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.
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.
//
// If the underlying database driver has the concept of a connection
// and per-connection session state, the sql package manages creating
// and freeing connections automatically, including maintaining a free
// pool of idle connections. If observing session state is required,
// either do not share a *DB between multiple concurrent goroutines or
// create and observe all state only within a transaction. Once
// DB.Open is called, the returned Tx is bound to a single isolated
// connection. Once Tx.Commit or Tx.Rollback is called, that
// connection is returned to DB's idle connection pool.
type DB struct {
	driver driver.Driver
	dsn    string

	mu        sync.Mutex           // protects following fields
	outConn   map[driver.Conn]bool // whether the conn is in use
	freeConn  []driver.Conn
	closed    bool
	dep       map[finalCloser]depSet
	onConnPut map[driver.Conn][]func() // code (with mu held) run when conn is next returned
	lastPut   map[driver.Conn]string   // stacktrace of last conn's put; debug only
}

// 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()
	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 {
	//println(fmt.Sprintf("removeDep(%T %p, %T %p)", x, x, dep, dep))
	done := false

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

	if !done {
		return nil
	}
	//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.
//
// 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[driver.Conn]bool),
		lastPut:   make(map[driver.Conn]string),
		onConnPut: make(map[driver.Conn][]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.
	c, err := db.conn()
	if err != nil {
		return err
	}
	db.putConn(c, 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 _, c := range db.freeConn {
		err1 := c.Close()
		if err1 != nil {
			err = err1
		}
	}
	db.freeConn = nil
	db.closed = true
	return err
}

func (db *DB) maxIdleConns() int {
	const defaultMaxIdleConns = 2
	// TODO(bradfitz): ask driver, if supported, for its default preference
	// TODO(bradfitz): let users override?
	return defaultMaxIdleConns
}

// conn returns a newly-opened or cached driver.Conn
func (db *DB) conn() (driver.Conn, 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()
	conn, err := db.driver.Open(db.dsn)
	if err == nil {
		db.mu.Lock()
		db.outConn[conn] = true
		db.mu.Unlock()
	}
	return conn, err
}

// 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 driver.Conn) (conn driver.Conn, 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, driver.Conn)

// 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 driver.Conn, 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(c driver.Conn, err error) {
	db.mu.Lock()
	if !db.outConn[c] {
		if debugGetPut {
			fmt.Printf("putConn(%v) DUPLICATE was: %s\n\nPREVIOUS was: %s", c, stack(), db.lastPut[c])
		}
		panic("sql: connection returned that was never out")
	}
	if debugGetPut {
		db.lastPut[c] = stack()
	}
	delete(db.outConn, c)

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

	if err == driver.ErrBadConn {
		// Don't reuse bad connections.
		db.mu.Unlock()
		return
	}
	if putConnHook != nil {
		putConnHook(db, c)
	}
	if n := len(db.freeConn); !db.closed && n < db.maxIdleConns() {
		db.freeConn = append(db.freeConn, c)
		db.mu.Unlock()
		return
	}
	// TODO: check to see if we need this Conn for any prepared
	// statements which are still active?
	db.mu.Unlock()
	c.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.
	ci, err := db.conn()
	if err != nil {
		return nil, err
	}
	si, err := ci.Prepare(query)
	if err != nil {
		db.putConn(ci, err)
		return nil, err
	}
	stmt := &Stmt{
		db:    db,
		query: query,
		css:   []connStmt{{ci, si}},
	}
	db.addDep(stmt, stmt)
	db.putConn(ci, 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) {
	ci, err := db.conn()
	if err != nil {
		return nil, err
	}
	defer func() {
		db.putConn(ci, err)
	}()

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

	sti, err := ci.Prepare(query)
	if err != nil {
		return nil, err
	}
	defer sti.Close()

	return resultFromStatement(sti, 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(ci driver.Conn, releaseConn func(error), query string, args []interface{}) (*Rows, error) {
	if queryer, ok := ci.(driver.Queryer); ok {
		dargs, err := driverArgs(nil, args)
		if err != nil {
			releaseConn(err)
			return nil, err
		}
		rowsi, err := queryer.Query(query, dargs)
		if err != driver.ErrSkip {
			if err != nil {
				releaseConn(err)
				return nil, err
			}
			// Note: ownership of ci passes to the *Rows, to be freed
			// with releaseConn.
			rows := &Rows{
				db:          db,
				ci:          ci,
				releaseConn: releaseConn,
				rowsi:       rowsi,
			}
			return rows, nil
		}
	}

	sti, err := ci.Prepare(query)
	if err != nil {
		releaseConn(err)
		return nil, err
	}

	rowsi, err := rowsiFromStatement(sti, args...)
	if err != nil {
		releaseConn(err)
		sti.Close()
		return nil, err
	}

	// Note: ownership of ci passes to the *Rows, to be freed
	// with releaseConn.
	rows := &Rows{
		db:          db,
		ci:          ci,
		releaseConn: releaseConn,
		rowsi:       rowsi,
		closeStmt:   sti,
	}
	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) {
	ci, err := db.conn()
	if err != nil {
		return nil, err
	}
	txi, err := ci.Begin()
	if err != nil {
		db.putConn(ci, err)
		return nil, err
	}
	return &Tx{
		db:  db,
		ci:  ci,
		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

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

	// cimu is held while somebody is using ci (between grabConn
	// and releaseConn)
	cimu sync.Mutex

