text: // 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.
  //
  // For more usage examples, see the wiki page at
  // http://golang.org/s/sqlwiki.
  package sql
  
  import (
  	"database/sql/driver"
  	"errors"
  	"fmt"
  	"io"
  	"runtime"
  	"sort"
  	"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
  }
  
  func unregisterAllDrivers() {
  	// For tests.
  	drivers = make(map[string]driver.Driver)
  }
  
  // Drivers returns a sorted list of the names of the registered drivers.
  func Drivers() []string {
  	var list []string
  	for name := range drivers {
  		list = append(list, name)
  	}
  	sort.Strings(list)
  	return list
  }
  
  // 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 representing a pool of zero or more
  // underlying connections. 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     []*driverConn
  	connRequests []chan connRequest
  	numOpen      int
  	pendingOpens int
  	// Used to signal 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
  }
  
  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 1.2, 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 satisfy 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.
  //
  // The returned DB is safe for concurrent use by multiple goroutines
  // and maintains its own pool of idle connections. Thus, the Open
  // function should be called just once. It is rarely necessary to
  // close a DB.
  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),
  	}
  	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.
  //
  // It is rare to Close a DB, as the DB handle is meant to be
  // long-lived and shared between many goroutines.
  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, len(db.freeConn))
  	for _, dc := range db.freeConn {
  		fns = append(fns, dc.closeDBLocked())
  	}
  	db.freeConn = nil
  	db.closed = true
  	for _, req := range db.connRequests {
  		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
  	idleCount := len(db.freeConn)
  	maxIdle := db.maxIdleConnsLocked()
  	if idleCount > maxIdle {
  		closing = db.freeConn[maxIdle:]
  		db.freeConn = db.freeConn[:maxIdle]
  	}
  	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 := len(db.connRequests) - 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 separate 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 struct {
  	conn *driverConn
  	err  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
  	// and there are no free connection, make a request and wait.
  	if db.maxOpen > 0 && db.numOpen >= db.maxOpen && len(db.freeConn) == 0 {
  		// Make the connRequest channel. It's buffered so that the
  		// connectionOpener doesn't block while waiting for the req to be read.
  		req := make(chan connRequest, 1)
  		db.connRequests = append(db.connRequests, req)
  		db.maybeOpenNewConnections()
  		db.mu.Unlock()
  		ret := <-req
  		return ret.conn, ret.err
  	}
  
  	if c := len(db.freeConn); c > 0 {
  		conn := db.freeConn[0]
  		copy(db.freeConn, db.freeConn[1:])
  		db.freeConn = db.freeConn[:c-1]
  		conn.inUse = true
  		db.mu.Unlock()
  		return conn, nil
  	}
  
  	db.numOpen++ // optimistically
  	db.mu.Unlock()
  	ci, err := db.driver.Open(db.dsn)
  	if err != nil {
  		db.mu.Lock()
  		db.numOpen-- // correct for earlier optimism
  		db.mu.Unlock()
  		return nil, err
  	}
  	db.mu.Lock()
  	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
  	}
  	idx := -1
  	for ii, v := range db.freeConn {
  		if v == wanted {
  			idx = ii
  			break
  		}
  	}
  	if idx >= 0 {
  		db.freeConn = append(db.freeConn[:idx], db.freeConn[idx+1:]...)
  		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 will not be exceeded.
  // If err != nil, the value of dc is ignored.
  // If err == nil, then dc must not equal nil.
  // If a connRequest was fulfilled 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 c := len(db.connRequests); c > 0 {
  		req := db.connRequests[0]
  		// This copy is O(n) but in practice faster than a linked list.
  		// TODO: consider compacting it down less often and
  		// moving the base instead?
  		copy(db.connRequests, db.connRequests[1:])
  		db.connRequests = db.connRequests[:c-1]
  		if err == nil {
  			dc.inUse = true
  		}
  		req <- connRequest{
  			conn: dc,
  			err:  err,
  		}
  		return true
  	} else if err == nil && !db.closed && db.maxIdleConnsLocked() > len(db.freeConn) {
  		db.freeConn = append(db.freeConn, dc)
  		return true
  	}
  	return false
  }
  
