sql.go

		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 db.closed {
		return false
	}
	if db.maxOpen > 0 && db.numOpen > db.maxOpen {
		return false
	}
	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)
		db.startCleanerLocked()
		return true
	}
	return false
}

// maxBadConnRetries is the number of maximum retries if the driver returns
// driver.ErrBadConn to signal a broken connection before forcing a new
// connection to be opened.
const maxBadConnRetries = 2

// PrepareContext creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
// The caller must call the statement's Close method
// when the statement is no longer needed.
//
// The provided context is used for the preparation of the statement, not for the
// execution of the statement.
func (db *DB) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
	var stmt *Stmt
	var err error
	for i := 0; i < maxBadConnRetries; i++ {
		stmt, err = db.prepare(ctx, query, cachedOrNewConn)
		if err != driver.ErrBadConn {
			break
		}
	}
	if err == driver.ErrBadConn {
		return db.prepare(ctx, query, alwaysNewConn)
	}
	return stmt, err
}

// Prepare creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
// The caller must call the statement's Close method
// when the statement is no longer needed.
func (db *DB) Prepare(query string) (*Stmt, error) {
	return db.PrepareContext(context.Background(), query)
}

func (db *DB) prepare(ctx context.Context, query string, strategy connReuseStrategy) (*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(ctx, strategy)
	if err != nil {
		return nil, err
	}
	var ds *driverStmt
	withLock(dc, func() {
		ds, err = dc.prepareLocked(ctx, query)
	})
	if err != nil {
		db.putConn(dc, err)
		return nil, err
	}
	stmt := &Stmt{
		db:            db,
		query:         query,
		css:           []connStmt{{dc, ds}},
		lastNumClosed: atomic.LoadUint64(&db.numClosed),
	}
	db.addDep(stmt, stmt)
	db.putConn(dc, nil)
	return stmt, nil
}

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

// 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) {
	return db.ExecContext(context.Background(), query, args...)
}

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

	if execer, ok := dc.ci.(driver.Execer); ok {
		var dargs []driver.NamedValue
		dargs, err = driverArgs(nil, args)
		if err != nil {
			return nil, err
		}
		var resi driver.Result
		withLock(dc, func() {
			resi, err = ctxDriverExec(ctx, execer, query, dargs)
		})
		if err != driver.ErrSkip {
			if err != nil {
				return nil, err
			}
			return driverResult{dc, resi}, nil
		}
	}

	var si driver.Stmt
	withLock(dc, func() {
		si, err = ctxDriverPrepare(ctx, dc.ci, query)
	})
	if err != nil {
		return nil, err
	}
	ds := &driverStmt{Locker: dc, si: si}
	defer ds.Close()
	return resultFromStatement(ctx, ds, args...)
}

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

// 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) {
	return db.QueryContext(context.Background(), query, args...)
}

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

	return db.queryConn(ctx, 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(ctx context.Context, 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
		}
		var rowsi driver.Rows
		withLock(dc, func() {
			rowsi, err = ctxDriverQuery(ctx, queryer, query, dargs)
		})
		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,
			}
			rows.initContextClose(ctx)
			return rows, nil
		}
	}

	var si driver.Stmt
	var err error
	withLock(dc, func() {
		si, err = ctxDriverPrepare(ctx, dc.ci, query)
	})
	if err != nil {
		releaseConn(err)
		return nil, err
	}

	ds := &driverStmt{Locker: dc, si: si}
	rowsi, err := rowsiFromStatement(ctx, ds, args...)
	if err != nil {
		ds.Close()
		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:   ds,
	}
	rows.initContextClose(ctx)
	return rows, nil
}

