sql.go

		c.Unlock()
		if !fc {
			ds.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 db.closed {
		return false
	}
	if db.maxOpen > 0 && db.numOpen > db.maxOpen {
		return false
	}
	if c := len(db.connRequests); c > 0 {
		var req chan connRequest
		var reqKey uint64
		for reqKey, req = range db.connRequests {
			break
		}
		delete(db.connRequests, reqKey) // Remove from pending requests.
		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
	}
	return db.prepareDC(ctx, dc, dc.releaseConn, query)
}

func (db *DB) prepareDC(ctx context.Context, dc *driverConn, release func(error), query string) (*Stmt, error) {
	var ds *driverStmt
	var err error
	defer func() {
		release(err)
	}()
	withLock(dc, func() {
		ds, err = dc.prepareLocked(ctx, query)
	})
	if err != nil {
		return nil, err
	}
	stmt := &Stmt{
		db:            db,
		query:         query,
		css:           []connStmt{{dc, ds}},
		lastNumClosed: atomic.LoadUint64(&db.numClosed),
	}
	db.addDep(stmt, stmt)
	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) (Result, error) {
	dc, err := db.conn(ctx, strategy)
	if err != nil {
		return nil, err
	}
	return db.execDC(ctx, dc, dc.releaseConn, query, args)
}

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

	return db.queryDC(ctx, dc, dc.releaseConn, query, args)
}

// queryDC executes a query on the given connection.
// The connection gets released by the releaseConn function.
func (db *DB) queryDC(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...)
}

// BeginTx 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
// BeginTx is canceled.
//
// The provided TxOptions is optional and may be nil if defaults should be used.
// If a non-default isolation level is used that the driver doesn't support,
// an error will be returned.
func (db *DB) BeginTx(ctx context.Context, opts *TxOptions) (*Tx, error) {
	var tx *Tx
	var err error
	for i := 0; i < maxBadConnRetries; i++ {
		tx, err = db.begin(ctx, opts, cachedOrNewConn)
		if err != driver.ErrBadConn {
			break
		}
	}
	if err == driver.ErrBadConn {
		return db.begin(ctx, opts, 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.BeginTx(context.Background(), nil)
}

func (db *DB) begin(ctx context.Context, opts *TxOptions, strategy connReuseStrategy) (tx *Tx, err error) {
	dc, err := db.conn(ctx, strategy)
	if err != nil {
		return nil, err
	}
	return db.beginDC(ctx, dc, dc.releaseConn, opts)
}

// beginDC starts a transaction. The provided dc must be valid and ready to use.
func (db *DB) beginDC(ctx context.Context, dc *driverConn, release func(error), opts *TxOptions) (tx *Tx, err error) {
	var txi driver.Tx
	withLock(dc, func() {
		txi, err = ctxDriverBegin(ctx, opts, dc.ci)
	})
	if err != nil {
		release(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,
		releaseConn: release,
		txi:         txi,
		cancel:      cancel,
		ctx:         ctx,
	}
	go tx.awaitDone()
	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

	// closemu prevents the transaction from closing while there
	// is an active query. It is held for read during queries
	// and exclusively during close.
	closemu sync.RWMutex

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

	// releaseConn is called once the Tx is closed to release
	// any held driverConn back to the pool.
	releaseConn func(error)

	// 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 0 to 1.
	cancel func()

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

// awaitDone blocks until the context in Tx is canceled and rolls back
// the transaction if it's not already done.
func (tx *Tx) awaitDone() {
	// Wait for either the transaction to be committed or rolled
	// back, or for the associated context to be closed.
	<-tx.ctx.Done()

	// Discard and close the connection used to ensure the
	// transaction is closed and the resources are released.  This
	// rollback does nothing if the transaction has already been
	// committed or rolled back.
	tx.rollback(true)
}

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")