  // maxBadConnRetries is the number of maximum retries if the driver returns
  // driver.ErrBadConn to signal a broken connection.
  const maxBadConnRetries = 10
  
  // 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 < maxBadConnRetries; 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 < maxBadConnRetries; 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 < maxBadConnRetries; 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 < maxBadConnRetries; 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
  
  	// All Stmts prepared for this transaction.  These will be closed after the
  	// transaction has been committed or rolled back.
  	stmts struct {
  		sync.Mutex
  		v []*Stmt
  	}
  }
  
  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
  }
  
  // Closes all Stmts prepared for this transaction.
  func (tx *Tx) closePrepared() {
  	tx.stmts.Lock()
  	for _, stmt := range tx.stmts.v {
  		stmt.Close()
  	}
  	tx.stmts.Unlock()
  }
  
  // Commit commits the transaction.
  func (tx *Tx) Commit() error {
  	if tx.done {
  		return ErrTxDone
  	}
  	defer tx.close()
  	tx.dc.Lock()
  	err := tx.txi.Commit()
  	tx.dc.Unlock()
  	if err != driver.ErrBadConn {
  		tx.closePrepared()
  	}
  	return err
  }
  
  // Rollback aborts the transaction.
  func (tx *Tx) Rollback() error {
  	if tx.done {
  		return ErrTxDone
  	}
  	defer tx.close()
  	tx.dc.Lock()
  	err := tx.txi.Rollback()
  	tx.dc.Unlock()
  	if err != driver.ErrBadConn {
  		tx.closePrepared()
  	}
  	return err
  }
  
  // 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,
  	}
  	tx.stmts.Lock()
  	tx.stmts.v = append(tx.stmts.v, stmt)
  	tx.stmts.Unlock()
  	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()
  	txs := &Stmt{
  		db: tx.db,
  		tx: tx,
  		txsi: &driverStmt{
  			Locker: dc,
  			si:     si,
  		},
  		query:     stmt.query,
  		stickyErr: err,
  	}
  	tx.stmts.Lock()
  	tx.stmts.v = append(tx.stmts.v, txs)
  	tx.stmts.Unlock()
  	return txs
  }
  
  // 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()
  
  	var res Result
  	for i := 0; i < maxBadConnRetries; i++ {
  		dc, releaseConn, si, err := s.connStmt()
  		if err != nil {
  			if err == driver.ErrBadConn {
  				continue
  			}
  			return nil, err
  		}
  
  		res, err = resultFromStatement(driverStmt{dc, si}, args...)
  		releaseConn(err)
  		if err != driver.ErrBadConn {
  			return res, err
  		}
  	}
  	return nil, driver.ErrBadConn
  }
  
  func resultFromStatement(ds driverStmt, args ...interface{}) (Result, error) {
  	ds.Lock()
  	want := ds.si.NumInput()
  	ds.Unlock()
  
  	// -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 != -1 && len(args) != want {
  		return nil, fmt.Errorf("sql: expected %d arguments, got %d", want, len(args))
  	}
  
  	dargs, err := driverArgs(&ds, args)
  	if err != nil {
  		return nil, err
  	}
  
  	ds.Lock()
  	resi, err := ds.si.Exec(dargs)
  	ds.Unlock()
  	if err != nil {
  		return nil, err
  	}
  	return driverResult{ds.Locker, 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 *driverConn, 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) {}
  		return ci, releaseConn, s.txsi.si, nil
  	}
  
  	for i := 0; i < len(s.css); i++ {
  		v := s.css[i]
  		_, err := s.db.connIfFree(v.dc)
  		if err == nil {
  			s.mu.Unlock()
  			return v.dc, v.dc.releaseConn, v.si, nil
  		}
  		if err == errConnClosed {
  			// Lazily remove dead conn from our freelist.
  			s.css[i] = s.css[len(s.css)-1]
  			s.css = s.css[:len(s.css)-1]
  			i--
  		}
  