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

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

// BeginContext starts a transaction.
//
// The provided context is used until the transaction is committed or rolled back.
// If the context is canceled, the sql package will roll back
// the transaction. Tx.Commit will return an error if the context provided to
// BeginContext is canceled.
//
// An isolation level may be set by setting the value in the context
// before calling this. If a non-default isolation level is used
// that the driver doesn't support an error will be returned. Different drivers
// may have slightly different meanings for the same isolation level.
func (db *DB) BeginContext(ctx context.Context) (*Tx, error) {
	var tx *Tx
	var err error
	for i := 0; i < maxBadConnRetries; i++ {
		tx, err = db.begin(ctx, cachedOrNewConn)
		if err != driver.ErrBadConn {
			break
		}
	}
	if err == driver.ErrBadConn {
		return db.begin(ctx, alwaysNewConn)
	}
	return tx, err
}

// Begin starts a transaction. The default isolation level is dependent on
// the driver.
func (db *DB) Begin() (*Tx, error) {
	return db.BeginContext(context.Background())
}

func (db *DB) begin(ctx context.Context, strategy connReuseStrategy) (tx *Tx, err error) {
	dc, err := db.conn(ctx, strategy)
	if err != nil {
		return nil, err
	}
	var txi driver.Tx
	withLock(dc, func() {
		txi, err = ctxDriverBegin(ctx, dc.ci)
	})
	if err != nil {
		db.putConn(dc, err)
		return nil, err
	}

	// Schedule the transaction to rollback when the context is cancelled.
	// The cancel function in Tx will be called after done is set to true.
	ctx, cancel := context.WithCancel(ctx)
	tx = &Tx{
		db:     db,
		dc:     dc,
		txi:    txi,
		cancel: cancel,
		ctx:    ctx,
	}
	go func(tx *Tx) {
		select {
		case <-tx.ctx.Done():
			if !tx.isDone() {
				// Discard and close the connection used to ensure the transaction
				// is closed and the resources are released.
				tx.rollback(true)
			}
		}
	}(tx)
	return tx, 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.
//
// The statements prepared for a transaction by calling
// the transaction's Prepare or Stmt methods are closed
// by the call to Commit or Rollback.
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 0 to 1 exactly once, on Commit
	// or Rollback. once done, all operations fail with
	// ErrTxDone.
	// Use atomic operations on value when checking value.
	done int32

	// 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
	}

	// cancel is called after done transitions from false to true.
	cancel func()

	// ctx lives for the life of the transaction.
	ctx context.Context
}

func (tx *Tx) isDone() bool {
	return atomic.LoadInt32(&tx.done) != 0
}

// ErrTxDone is returned by any operation that is performed on a transaction
// that has already been committed or rolled back.
var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")

func (tx *Tx) close(err error) {
	if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
		panic("double close") // internal error
	}
	tx.db.putConn(tx.dc, err)
	tx.cancel()
	tx.dc = nil
	tx.txi = nil
}

func (tx *Tx) grabConn(ctx context.Context) (*driverConn, error) {
	if tx.isDone() {
		return nil, ErrTxDone
	}
	return tx.dc, nil
}

// Closes all Stmts prepared for this transaction.
func (tx *Tx) closePrepared() {
	tx.stmts.Lock()
	defer tx.stmts.Unlock()
	for _, stmt := range tx.stmts.v {
		stmt.Close()
	}
}

// Commit commits the transaction.
func (tx *Tx) Commit() error {
	select {
	default:
	case <-tx.ctx.Done():
		return tx.ctx.Err()
	}
	if tx.isDone() {
		return ErrTxDone
	}
	var err error
	withLock(tx.dc, func() {
		err = tx.txi.Commit()
	})
	if err != driver.ErrBadConn {
		tx.closePrepared()
	}
	tx.close(err)
	return err
}

// rollback aborts the transaction and optionally forces the pool to discard
// the connection.
func (tx *Tx) rollback(discardConn bool) error {
	if tx.isDone() {
		return ErrTxDone
	}
	var err error
	withLock(tx.dc, func() {
		err = tx.txi.Rollback()
	})
	if err != driver.ErrBadConn {
		tx.closePrepared()
	}
	if discardConn {
		err = driver.ErrBadConn
	}
	tx.close(err)
	return err
}

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
	return tx.rollback(false)
}

