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 (
	"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
	outConn   map[*driverConn]bool // whether the conn is in use
	freeConn  []*driverConn
	closed    bool
	dep       map[finalCloser]depSet
	onConnPut map[*driverConn][]func() // code (with mu held) run when conn is next returned
	lastPut   map[*driverConn]string   // stacktrace of last conn's put; debug only
	maxIdle   int                      // zero means defaultMaxIdleConns; negative means 0
}

// 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
}

// the dc.db's Mutex is held.
func (dc *driverConn) closeDBLocked() 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
	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
	return dc.db.removeDep(dc, dc)
}

func (dc *driverConn) finalClose() error {
	dc.Lock()
	err := dc.ci.Close()
	dc.ci = nil
	dc.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 (*DB).get
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))
	done := false

	xdep := db.dep[x]
	if xdep != nil {
		delete(xdep, dep)
		if len(xdep) == 0 {
			delete(db.dep, x)
			done = true
		}
	}

	if !done {
		return func() error { return nil }
	}
	return func() error {
		//println(fmt.Sprintf("calling final close on %T %v (%#v)", x, x, x))
		return x.finalClose()
	}
}

// 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,
		outConn:   make(map[*driverConn]bool),
		lastPut:   make(map[*driverConn]string),
		onConnPut: make(map[*driverConn][]func()),
	}
	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()
	defer db.mu.Unlock()
	var err error
	for _, dc := range db.freeConn {
		err1 := dc.closeDBLocked()
		if err1 != nil {
			err = err1
		}
	}
	db.freeConn = nil
	db.closed = true
	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 n <= 0, no idle connections are retained.
func (db *DB) SetMaxIdleConns(n int) {
	db.mu.Lock()
	defer db.mu.Unlock()
	if n > 0 {
		db.maxIdle = n
	} else {
		// No idle connections.
		db.maxIdle = -1
	}
	for len(db.freeConn) > 0 && len(db.freeConn) > n {
		nfree := len(db.freeConn)
		dc := db.freeConn[nfree-1]
		db.freeConn[nfree-1] = nil
		db.freeConn = db.freeConn[:nfree-1]
		go dc.Close()
	}
}

// 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, errors.New("sql: database is closed")
	}
	if n := len(db.freeConn); n > 0 {
		conn := db.freeConn[n-1]
		db.freeConn = db.freeConn[:n-1]
		db.outConn[conn] = true
		db.mu.Unlock()
		return conn, nil
	}
	db.mu.Unlock()

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

// connIfFree returns (wanted, true) if wanted is still a valid conn and
// isn't in use.
//
// If wanted is valid but in use, connIfFree returns (wanted, false).
// If wanted is invalid, connIfFre returns (nil, false).
func (db *DB) connIfFree(wanted *driverConn) (conn *driverConn, ok bool) {
	db.mu.Lock()
	defer db.mu.Unlock()
	if db.outConn[wanted] {
		return conn, false
	}
	for i, conn := range db.freeConn {
		if conn != wanted {
			continue
		}
		db.freeConn[i] = db.freeConn[len(db.freeConn)-1]
		db.freeConn = db.freeConn[:len(db.freeConn)-1]
		db.outConn[wanted] = true
		return wanted, true
	}
	return nil, false
}

// 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 db.outConn[c] {
		db.onConnPut[c] = append(db.onConnPut[c], func() {
			si.Close()
		})
	} else {
		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 !db.outConn[dc] {
		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()
	}
	delete(db.outConn, dc)

	if fns, ok := db.onConnPut[dc]; ok {
		for _, fn := range fns {
			fn()
		}
		delete(db.onConnPut, dc)
	}

	if err == driver.ErrBadConn {
		// Don't reuse bad connections.
		db.mu.Unlock()
		return
	}
	if putConnHook != nil {
		putConnHook(db, dc)
	}
	if n := len(db.freeConn); !db.closed && n < db.maxIdleConnsLocked() {
		db.freeConn = append(db.freeConn, dc)
		db.mu.Unlock()
		return
	}
	// TODO: check to see if we need this Conn for any prepared
	// statements which are still active?
	db.mu.Unlock()

	dc.Close()
}

// 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.ci.Prepare(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.addDep(dc, 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
	}

	releaseConn := func(err error) { db.putConn(ci, err) }

	return db.queryConn(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{
				db:          db,
				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 {
		releaseConn(err)
		dc.Lock()
		si.Close()
		dc.Unlock()
		return nil, err
	}

	// Note: ownership of ci passes to the *Rows, to be freed
	// with releaseConn.
	rows := &Rows{
		db:          db,
		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 {
		db.putConn(dc, err)
		return nil, err
	}
	return &Tx{
		db:  db,
		dc:  dc,
		txi: txi,
	}, nil
}

// Driver returns the database's underlying driver.
func (db *DB) Driver() driver.Driver {
	return db.driver
}

// Tx is an in-progress database transaction.
//
// A transaction must end with a call to Commit or Rollback.
//
// After a call to Commit or Rollback, all operations on the
// transaction fail with ErrTxDone.
type Tx struct {
	db *DB

	// dc is owned exclusively until Commit or Rollback, at which point
	// it's returned with putConn.
	dc  *driverConn
	txi driver.Tx

