmt as a pointer
		// everywhere, and making it a finalCloser.
		if dc.openStmt == nil {
			dc.openStmt = make(map[driver.Stmt]bool)
		}
		dc.openStmt[si] = true
	}
	return si, err
}

// the dc.db's Mutex is held.
func (dc *driverConn) closeDBLocked() func() error {
	dc.Lock()
	defer dc.Unlock()
	if dc.closed {
		return func() error { return errors.New("sql: duplicate driverConn close") }
	}
	dc.closed = true
	return dc.db.removeDepLocked(dc, dc)
}

func (dc *driverConn) Close() error {
	dc.Lock()
	if dc.closed {
		dc.Unlock()
		return errors.New("sql: duplicate driverConn close")
	}
	dc.closed = true
	dc.Unlock() // not defer; removeDep finalClose calls may need to lock

	// And now updates that require holding dc.mu.Lock.
	dc.db.mu.Lock()
	dc.dbmuClosed = true
	fn := dc.db.removeDepLocked(dc, dc)
	dc.db.mu.Unlock()
	return fn()
}

func (dc *driverConn) finalClose() error {
	dc.Lock()

	for si := range dc.openStmt {
		si.Close()
	}
	dc.openStmt = nil

	err := dc.ci.Close()
	dc.ci = nil
	dc.finalClosed = true
	dc.Unlock()

	dc.db.mu.Lock()
	dc.db.numOpen--
	dc.db.maybeOpenNewConnections()
	dc.db.mu.Unlock()

	return err
}

// driverStmt associates a driver.Stmt with the
// *driverConn from which it came, so the driverConn's lock can be
// held during calls.
type driverStmt struct {
	sync.Locker // the *driverConn
	si          driver.Stmt
}

func (ds *driverStmt) Close() error {
	ds.Lock()
	defer ds.Unlock()
	return ds.si.Close()
}

// depSet is a finalCloser's outstanding dependencies
type depSet map[interface{}]bool // set of true bools

// The finalCloser interface is used by (*DB).addDep and related
// dependency reference counting.
type finalCloser interface {
	// finalClose is called when the reference count of an object
	// goes to zero. (*DB).mu is not held while calling it.
	finalClose() error
}

// addDep notes that x now depends on dep, and x's finalClose won't be
// called until all of x's dependencies are removed with removeDep.
func (db *DB) addDep(x finalCloser, dep interface{}) {
	//println(fmt.Sprintf("addDep(%T %p, %T %p)", x, x, dep, dep))
	db.mu.Lock()
	defer db.mu.Unlock()
	db.addDepLocked(x, dep)
}

func (db *DB) addDepLocked(x finalCloser, dep interface{}) {
	if db.dep == nil {
		db.dep = make(map[finalCloser]depSet)
	}
	xdep := db.dep[x]
	if xdep == nil {
		xdep = make(depSet)
		db.dep[x] = xdep
	}
	xdep[dep] = true
}

// removeDep notes that x no longer depends on dep.
// If x still has dependencies, nil is returned.
// If x no longer has any dependencies, its finalClose method will be
// called and its error value will be returned.
func (db *DB) removeDep(x finalCloser, dep interface{}) error {
	db.mu.Lock()
	fn := db.removeDepLocked(x, dep)
	db.mu.Unlock()
	return fn()
}

func (db *DB) removeDepLocked(x finalCloser, dep interface{}) func() error {
	//println(fmt.Sprintf("removeDep(%T %p, %T %p)", x, x, dep, dep))

	xdep, ok := db.dep[x]
	if !ok {
		panic(fmt.Sprintf("unpaired removeDep: no deps for %T", x))
	}

	l0 := len(xdep)
	delete(xdep, dep)

	switch len(xdep) {
	case l0:
		// Nothing removed. Shouldn't happen.
		panic(fmt.Sprintf("unpaired removeDep: no %T dep on %T", dep, x))
	case 0:
		// No more dependencies.
		delete(db.dep, x)
		return x.finalClose
	default:
		// Dependencies remain.
		return func() error { return nil }
	}
}

// This is the size of the connectionOpener request chan (dn.openerCh).
// This value should be larger than the maximum typical value
// used for db.maxOpen. If maxOpen is significantly larger than
// connectionRequestQueueSize then it is possible for ALL calls into the *DB
// to block until the connectionOpener can satify the backlog of requests.
var connectionRequestQueueSize = 1000000