// close returns the connection to the pool and
// must only be called by Tx.rollback or Tx.Commit.
func (tx *Tx) close(err error) {
	tx.closemu.Lock()
	defer tx.closemu.Unlock()

	tx.releaseConn(err)
	tx.cancel()
	tx.dc = nil
	tx.txi = nil
}

// hookTxGrabConn specifies an optional hook to be called on
// a successful call to (*Tx).grabConn. For tests.
var hookTxGrabConn func()

func (tx *Tx) grabConn(ctx context.Context) (*driverConn, error) {
	select {
	default:
	case <-ctx.Done():
		return nil, ctx.Err()
	}
	if tx.isDone() {
		return nil, ErrTxDone
	}
	if hookTxGrabConn != nil { // test hook
		hookTxGrabConn()
	}
	return tx.dc, nil
}

// closemuRUnlockRelease is used as a func(error) method value in
// ExecContext and QueryContext. Unlocking in the releaseConn keeps
// the driver conn from being returned to the connection pool until
// the Rows has been closed.
func (tx *Tx) closemuRUnlockRelease(error) {
	tx.closemu.RUnlock()
}

// 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 {
	if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
		return ErrTxDone
	}
	select {
	default:
	case <-tx.ctx.Done():
		return tx.ctx.Err()
	}
	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 !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
		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) {
	tx.closemu.RLock()
	defer tx.closemu.RUnlock()

	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 {
	tx.closemu.RLock()
	defer tx.closemu.RUnlock()

	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
	var parentStmt *Stmt
	stmt.mu.Lock()
	if stmt.closed || stmt.tx != nil {
		// If the statement has been closed or already belongs to a
		// transaction, we can't reuse it in this connection.
		// Since tx.StmtContext should never need to be called with a
		// Stmt already belonging to tx, we ignore this edge case and
		// re-prepare the statement in this case. No need to add
		// code-complexity for this.
		stmt.mu.Unlock()
		withLock(dc, func() {
			si, err = ctxDriverPrepare(ctx, dc.ci, stmt.query)
		})
		if err != nil {
			return &Stmt{stickyErr: err}
		}
	} else {
		stmt.removeClosedStmtLocked()
		// See if the statement has already been prepared on this connection,
		// and reuse it if possible.
		for _, v := range stmt.css {
			if v.dc == dc {
				si = v.ds.si
				break
			}
		}

		stmt.mu.Unlock()

		if si == nil {
			cs, err := stmt.prepareOnConnLocked(ctx, dc)
			if err != nil {
				return &Stmt{stickyErr: err}
			}
			si = cs.si
		}
		parentStmt = stmt
	}

	txs := &Stmt{
		db: tx.db,
		tx: tx,
		txds: &driverStmt{
			Locker: dc,
			si:     si,
		},
		parentStmt: parentStmt,
		query:      stmt.query,
	}
	if parentStmt != nil {
		tx.db.addDep(parentStmt, txs)
	}
	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) {
	tx.closemu.RLock()

	dc, err := tx.grabConn(ctx)
	if err != nil {
		tx.closemu.RUnlock()
		return nil, err
	}
	return tx.db.execDC(ctx, dc, tx.closemuRUnlockRelease, query, 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) {
	tx.closemu.RLock()

	dc, err := tx.grabConn(ctx)
	if err != nil {
		tx.closemu.RUnlock()
		return nil, err
	}

	return tx.db.queryDC(ctx, dc, tx.closemuRUnlockRelease, 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

	// parentStmt is set when a transaction-specific statement
	// is requested from an identical statement prepared on the same
	// conn. parentStmt is used to track the dependency of this statement
	// on its originating ("parent") statement so that parentStmt may
	// be closed by the user without them having to know whether or not
	// any transactions are still using it.
	parentStmt *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, txds 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 = s.prepareOnConnLocked(ctx, dc)
	})
	if err != nil {
		dc.releaseConn(err)
		return nil, nil, nil, err
	}

	return dc, dc.releaseConn, ds, nil
}

// prepareOnConnLocked prepares the query in Stmt s on dc and adds it to the list of
// open connStmt on the statement. It assumes the caller is holding the lock on dc.
func (s *Stmt) prepareOnConnLocked(ctx context.Context, dc *driverConn) (*driverStmt, error) {
	si, err := dc.prepareLocked(ctx, s.query)
	if err != nil {
		return nil, err
	}
	cs := connStmt{dc, si}
	s.mu.Lock()
	s.css = append(s.css, cs)
	s.mu.Unlock()
	return cs.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)