// Prepare creates a prepared statement for use within a transaction.
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
//
// To use an existing prepared statement on this transaction, see Tx.Stmt.
//
// The provided context will be used for the preparation of the context, not
// for the execution of the returned statement. The returned statement
// will run in the transaction context.
func (tx *Tx) PrepareContext(ctx context.Context, 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(ctx)
	if err != nil {
		return nil, err
	}

	var si driver.Stmt
	withLock(dc, func() {
		si, err = ctxDriverPrepare(ctx, dc.ci, query)
	})
	if err != nil {
		return nil, err
	}

	stmt := &Stmt{
		db: tx.db,
		tx: tx,
		txds: &driverStmt{
			Locker: dc,
			si:     si,
		},
		query: query,
	}
	tx.stmts.Lock()
	tx.stmts.v = append(tx.stmts.v, stmt)
	tx.stmts.Unlock()
	return stmt, nil
}

// 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) {
	return tx.PrepareContext(context.Background(), query)
}

// StmtContext 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.StmtContext(ctx, updateMoney).Exec(123.45, 98293203)
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
func (tx *Tx) StmtContext(ctx context.Context, 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(ctx)
	if err != nil {
		return &Stmt{stickyErr: err}
	}
	var si driver.Stmt
	withLock(dc, func() {
		si, err = ctxDriverPrepare(ctx, dc.ci, stmt.query)
	})
	txs := &Stmt{
		db: tx.db,
		tx: tx,
		txds: &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
}

// 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)
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
	return tx.StmtContext(context.Background(), stmt)
}

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

	var si driver.Stmt
	withLock(dc, func() {
		si, err = ctxDriverPrepare(ctx, dc.ci, query)
	})
	if err != nil {
		return nil, err
	}
	ds := &driverStmt{Locker: dc, si: si}
	defer ds.Close()

	return resultFromStatement(ctx, ds, args...)
}

// 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) {
	return tx.ExecContext(context.Background(), query, args...)
}

// QueryContext executes a query that returns rows, typically a SELECT.
func (tx *Tx) QueryContext(ctx context.Context, query string, args ...interface{}) (*Rows, error) {
	dc, err := tx.grabConn(ctx)
	if err != nil {
		return nil, err
	}
	releaseConn := func(error) {}
	return tx.db.queryConn(ctx, dc, releaseConn, query, args)
}

// Query executes a query that returns rows, typically a SELECT.
func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
	return tx.QueryContext(context.Background(), query, args...)
}

// QueryRowContext executes a query that is expected to return at most one row.
// QueryRowContext always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
func (tx *Tx) QueryRowContext(ctx context.Context, query string, args ...interface{}) *Row {
	rows, err := tx.QueryContext(ctx, query, args...)
	return &Row{rows: rows, err: err}
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
func (tx *Tx) QueryRow(query string, args ...interface{}) *Row {
	return tx.QueryRowContext(context.Background(), query, args...)
}

// connStmt is a prepared statement on a particular connection.
type connStmt struct {
	dc *driverConn
	ds *driverStmt
}

// Stmt is a prepared statement.
// A 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
	txds *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

	// lastNumClosed is copied from db.numClosed when Stmt is created
	// without tx and closed connections in css are removed.
	lastNumClosed uint64
}

// ExecContext executes a prepared statement with the given arguments and
// returns a Result summarizing the effect of the statement.
func (s *Stmt) ExecContext(ctx context.Context, args ...interface{}) (Result, error) {
	s.closemu.RLock()
	defer s.closemu.RUnlock()

	var res Result
	for i := 0; i < maxBadConnRetries; i++ {
		_, releaseConn, ds, err := s.connStmt(ctx)
		if err != nil {
			if err == driver.ErrBadConn {
				continue
			}
			return nil, err
		}

		res, err = resultFromStatement(ctx, ds, args...)
		releaseConn(err)
		if err != driver.ErrBadConn {
			return res, err
		}
	}
	return nil, driver.ErrBadConn
}