  	}
  	s.mu.Unlock()
  
  	// If all connections are busy, either wait for one to become available (if
  	// we've already hit the maximum number of open connections) or create a
  	// new one.
  	//
  	// TODO(bradfitz): or always wait for one? make configurable later?
  	dc, err := s.db.conn()
  	if err != nil {
  		return nil, nil, nil, err
  	}
  
  	// Do another pass over the list to see whether this statement has
  	// already been prepared on the connection assigned to us.
  	s.mu.Lock()
  	for _, v := range s.css {
  		if v.dc == dc {
  			s.mu.Unlock()
  			return dc, dc.releaseConn, v.si, nil
  		}
  	}
  	s.mu.Unlock()
  
  	// No luck; we need to prepare the statement on this connection
  	dc.Lock()
  	si, err = dc.prepareLocked(s.query)
  	dc.Unlock()
  	if err != nil {
  		s.db.putConn(dc, err)
  		return nil, nil, nil, err
  	}
  	s.mu.Lock()
  	cs := connStmt{dc, si}
  	s.css = append(s.css, cs)
  	s.mu.Unlock()
  
  	return dc, dc.releaseConn, 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()
  
  	var rowsi driver.Rows
  	for i := 0; i < maxBadConnRetries; i++ {
  		dc, releaseConn, si, err := s.connStmt()
  		if err != nil {
  			if err == driver.ErrBadConn {
  				continue
  			}
  			return nil, err
  		}
  
  		rowsi, err = rowsiFromStatement(driverStmt{dc, si}, args...)
  		if err == nil {
  			// Note: ownership of ci passes to the *Rows, to be freed
  			// with releaseConn.
  			rows := &Rows{
  				dc:    dc,
  				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
  		}
  
  		releaseConn(err)
  		if err != driver.ErrBadConn {
  			return nil, err
  		}
  	}
  	return nil, driver.ErrBadConn
  }
  
  func rowsiFromStatement(ds driverStmt, args ...interface{}) (driver.Rows, error) {
  	ds.Lock()
  	want := ds.si.NumInput()
  	ds.Unlock()
  
  	// -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 != -1 && len(args) != want {
  		return nil, fmt.Errorf("sql: statement expects %d inputs; got %d", want, len(args))
  	}
  
  	dargs, err := driverArgs(&ds, args)
  	if err != nil {
  		return nil, err
  	}
  
  	ds.Lock()
  	rowsi, err := ds.si.Query(dargs)
  	ds.Unlock()
  	if err != nil {
  		return nil, err
  	}
  	return rowsi, nil
  }
  
  // QueryRow executes a prepared query statement with the given arguments.
  // If an error occurs during the execution of the statement, that error will
  // be returned by a call to Scan on the returned *Row, which is always non-nil.
  // If the query selects no rows, the *Row's Scan will return ErrNoRows.
  // Otherwise, the *Row's Scan scans the first selected row and discards
  // the rest.
  //
  // Example usage:
  //
  //  var name string
  //  err := nameByUseridStmt.QueryRow(id).Scan(&name)
  func (s *Stmt) QueryRow(args ...interface{}) *Row {
  	rows, err := s.Query(args...)
  	if err != nil {
  		return &Row{err: err}
  	}
  	return &Row{rows: rows}
  }
  
