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// 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"
        "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 ScannerInto 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
}

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

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

// NullInt64 represents an int64 that may be null.
// NullInt64 implements the ScannerInto 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
}

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

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

// NullFloat64 represents a float64 that may be null.
// NullFloat64 implements the ScannerInto 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
}

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

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

// NullBool represents a bool that may be null.
// NullBool implements the ScannerInto 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
}

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

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

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

        mu       sync.Mutex // protects freeConn and closed
        freeConn []driver.Conn
        closed   bool
}

// 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.
func Open(driverName, dataSourceName string) (*DB, error) {
        driver, ok := drivers[driverName]
        if !ok {
                return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
        }
        return &DB{driver: driver, dsn: dataSourceName}, 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.mu.Unlock()
                return conn, nil
        }
        db.mu.Unlock()
        return db.driver.Open(db.dsn)
}

func (db *DB) connIfFree(wanted driver.Conn) (conn driver.Conn, ok bool) {
        db.mu.Lock()
        defer db.mu.Unlock()
        for n, conn := range db.freeConn {
                if conn == wanted {
                        db.freeConn[n] = db.freeConn[len(db.freeConn)-1]
                        db.freeConn = db.freeConn[:len(db.freeConn)-1]
                        return wanted, true
                }
        }
        return nil, false
}

func (db *DB) putConn(c driver.Conn) {
        db.mu.Lock()
        defer db.mu.Unlock()
        if n := len(db.freeConn); !db.closed && n < db.maxIdleConns() {
                db.freeConn = append(db.freeConn, c)
                return
        }
        db.closeConn(c) // TODO(bradfitz): release lock before calling this?
}

func (db *DB) closeConn(c driver.Conn) {
        // TODO: check to see if we need this Conn for any prepared statements
        // that are active.
        c.Close()
}

// Prepare creates a prepared statement for later execution.
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
        }
        defer db.putConn(ci)
        si, err := ci.Prepare(query)
        if err != nil {
                return nil, err
        }
        stmt := &Stmt{
                db:    db,
                query: query,
                css:   []connStmt{{ci, si}},
        }
        return stmt, nil
}

// Exec executes a query without returning any rows.
func (db *DB) Exec(query string, args ...interface{}) (Result, error) {
        sargs, err := subsetTypeArgs(args)
        if err != nil {
                return nil, err
        }

        ci, err := db.conn()
        if err != nil {
                return nil, err
        }
        defer db.putConn(ci)

        if execer, ok := ci.(driver.Execer); ok {
                resi, err := execer.Exec(query, sargs)
                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()

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

// Query executes a query that returns rows, typically a SELECT.
func (db *DB) Query(query string, args ...interface{}) (*Rows, error) {
        stmt, err := db.Prepare(query)
        if err != nil {
                return nil, err
        }
        rows, err := stmt.Query(args...)
        if err != nil {
                stmt.Close()
                return nil, err
        }
        rows.closeStmt = stmt
        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) {
        ci, err := db.conn()
        if err != nil {
                return nil, err
        }
        txi, err := ci.Begin()
        if err != nil {
                db.putConn(ci)
                return nil, fmt.Errorf("sql: failed to Begin transaction: %v", err)
        }
        return &Tx{
                db:  db,
                ci:  ci,
                txi: txi,
        }, nil
}

// DriverDatabase 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 ErrTransactionFinished.
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
        // ErrTransactionFinished.
        done bool
}

var ErrTransactionFinished = 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)
        tx.ci = nil
        tx.txi = nil
}

func (tx *Tx) grabConn() (driver.Conn, error) {
        if tx.done {
                return nil, ErrTransactionFinished
        }
        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 ErrTransactionFinished
        }
        defer tx.close()
        return tx.txi.Commit()
}

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
        if tx.done {
                return ErrTransactionFinished
        }
        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 {
                resi, err := execer.Exec(query, args)
                if err != nil {
                        return nil, err
                }
                return result{resi}, nil
        }

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

        sargs, err := subsetTypeArgs(args)
        if err != nil {
                return nil, err
        }

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

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

// 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

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

        // -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))
        }

        // Convert args to subset types.
        if cc, ok := si.(driver.ColumnConverter); ok {
                for n, arg := range args {
                        // First, see if the value itself knows how to convert
                        // itself to a driver type.  For example, a NullString
                        // struct changing into a string or nil.
                        if svi, ok := arg.(driver.SubsetValuer); ok {
                                sv, err := svi.SubsetValue()
                                if err != nil {
                                        return nil, fmt.Errorf("sql: argument index %d from SubsetValue: %v", n, err)
                                }
                                if !driver.IsParameterSubsetType(sv) {
                                        return nil, fmt.Errorf("sql: argument index %d: non-subset type %T returned from SubsetValue", n, sv)
                                }
                                arg = sv
                        }