// Open opens a database specified by its database driver name and a
// driver-specific data source name, usually consisting of at least a
// database name and connection information.
//
// Most users will open a database via a driver-specific connection
// helper function that returns a *DB. No database drivers are included
// in the Go standard library. See http://golang.org/s/sqldrivers for
// a list of third-party drivers.
//
// Open may just validate its arguments without creating a connection
// to the database. To verify that the data source name is valid, call
// Ping.
func Open(driverName, dataSourceName string) (*DB, error) {
	driveri, ok := drivers[driverName]
	if !ok {
		return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
	}
	db := &DB{
		driver:   driveri,
		dsn:      dataSourceName,
		openerCh: make(chan struct{}, connectionRequestQueueSize),
		lastPut:  make(map[*driverConn]string),
	}
	db.freeConn = list.New()
	db.connRequests = list.New()
	go db.connectionOpener()
	return db, nil
}

// Ping verifies a connection to the database is still alive,
// establishing a connection if necessary.
func (db *DB) Ping() error {
	// TODO(bradfitz): give drivers an optional hook to implement
	// this in a more efficient or more reliable way, if they
	// have one.
	dc, err := db.conn()
	if err != nil {
		return err
	}
	db.putConn(dc, nil)
	return nil
}

// Close closes the database, releasing any open resources.
func (db *DB) Close() error {
	db.mu.Lock()
	if db.closed { // Make DB.Close idempotent
		db.mu.Unlock()
		return nil
	}
	close(db.openerCh)
	var err error
	fns := make([]func() error, 0, db.freeConn.Len())
	for db.freeConn.Front() != nil {
		dc := db.freeConn.Front().Value.(*driverConn)
		dc.listElem = nil
		fns = append(fns, dc.closeDBLocked())
		db.freeConn.Remove(db.freeConn.Front())
	}
	db.closed = true
	for db.connRequests.Front() != nil {
		req := db.connRequests.Front().Value.(connRequest)
		db.connRequests.Remove(db.connRequests.Front())
		close(req)
	}
	db.mu.Unlock()
	for _, fn := range fns {
		err1 := fn()
		if err1 != nil {
			err = err1
		}
	}
	return err
}

const defaultMaxIdleConns = 2

func (db *DB) maxIdleConnsLocked() int {
	n := db.maxIdle
	switch {
	case n == 0:
		// TODO(bradfitz): ask driver, if supported, for its default preference
		return defaultMaxIdleConns
	case n < 0:
		return 0
	default:
		return n
	}
}

// SetMaxIdleConns sets the maximum number of connections in the idle
// connection pool.
//
// If MaxOpenConns is greater than 0 but less than the new MaxIdleConns
// then the new MaxIdleConns will be reduced to match the MaxOpenConns limit
//
// If n <= 0, no idle connections are retained.
func (db *DB) SetMaxIdleConns(n int) {
	db.mu.Lock()
	if n > 0 {
		db.maxIdle = n
	} else {
		// No idle connections.
		db.maxIdle = -1
	}
	// Make sure maxIdle doesn't exceed maxOpen
	if db.maxOpen > 0 && db.maxIdleConnsLocked() > db.maxOpen {
		db.maxIdle = db.maxOpen
	}
	var closing []*driverConn
	for db.freeConn.Len() > db.maxIdleConnsLocked() {
		dc := db.freeConn.Back().Value.(*driverConn)
		dc.listElem = nil
		db.freeConn.Remove(db.freeConn.Back())
		closing = append(closing, dc)
	}
	db.mu.Unlock()
	for _, c := range closing {
		c.Close()
	}
}

// SetMaxOpenConns sets the maximum number of open connections to the database.
//
// If MaxIdleConns is greater than 0 and the new MaxOpenConns is less than
// MaxIdleConns, then MaxIdleConns will be reduced to match the new
// MaxOpenConns limit
//
// If n <= 0, then there is no limit on the number of open connections.
// The default is 0 (unlimited).
func (db *DB) SetMaxOpenConns(n int) {
	db.mu.Lock()
	db.maxOpen = n
	if n < 0 {
		db.maxOpen = 0
	}
	syncMaxIdle := db.maxOpen > 0 && db.maxIdleConnsLocked() > db.maxOpen
	db.mu.Unlock()
	if syncMaxIdle {
		db.SetMaxIdleConns(n)
	}
}