	// done transitions from false to true exactly once, on Commit
	// or Rollback. once done, all operations fail with
	// ErrTxDone.
	done bool
}

var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")

func (tx *Tx) close() {
	if tx.done {
		panic("double close") // internal error
	}
	tx.done = true
	tx.db.putConn(tx.dc, nil)
	tx.dc = nil
	tx.txi = nil
}

func (tx *Tx) grabConn() (*driverConn, error) {
	if tx.done {
		return nil, ErrTxDone
	}
	return tx.dc, nil
}

// Commit commits the transaction.
func (tx *Tx) Commit() error {
	if tx.done {
		return ErrTxDone
	}
	defer tx.close()
	tx.dc.Lock()
	defer tx.dc.Unlock()
	return tx.txi.Commit()
}

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
	if tx.done {
		return ErrTxDone
	}
	defer tx.close()
	tx.dc.Lock()
	defer tx.dc.Unlock()
	return tx.txi.Rollback()
}

// Prepare creates a prepared statement for use within a transaction.
//
// The returned statement operates within the transaction and can no longer
// be used once the transaction has been committed or rolled back.
//
// To use an existing prepared statement on this transaction, see Tx.Stmt.
func (tx *Tx) Prepare(query string) (*Stmt, error) {
	// TODO(bradfitz): We could be more efficient here and either
	// provide a method to take an existing Stmt (created on
	// perhaps a different Conn), and re-create it on this Conn if
	// necessary. Or, better: keep a map in DB of query string to
	// Stmts, and have Stmt.Execute do the right thing and
	// re-prepare if the Conn in use doesn't have that prepared
	// statement.  But we'll want to avoid caching the statement
	// in the case where we only call conn.Prepare implicitly
	// (such as in db.Exec or tx.Exec), but the caller package
	// can't be holding a reference to the returned statement.
	// Perhaps just looking at the reference count (by noting
	// Stmt.Close) would be enough. We might also want a finalizer
	// on Stmt to drop the reference count.
	dc, err := tx.grabConn()
	if err != nil {
		return nil, err
	}

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

	stmt := &Stmt{
		db: tx.db,
		tx: tx,
		txsi: &driverStmt{
			Locker: dc,
			si:     si,
		},
		query: query,
	}
	return stmt, nil
}

// Stmt returns a transaction-specific prepared statement from
// an existing statement.
//
// Example:
//  updateMoney, err := db.Prepare("UPDATE balance SET money=money+? WHERE id=?")
//  ...
//  tx, err := db.Begin()
//  ...
//  res, err := tx.Stmt(updateMoney).Exec(123.45, 98293203)
func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
	// TODO(bradfitz): optimize this. Currently this re-prepares
	// each time.  This is fine for now to illustrate the API but
	// we should really cache already-prepared statements
	// per-Conn. See also the big comment in Tx.Prepare.

	if tx.db != stmt.db {
		return &Stmt{stickyErr: errors.New("sql: Tx.Stmt: statement from different database used")}
	}
	dc, err := tx.grabConn()
	if err != nil {
		return &Stmt{stickyErr: err}
	}
	dc.Lock()
	si, err := dc.ci.Prepare(stmt.query)
	dc.Unlock()
	return &Stmt{
		db: tx.db,
		tx: tx,
		txsi: &driverStmt{
			Locker: dc,
			si:     si,
		},
		query:     stmt.query,
		stickyErr: err,
	}
}

// Exec executes a query that doesn't return rows.
// For example: an INSERT and UPDATE.
func (tx *Tx) Exec(query string, args ...interface{}) (Result, error) {
	dc, err := tx.grabConn()
	if err != nil {
		return nil, 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 == nil {
			return driverResult{dc, resi}, nil
		}
		if err != driver.ErrSkip {
			return nil, err
		}
	}

	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.
func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
	dc, err := tx.grabConn()
	if err != nil {
		return nil, err
	}
	releaseConn := func(error) {}
	return tx.db.queryConn(dc, releaseConn, query, args)
}

// 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 (tx *Tx) QueryRow(query string, args ...interface{}) *Row {
	rows, err := tx.Query(query, args...)
	return &Row{rows: rows, err: err}
}

// connStmt is a prepared statement on a particular connection.
type connStmt struct {
	dc *driverConn
	si driver.Stmt
}

// Stmt is a prepared statement. Stmt is safe for concurrent use by multiple goroutines.
type Stmt struct {
	// Immutable:
	db        *DB    // where we came from
	query     string // that created the Stmt
	stickyErr error  // if non-nil, this error is returned for all operations

	closemu sync.RWMutex // held exclusively during close, for read otherwise.

	// If in a transaction, else both nil:
	tx   *Tx
	txsi *driverStmt

	mu     sync.Mutex // protects the rest of the fields
	closed bool

	// css is a list of underlying driver statement interfaces
	// that are valid on particular connections.  This is only
	// used if tx == nil and one is found that has idle
	// connections.  If tx != nil, txsi is always used.
	css []connStmt
}

// Exec executes a prepared statement with the given arguments and
// returns a Result summarizing the effect of the statement.
func (s *Stmt) Exec(args ...interface{}) (Result, error) {
	s.closemu.RLock()
	defer s.closemu.RUnlock()
	dc, releaseConn, si, err := s.connStmt()
	if err != nil {
		return nil, err
	}
	defer releaseConn(nil)

	return resultFromStatement(driverStmt{dc, si}, args...)
}

func resultFromStatement(ds driverStmt, args ...interface{}) (Result, error) {