  // Close closes the statement.
  func (s *Stmt) Close() error {
  	s.closemu.Lock()
  	defer s.closemu.Unlock()
  
  	if s.stickyErr != nil {
  		return s.stickyErr
  	}
  	s.mu.Lock()
  	if s.closed {
  		s.mu.Unlock()
  		return nil
  	}
  	s.closed = true
  
  	if s.tx != nil {
  		s.txsi.Close()
  		s.mu.Unlock()
  		return nil
  	}
  	s.mu.Unlock()
  
  	return s.db.removeDep(s, s)
  }
  
  func (s *Stmt) finalClose() error {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	if s.css != nil {
  		for _, v := range s.css {
  			s.db.noteUnusedDriverStatement(v.dc, v.si)
  			v.dc.removeOpenStmt(v.si)
  		}
  		s.css = nil
  	}
  	return nil
  }
  
  // Rows is the result of a query. Its cursor starts before the first row
  // of the result set. Use Next to advance through the rows:
  //
  //     rows, err := db.Query("SELECT ...")
  //     ...
  //     defer rows.Close()
  //     for rows.Next() {
  //         var id int
  //         var name string
  //         err = rows.Scan(&id, &name)
  //         ...
  //     }
  //     err = rows.Err() // get any error encountered during iteration
  //     ...
  type Rows struct {
  	dc          *driverConn // owned; must call releaseConn when closed to release
  	releaseConn func(error)
  	rowsi       driver.Rows
  
  	closed    bool
  	lastcols  []driver.Value
  	lasterr   error       // non-nil only if closed is true
  	closeStmt driver.Stmt // if non-nil, statement to Close on close
  }
  
  // Next prepares the next result row for reading with the Scan method.  It
  // returns true on success, or false if there is no next result row or an error
  // happened while preparing it.  Err should be consulted to distinguish between
  // the two cases.
  //
  // Every call to Scan, even the first one, must be preceded by a call to Next.
  func (rs *Rows) Next() bool {
  	if rs.closed {
  		return false
  	}
  	if rs.lastcols == nil {
  		rs.lastcols = make([]driver.Value, len(rs.rowsi.Columns()))
  	}
  	rs.lasterr = rs.rowsi.Next(rs.lastcols)
  	if rs.lasterr != nil {
  		rs.Close()
  		return false
  	}
  	return true
  }
  
  // Err returns the error, if any, that was encountered during iteration.
  // Err may be called after an explicit or implicit Close.
  func (rs *Rows) Err() error {
  	if rs.lasterr == io.EOF {
  		return nil
  	}
  	return rs.lasterr
  }
  
  // Columns returns the column names.
  // Columns returns an error if the rows are closed, or if the rows
  // are from QueryRow and there was a deferred error.
  func (rs *Rows) Columns() ([]string, error) {
  	if rs.closed {
  		return nil, errors.New("sql: Rows are closed")
  	}
  	if rs.rowsi == nil {
  		return nil, errors.New("sql: no Rows available")
  	}
  	return rs.rowsi.Columns(), nil
  }
  
  // Scan copies the columns in the current row into the values pointed
  // at by dest.
  //
  // If an argument has type *[]byte, Scan saves in that argument a copy
  // of the corresponding data. The copy is owned by the caller and can
  // be modified and held indefinitely. The copy can be avoided by using
  // an argument of type *RawBytes instead; see the documentation for
  // RawBytes for restrictions on its use.
  //
  // If an argument has type *interface{}, Scan copies the value
  // provided by the underlying driver without conversion. If the value
  // is of type []byte, a copy is made and the caller owns the result.
  func (rs *Rows) Scan(dest ...interface{}) error {
  	if rs.closed {
  		return errors.New("sql: Rows are closed")
  	}
  	if rs.lastcols == nil {
  		return errors.New("sql: Scan called without calling Next")
  	}
  	if len(dest) != len(rs.lastcols) {
  		return fmt.Errorf("sql: expected %d destination arguments in Scan, not %d", len(rs.lastcols), len(dest))
  	}
  	for i, sv := range rs.lastcols {
  		err := convertAssign(dest[i], sv)
  		if err != nil {
  			return fmt.Errorf("sql: Scan error on column index %d: %v", i, err)
  		}
  	}
  	return nil
  }
  
  var rowsCloseHook func(*Rows, *error)
  