// Assumes db.mu is locked.
// If there are connRequests and the connection limit hasn't been reached,
// then tell the connectionOpener to open new connections.
func (db *DB) maybeOpenNewConnections() {
	numRequests := db.connRequests.Len() - db.pendingOpens
	if db.maxOpen > 0 {
		numCanOpen := db.maxOpen - (db.numOpen + db.pendingOpens)
		if numRequests > numCanOpen {
			numRequests = numCanOpen
		}
	}
	for numRequests > 0 {
		db.pendingOpens++
		numRequests--
		db.openerCh <- struct{}{}
	}
}

// Runs in a seperate goroutine, opens new connections when requested.
func (db *DB) connectionOpener() {
	for _ = range db.openerCh {
		db.openNewConnection()
	}
}

// Open one new connection
func (db *DB) openNewConnection() {
	ci, err := db.driver.Open(db.dsn)
	db.mu.Lock()
	defer db.mu.Unlock()
	if db.closed {
		if err == nil {
			ci.Close()
		}
		return
	}
	db.pendingOpens--
	if err != nil {
		db.putConnDBLocked(nil, err)
		return
	}
	dc := &driverConn{
		db: db,
		ci: ci,
	}
	if db.putConnDBLocked(dc, err) {
		db.addDepLocked(dc, dc)
		db.numOpen++
	} else {
		ci.Close()
	}
}

// connRequest represents one request for a new connection
// When there are no idle connections available, DB.conn will create
// a new connRequest and put it on the db.connRequests list.
type connRequest chan<- interface{} // takes either a *driverConn or an error

var errDBClosed = errors.New("sql: database is closed")

// conn returns a newly-opened or cached *driverConn
func (db *DB) conn() (*driverConn, error) {
	db.mu.Lock()
	if db.closed {
		db.mu.Unlock()
		return nil, errDBClosed
	}

	// If db.maxOpen > 0 and the number of open connections is over the limit
	// or there are no free connection, then make a request and wait.
	if db.maxOpen > 0 && (db.numOpen >= db.maxOpen || db.freeConn.Len() == 0) {
		// Make the connRequest channel. It's buffered so that the
		// connectionOpener doesn't block while waiting for the req to be read.
		ch := make(chan interface{}, 1)
		req := connRequest(ch)
		db.connRequests.PushBack(req)
		db.maybeOpenNewConnections()
		db.mu.Unlock()
		ret, ok := <-ch
		if !ok {
			return nil, errDBClosed
		}
		switch ret.(type) {
		case *driverConn:
			return ret.(*driverConn), nil
		case error:
			return nil, ret.(error)
		default:
			panic("sql: Unexpected type passed through connRequest.ch")
		}
	}

	if f := db.freeConn.Front(); f != nil {
		conn := f.Value.(*driverConn)
		conn.listElem = nil
		db.freeConn.Remove(f)
		conn.inUse = true
		db.mu.Unlock()
		return conn, nil
	}

	db.mu.Unlock()
	ci, err := db.driver.Open(db.dsn)
	if err != nil {
		return nil, err
	}
	db.mu.Lock()
	db.numOpen++
	dc := &driverConn{
		db: db,
		ci: ci,
	}
	db.addDepLocked(dc, dc)
	dc.inUse = true
	db.mu.Unlock()
	return dc, nil
}

var (
	errConnClosed = errors.New("database/sql: internal sentinel error: conn is closed")
	errConnBusy   = errors.New("database/sql: internal sentinel error: conn is busy")
)

// connIfFree returns (wanted, nil) if wanted is still a valid conn and
// isn't in use.
//
// The error is errConnClosed if the connection if the requested connection
// is invalid because it's been closed.
//
// The error is errConnBusy if the connection is in use.
func (db *DB) connIfFree(wanted *driverConn) (*driverConn, error) {
	db.mu.Lock()
	defer db.mu.Unlock()
	if wanted.dbmuClosed {
		return nil, errConnClosed
	}
	if wanted.inUse {
		return nil, errConnBusy
	}
	if wanted.listElem != nil {
		db.freeConn.Remove(wanted.listElem)
		wanted.listElem = nil
		wanted.inUse = true
		return wanted, nil
	}
	// TODO(bradfitz): shouldn't get here. After Go 1.1, change this to:
	// panic("connIfFree call requested a non-closed, non-busy, non-free conn")
	// Which passes all the tests, but I'm too paranoid to include this
	// late in Go 1.1.
	// Instead, treat it like a busy connection:
	return nil, errConnBusy
}