// 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) {
	return s.ExecContext(context.Background(), args...)
}

func driverNumInput(ds *driverStmt) int {
	ds.Lock()
	defer ds.Unlock() // in case NumInput panics
	return ds.si.NumInput()
}

func resultFromStatement(ctx context.Context, ds *driverStmt, args ...interface{}) (Result, error) {
	want := driverNumInput(ds)

	// -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()
	defer ds.Unlock()

	resi, err := ctxDriverStmtExec(ctx, ds.si, dargs)
	if err != nil {
		return nil, err
	}
	return driverResult{ds.Locker, resi}, nil
}

// removeClosedStmtLocked removes closed conns in s.css.
//
// To avoid lock contention on DB.mu, we do it only when
// s.db.numClosed - s.lastNum is large enough.
func (s *Stmt) removeClosedStmtLocked() {
	t := len(s.css)/2 + 1
	if t > 10 {
		t = 10
	}
	dbClosed := atomic.LoadUint64(&s.db.numClosed)
	if dbClosed-s.lastNumClosed < uint64(t) {
		return
	}

	s.db.mu.Lock()
	for i := 0; i < len(s.css); i++ {
		if s.css[i].dc.dbmuClosed {
			s.css[i] = s.css[len(s.css)-1]
			s.css = s.css[:len(s.css)-1]
			i--
		}
	}
	s.db.mu.Unlock()
	s.lastNumClosed = dbClosed
}

// 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(ctx context.Context) (ci *driverConn, releaseConn func(error), ds *driverStmt, 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(ctx) // blocks, waiting for the connection.
		if err != nil {
			return
		}
		releaseConn = func(error) {}
		return ci, releaseConn, s.txds, nil
	}

	s.removeClosedStmtLocked()
	s.mu.Unlock()

	dc, err := s.db.conn(ctx, cachedOrNewConn)
	if err != nil {
		return nil, nil, nil, err
	}

	s.mu.Lock()
	for _, v := range s.css {
		if v.dc == dc {
			s.mu.Unlock()
			return dc, dc.releaseConn, v.ds, nil
		}
	}
	s.mu.Unlock()

	// No luck; we need to prepare the statement on this connection
	withLock(dc, func() {
		ds, err = dc.prepareLocked(ctx, s.query)
	})
	if err != nil {
		s.db.putConn(dc, err)
		return nil, nil, nil, err
	}
	s.mu.Lock()
	cs := connStmt{dc, ds}
	s.css = append(s.css, cs)
	s.mu.Unlock()

	return dc, dc.releaseConn, ds, nil
}

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

	var rowsi driver.Rows
	for i := 0; i < maxBadConnRetries; i++ {
		dc, releaseConn, ds, err := s.connStmt(ctx)
		if err != nil {
			if err == driver.ErrBadConn {
				continue
			}
			return nil, err
		}

		rowsi, err = rowsiFromStatement(ctx, ds, 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
			}
			rows.initContextClose(ctx)
			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
}

// 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) {
	return s.QueryContext(context.Background(), args...)
}

func rowsiFromStatement(ctx context.Context, ds *driverStmt, args ...interface{}) (driver.Rows, error) {
	var want int
	withLock(ds, func() {
		want = ds.si.NumInput()
	})

	// -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()
	defer ds.Unlock()

	rowsi, err := ctxDriverStmtQuery(ctx, ds.si, dargs)
	if err != nil {
		return nil, err
	}
	return rowsi, nil
}

// QueryRowContext 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.QueryRowContext(ctx, id).Scan(&name)
func (s *Stmt) QueryRowContext(ctx context.Context, args ...interface{}) *Row {
	rows, err := s.QueryContext(ctx, args...)
	if err != nil {
		return &Row{err: err}
	}
	return &Row{rows: rows}
}

// 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 {
	return s.QueryRowContext(context.Background(), args...)
}

// 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
	s.mu.Unlock()

	if s.tx != nil {
		return s.txds.Close()
	}

	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.ds)
			v.dc.removeOpenStmt(v.ds)