  // Close closes the Rows, preventing further enumeration. If Next returns
  // false, the Rows are closed automatically and it will suffice to check the
  // result of Err. Close is idempotent and does not affect the result of Err.
  func (rs *Rows) Close() error {
  	if rs.closed {
  		return nil
  	}
  	rs.closed = true
  	err := rs.rowsi.Close()
  	if fn := rowsCloseHook; fn != nil {
  		fn(rs, &err)
  	}
  	if rs.closeStmt != nil {
  		rs.closeStmt.Close()
  	}
  	rs.releaseConn(err)
  	return err
  }
  
  // Row is the result of calling QueryRow to select a single row.
  type Row struct {
  	// One of these two will be non-nil:
  	err  error // deferred error for easy chaining
  	rows *Rows
  }
  
  // Scan copies the columns from the matched row into the values
  // pointed at by dest.  If more than one row matches the query,
  // Scan uses the first row and discards the rest.  If no row matches
  // the query, Scan returns ErrNoRows.
  func (r *Row) Scan(dest ...interface{}) error {
  	if r.err != nil {
  		return r.err
  	}
  
  	// TODO(bradfitz): for now we need to defensively clone all
  	// []byte that the driver returned (not permitting
  	// *RawBytes in Rows.Scan), since we're about to close
  	// the Rows in our defer, when we return from this function.
  	// the contract with the driver.Next(...) interface is that it
  	// can return slices into read-only temporary memory that's
  	// only valid until the next Scan/Close.  But the TODO is that
  	// for a lot of drivers, this copy will be unnecessary.  We
  	// should provide an optional interface for drivers to
  	// implement to say, "don't worry, the []bytes that I return
  	// from Next will not be modified again." (for instance, if
  	// they were obtained from the network anyway) But for now we
  	// don't care.
  	defer r.rows.Close()
  	for _, dp := range dest {
  		if _, ok := dp.(*RawBytes); ok {
  			return errors.New("sql: RawBytes isn't allowed on Row.Scan")
  		}
  	}
  
  	if !r.rows.Next() {
  		if err := r.rows.Err(); err != nil {
  			return err
  		}
  		return ErrNoRows
  	}
  	err := r.rows.Scan(dest...)
  	if err != nil {
  		return err
  	}
  	// Make sure the query can be processed to completion with no errors.
  	if err := r.rows.Close(); err != nil {
  		return err
  	}
  
  	return nil
  }
  
  // A Result summarizes an executed SQL command.
  type Result interface {
  	// LastInsertId returns the integer generated by the database
  	// in response to a command. Typically this will be from an
  	// "auto increment" column when inserting a new row. Not all
  	// databases support this feature, and the syntax of such
  	// statements varies.
  	LastInsertId() (int64, error)
  
  	// RowsAffected returns the number of rows affected by an
  	// update, insert, or delete. Not every database or database
  	// driver may support this.
  	RowsAffected() (int64, error)
  }
  
  type driverResult struct {
  	sync.Locker // the *driverConn
  	resi        driver.Result
  }
  
  func (dr driverResult) LastInsertId() (int64, error) {
  	dr.Lock()
  	defer dr.Unlock()
  	return dr.resi.LastInsertId()
  }
  
  func (dr driverResult) RowsAffected() (int64, error) {
  	dr.Lock()
  	defer dr.Unlock()
  	return dr.resi.RowsAffected()
  }
  
  func stack() string {
  	var buf [2 << 10]byte
  	return string(buf[:runtime.Stack(buf[:], false)])
  }
  
  // withLock runs while holding lk.
  func withLock(lk sync.Locker, fn func()) {
  	lk.Lock()
  	fn()
  	lk.Unlock()
  }
  