// putConnHook is a hook for testing.
var putConnHook func(*DB, *driverConn)

// noteUnusedDriverStatement notes that si is no longer used and should
// be closed whenever possible (when c is next not in use), unless c is
// already closed.
func (db *DB) noteUnusedDriverStatement(c *driverConn, si driver.Stmt) {
	db.mu.Lock()
	defer db.mu.Unlock()
	if c.inUse {
		c.onPut = append(c.onPut, func() {
			si.Close()
		})
	} else {
		c.Lock()
		defer c.Unlock()
		if !c.finalClosed {
			si.Close()
		}
	}
}

// debugGetPut determines whether getConn & putConn calls' stack traces
// are returned for more verbose crashes.
const debugGetPut = false

// putConn adds a connection to the db's free pool.
// err is optionally the last error that occurred on this connection.
func (db *DB) putConn(dc *driverConn, err error) {
	db.mu.Lock()
	if !dc.inUse {
		if debugGetPut {
			fmt.Printf("putConn(%v) DUPLICATE was: %s\n\nPREVIOUS was: %s", dc, stack(), db.lastPut[dc])
		}
		panic("sql: connection returned that was never out")
	}
	if debugGetPut {
		db.lastPut[dc] = stack()
	}
	dc.inUse = false

	for _, fn := range dc.onPut {
		fn()
	}
	dc.onPut = nil

	if err == driver.ErrBadConn {
		// Don't reuse bad connections.
		// Since the conn is considered bad and is being discarded, treat it
		// as closed. Don't decrement the open count here, finalClose will
		// take care of that.
		db.maybeOpenNewConnections()
		db.mu.Unlock()
		dc.Close()
		return
	}
	if putConnHook != nil {
		putConnHook(db, dc)
	}
	added := db.putConnDBLocked(dc, nil)
	db.mu.Unlock()

	if !added {
		dc.Close()
	}
}

// Satisfy a connRequest or put the driverConn in the idle pool and return true
// or return false.
// putConnDBLocked will satisfy a connRequest if there is one, or it will
// return the *driverConn to the freeConn list if err != nil and the idle
// connection limit would not be reached.
// If err != nil, the value of dc is ignored.
// If err == nil, then dc must not equal nil.
// If a connRequest was fullfilled or the *driverConn was placed in the
// freeConn list, then true is returned, otherwise false is returned.
func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
	if db.connRequests.Len() > 0 {
		req := db.connRequests.Front().Value.(connRequest)
		db.connRequests.Remove(db.connRequests.Front())
		if err != nil {
			req <- err
		} else {
			dc.inUse = true
			req <- dc
		}
		return true
	} else if err == nil && !db.closed && db.maxIdleConnsLocked() > 0 && db.maxIdleConnsLocked() > db.freeConn.Len() {
		dc.listElem = db.freeConn.PushFront(dc)
		return true
	}
	return false
}

// Prepare creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
func (db *DB) Prepare(query string) (*Stmt, error) {
	var stmt *Stmt
	var err error
	for i := 0; i < 10; i++ {
		stmt, err = db.prepare(query)
		if err != driver.ErrBadConn {
			break
		}
	}
	return stmt, err
}

func (db *DB) prepare(query string) (*Stmt, error) {
	// TODO: check if db.driver supports an optional
	// driver.Preparer interface and call that instead, if so,
	// otherwise we make a prepared statement that's bound
	// to a connection, and to execute this prepared statement
	// we either need to use this connection (if it's free), else
	// get a new connection + re-prepare + execute on that one.
	dc, err := db.conn()
	if err != nil {
		return nil, err
	}
	dc.Lock()
	si, err := dc.prepareLocked(query)
	dc.Unlock()
	if err != nil {
		db.putConn(dc, err)
		return nil, err
	}
	stmt := &Stmt{
		db:    db,
		query: query,
		css:   []connStmt{{dc, si}},
	}
	db.addDep(stmt, stmt)
	db.putConn(dc, nil)
	return stmt, nil
}