	// 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.ci, nil)
	tx.ci = nil
	tx.txi = nil
}

func (tx *Tx) grabConn() (driver.Conn, error) {
	if tx.done {
		return nil, ErrTxDone
	}
	tx.cimu.Lock()
	return tx.ci, nil
}

func (tx *Tx) releaseConn() {
	tx.cimu.Unlock()
}

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

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
	if tx.done {
		return ErrTxDone
	}
	defer tx.close()
	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.
	ci, err := tx.grabConn()
	if err != nil {
		return nil, err
	}
	defer tx.releaseConn()

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

	stmt := &Stmt{
		db:    tx.db,
		tx:    tx,
		txsi:  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")}
	}
	ci, err := tx.grabConn()
	if err != nil {
		return &Stmt{stickyErr: err}
	}
	defer tx.releaseConn()
	si, err := ci.Prepare(stmt.query)
	return &Stmt{
		db:        tx.db,
		tx:        tx,
		txsi:      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) {
	ci, err := tx.grabConn()
	if err != nil {
		return nil, err
	}
	defer tx.releaseConn()

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

	sti, err := ci.Prepare(query)
	if err != nil {
		return nil, err
	}
	defer sti.Close()

	return resultFromStatement(sti, args...)
}

// Query executes a query that returns rows, typically a SELECT.
func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
	ci, err := tx.grabConn()
	if err != nil {
		return nil, err
	}

	releaseConn := func(err error) { tx.releaseConn() }

	return tx.db.queryConn(ci, 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 {
	ci driver.Conn
	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 driver.Stmt

	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()
	_, releaseConn, si, err := s.connStmt()
	if err != nil {
		return nil, err
	}
	defer releaseConn(nil)

	return resultFromStatement(si, args...)
}

func resultFromStatement(si driver.Stmt, args ...interface{}) (Result, error) {
	// -1 means the driver doesn't know how to count the number of
	// placeholders, so we won't sanity check input here and instead let the
	// driver deal with errors.
	if want := si.NumInput(); want != -1 && len(args) != want {
		return nil, fmt.Errorf("sql: expected %d arguments, got %d", want, len(args))
	}

	dargs, err := driverArgs(si, args)
	if err != nil {
		return nil, err
	}

	resi, err := si.Exec(dargs)
	if err != nil {
		return nil, err
	}
	return result{resi}, nil
}

// connStmt returns a free driver connection on which to execute the
// statement, a function to call to release the connection, and a
// statement bound to that connection.
func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(error), si driver.Stmt, err error) {
	if err = s.stickyErr; err != nil {
		return
	}
	s.mu.Lock()
	if s.closed {
		s.mu.Unlock()
		err = errors.New("sql: statement is closed")
		return
	}

	// In a transaction, we always use the connection that the
	// transaction was created on.
	if s.tx != nil {
		s.mu.Unlock()
		ci, err = s.tx.grabConn() // blocks, waiting for the connection.
		if err != nil {
			return
		}
		releaseConn = func(error) { s.tx.releaseConn() }
		return ci, releaseConn, s.txsi, nil
	}

	var cs connStmt
	match := false
	for _, v := range s.css {
		// TODO(bradfitz): lazily clean up entries in this
		// list with dead conns while enumerating
		if _, match = s.db.connIfFree(v.ci); match {
			cs = v
			break
		}
	}
	s.mu.Unlock()

	// Make a new conn if all are busy.
	// TODO(bradfitz): or wait for one? make configurable later?
	if !match {
		for i := 0; ; i++ {
			ci, err := s.db.conn()
			if err != nil {
				return nil, nil, nil, err
			}
			si, err := ci.Prepare(s.query)
			if err == driver.ErrBadConn && i < 10 {
				continue
			}
			if err != nil {
				return nil, nil, nil, err
			}
			s.mu.Lock()
			cs = connStmt{ci, si}
			s.css = append(s.css, cs)
			s.mu.Unlock()
			break
		}
	}

	conn := cs.ci
	releaseConn = func(err error) { s.db.putConn(conn, err) }
	return conn, releaseConn, cs.si, nil
}

// Query executes a prepared query statement with the given arguments
// and returns the query results as a *Rows.
func (s *Stmt) Query(args ...interface{}) (*Rows, error) {
	s.closemu.RLock()
	defer s.closemu.RUnlock()

	ci, releaseConn, si, err := s.connStmt()
	if err != nil {
		return nil, err
	}

	rowsi, err := rowsiFromStatement(si, args...)
	if err != nil {
		releaseConn(err)
		return nil, err
	}

	// Note: ownership of ci passes to the *Rows, to be freed
	// with releaseConn.
	rows := &Rows{
		db:    s.db,
		ci:    ci,
		rowsi: rowsi,
		// releaseConn set below
	}
	s.db.addDep(s, rows)
	rows.releaseConn = func(err error) {
		releaseConn(err)
		s.db.removeDep(s, rows)
	}
	return rows, nil
}

func rowsiFromStatement(si driver.Stmt, args ...interface{}) (driver.Rows, error) {
	// -1 means the driver doesn't know how to count the number of
	// placeholders, so we won't sanity check input here and instead let the
	// driver deal with errors.
	if want := si.NumInput(); want != -1 && len(args) != want {
		return nil, fmt.Errorf("sql: statement expects %d inputs; got %d", si.NumInput(), len(args))
	}

	dargs, err := driverArgs(si, args)
	if err != nil {
		return nil, err
	}