                        // Second, ask the column to sanity check itself. For
                        // example, drivers might use this to make sure that
                        // an int64 values being inserted into a 16-bit
                        // integer field is in range (before getting
                        // truncated), or that a nil can't go into a NOT NULL
                        // column before going across the network to get the
                        // same error.
                        args[n], err = cc.ColumnConverter(n).ConvertValue(arg)
                        if err != nil {
                                return nil, fmt.Errorf("sql: converting Exec argument #%d's type: %v", n, err)
                        }
                        if !driver.IsParameterSubsetType(args[n]) {
                                return nil, fmt.Errorf("sql: driver ColumnConverter error converted %T to unsupported type %T",
                                        arg, args[n])
                        }
                }
        } else {
                for n, arg := range args {
                        args[n], err = driver.DefaultParameterConverter.ConvertValue(arg)
                        if err != nil {
                                return nil, fmt.Errorf("sql: converting Exec argument #%d's type: %v", n, err)
                        }
                }
        }

        resi, err := si.Exec(args)
        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(), 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() { 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(cs.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 {
                ci, err := s.db.conn()
                if err != nil {
                        return nil, nil, nil, err
                }
                si, err := ci.Prepare(s.query)
                if err != nil {
                        return nil, nil, nil, err
                }
                s.mu.Lock()
                cs = connStmt{ci, si}
                s.css = append(s.css, cs)
                s.mu.Unlock()
        }

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

        // -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))
        }
        sargs, err := subsetTypeArgs(args)
        if err != nil {
                return nil, err
        }
        rowsi, err := si.Query(sargs)
        if err != nil {
                s.db.putConn(ci)
                return nil, err
        }
        // Note: ownership of ci passes to the *Rows, to be freed
        // with releaseConn.
        rows := &Rows{
                db:          s.db,
                ci:          ci,
                releaseConn: releaseConn,
                rowsi:       rowsi,
        }
        return rows, 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(&s)
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 {
        if s.stickyErr != nil {
                return s.stickyErr
        }
        s.mu.Lock()
        defer s.mu.Unlock()
        if s.closed {
                return nil
        }
        s.closed = true

        if s.tx != nil {
                s.txsi.Close()
        } else {
                for _, v := range s.css {
                        if ci, match := s.db.connIfFree(v.ci); match {
                                v.si.Close()
                                s.db.putConn(ci)
                        } else {
                                // TODO(bradfitz): care that we can't close
                                // this statement because the statement's
                                // connection is in use?
                        }
                }
        }
        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 ...")
//     ...
//     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 {
        db          *DB
        ci          driver.Conn // owned; must call putconn when closed to release
        releaseConn func()
        rowsi       driver.Rows

        closed    bool
        lastcols  []interface{}
        lasterr   error
        closeStmt *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, false if there is no next result row.
// 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.lasterr != nil {
                return false
        }
        if rs.lastcols == nil {
                rs.lastcols = make([]interface{}, len(rs.rowsi.Columns()))
        }
        rs.lasterr = rs.rowsi.Next(rs.lastcols)
        if rs.lasterr == io.EOF {
                rs.Close()
        }
        return rs.lasterr == nil
}

// Err returns the error, if any, that was encountered during iteration.
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 closed")
        }
        if rs.lasterr != nil {
                return rs.lasterr
        }
        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)
                }
        }
        for _, dp := range dest {
                b, ok := dp.(*[]byte)
                if !ok {
                        continue
                }
                if *b == nil {
                        // If the []byte is now nil (for a NULL value),
                        // don't fall through to below which would
                        // turn it into a non-nil 0-length byte slice
                        continue
                }
                if _, ok = dp.(*RawBytes); ok {
                        continue
                }
                clone := make([]byte, len(*b))
                copy(clone, *b)
                *b = clone
        }
        return nil
}

// Close closes the Rows, preventing further enumeration. If the
// end is encountered, the Rows are closed automatically. Close
// is idempotent.
func (rs *Rows) Close() error {
        if rs.closed {
                return nil
        }
        rs.closed = true
        err := rs.rowsi.Close()
        rs.releaseConn()
        if rs.closeStmt != nil {
                rs.closeStmt.Close()
        }
        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.
        for _, dp := range dest {
                if _, ok := dp.(*RawBytes); ok {
                        return errors.New("sql: RawBytes isn't allowed on Row.Scan")
                }
        }

        defer r.rows.Close()
        if !r.rows.Next() {
                return ErrNoRows
        }
        err := r.rows.Scan(dest...)
        if err != nil {
                return err
        }

        return nil
}

// A Result summarizes an executed SQL command.
type Result interface {
        LastInsertId() (int64, error)
        RowsAffected() (int64, error)
}

type result struct {
        driver.Result
}