// Exec executes a query without returning any rows.
// The args are for any placeholder parameters in the query.
func (db *DB) Exec(query string, args ...interface{}) (Result, error) {
	var res Result
	var err error
	for i := 0; i < 10; i++ {
		res, err = db.exec(query, args)
		if err != driver.ErrBadConn {
			break
		}
	}
	return res, err
}

func (db *DB) exec(query string, args []interface{}) (res Result, err error) {
	dc, err := db.conn()
	if err != nil {
		return nil, err
	}
	defer func() {
		db.putConn(dc, err)
	}()

	if execer, ok := dc.ci.(driver.Execer); ok {
		dargs, err := driverArgs(nil, args)
		if err != nil {
			return nil, err
		}
		dc.Lock()
		resi, err := execer.Exec(query, dargs)
		dc.Unlock()
		if err != driver.ErrSkip {
			if err != nil {
				return nil, err
			}
			return driverResult{dc, resi}, nil
		}
	}

	dc.Lock()
	si, err := dc.ci.Prepare(query)
	dc.Unlock()
	if err != nil {
		return nil, err
	}
	defer withLock(dc, func() { si.Close() })
	return resultFromStatement(driverStmt{dc, si}, args...)
}

// Query executes a query that returns rows, typically a SELECT.
// The args are for any placeholder parameters in the query.
func (db *DB) Query(query string, args ...interface{}) (*Rows, error) {
	var rows *Rows
	var err error
	for i := 0; i < 10; i++ {
		rows, err = db.query(query, args)
		if err != driver.ErrBadConn {
			break
		}
	}
	return rows, err
}

func (db *DB) query(query string, args []interface{}) (*Rows, error) {
	ci, err := db.conn()
	if err != nil {
		return nil, err
	}

	return db.queryConn(ci, ci.releaseConn, query, args)
}

// queryConn executes a query on the given connection.
// The connection gets released by the releaseConn function.
func (db *DB) queryConn(dc *driverConn, releaseConn func(error), query string, args []interface{}) (*Rows, error) {
	if queryer, ok := dc.ci.(driver.Queryer); ok {
		dargs, err := driverArgs(nil, args)
		if err != nil {
			releaseConn(err)
			return nil, err
		}
		dc.Lock()
		rowsi, err := queryer.Query(query, dargs)
		dc.Unlock()
		if err != driver.ErrSkip {
			if err != nil {
				releaseConn(err)
				return nil, err
			}
			// Note: ownership of dc passes to the *Rows, to be freed
			// with releaseConn.
			rows := &Rows{
				dc:          dc,
				releaseConn: releaseConn,
				rowsi:       rowsi,
			}
			return rows, nil
		}
	}

	dc.Lock()
	si, err := dc.ci.Prepare(query)
	dc.Unlock()
	if err != nil {
		releaseConn(err)
		return nil, err
	}

	ds := driverStmt{dc, si}
	rowsi, err := rowsiFromStatement(ds, args...)
	if err != nil {
		dc.Lock()
		si.Close()
		dc.Unlock()
		releaseConn(err)
		return nil, err
	}

	// Note: ownership of ci passes to the *Rows, to be freed
	// with releaseConn.
	rows := &Rows{
		dc:          dc,
		releaseConn: releaseConn,
		rowsi:       rowsi,
		closeStmt:   si,
	}
	return rows, nil
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always return a non-nil value. Errors are deferred until
// Row's Scan method is called.
func (db *DB) QueryRow(query string, args ...interface{}) *Row {
	rows, err := db.Query(query, args...)
	return &Row{rows: rows, err: err}
}

// Begin starts a transaction. The isolation level is dependent on
// the driver.
func (db *DB) Begin() (*Tx, error) {
	var tx *Tx
	var err error
	for i := 0; i < 10; i++ {
		tx, err = db.begin()
		if err != driver.ErrBadConn {
			break
		}
	}
	return tx, err
}

func (db *DB) begin() (tx *Tx, err error) {
	dc, err := db.conn()
	if err != nil {
		return nil, err
	}
	dc.Lock()
	txi, err := dc.ci.Begin()
	dc.Unlock()
	if err != nil {