sql.go

// 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)
	if db.maxOpen > 0 {
		numCanOpen := db.maxOpen - db.numOpen
		if numRequests > numCanOpen {
			numRequests = numCanOpen
		}
	}
	for numRequests > 0 {
		db.numOpen++ // optimistically
		numRequests--
		if db.closed {
			return
		}
		db.openerCh <- struct{}{}
	}
}

// Runs in a separate goroutine, opens new connections when requested.
func (db *DB) connectionOpener(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case <-db.openerCh:
			db.openNewConnection(ctx)
		}
	}
}

// connectionResetter runs in a separate goroutine to reset connections async
// to exported API.
func (db *DB) connectionResetter(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			close(db.resetterCh)
			for dc := range db.resetterCh {
				dc.Unlock()
			}
			return
		case dc := <-db.resetterCh:
			dc.resetSession(ctx)
		}
	}
}

// Open one new connection
func (db *DB) openNewConnection(ctx context.Context) {
	// maybeOpenNewConnctions has already executed db.numOpen++ before it sent
	// on db.openerCh. This function must execute db.numOpen-- if the
	// connection fails or is closed before returning.
	ci, err := db.connector.Connect(ctx)
	db.mu.Lock()
	defer db.mu.Unlock()
	if db.closed {
		if err == nil {
			ci.Close()
		}
		db.numOpen--
		return
	}
	if err != nil {
		db.numOpen--
		db.putConnDBLocked(nil, err)
		db.maybeOpenNewConnections()
		return
	}
	dc := &driverConn{
		db:        db,
		createdAt: nowFunc(),
		ci:        ci,
	}
	if db.putConnDBLocked(dc, err) {
		db.addDepLocked(dc, dc)
	} else {
		db.numOpen--
		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")

// nextRequestKeyLocked returns the next connection request key.
// It is assumed that nextRequest will not overflow.
func (db *DB) nextRequestKeyLocked() uint64 {
	next := db.nextRequest
	db.nextRequest++
	return next
}

// conn returns a newly-opened or cached *driverConn.
func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn, error) {
	db.mu.Lock()
	if db.closed {
		db.mu.Unlock()
		return nil, errDBClosed
	}
	// Check if the context is expired.
	select {
	default:
	case <-ctx.Done():
		db.mu.Unlock()
		return nil, ctx.Err()
	}
	lifetime := db.maxLifetime

	// Prefer a free connection, if possible.
	numFree := len(db.freeConn)
	if strategy == cachedOrNewConn && numFree > 0 {
		conn := db.freeConn[0]
		copy(db.freeConn, db.freeConn[1:])
		db.freeConn = db.freeConn[:numFree-1]
		conn.inUse = true
		db.mu.Unlock()
		if conn.expired(lifetime) {
			conn.Close()
			return nil, driver.ErrBadConn
		}
		// Lock around reading lastErr to ensure the session resetter finished.
		conn.Lock()
		err := conn.lastErr
		conn.Unlock()
		if err == driver.ErrBadConn {
			conn.Close()
			return nil, driver.ErrBadConn
		}
		return conn, nil
	}

	// Out of free connections or we were asked not to use one. If we're not
	// allowed to open any more connections, make a request and wait.
	if db.maxOpen > 0 && db.numOpen >= db.maxOpen {
		// 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)
		reqKey := db.nextRequestKeyLocked()
		db.connRequests[reqKey] = req
		db.waitCount++
		db.mu.Unlock()

		waitStart := time.Now()

		// Timeout the connection request with the context.
		select {
		case <-ctx.Done():
			// Remove the connection request and ensure no value has been sent
			// on it after removing.
			db.mu.Lock()
			delete(db.connRequests, reqKey)
			db.mu.Unlock()

			atomic.AddInt64(&db.waitDuration, int64(time.Since(waitStart)))

			select {
			default:
			case ret, ok := <-req:
				if ok && ret.conn != nil {
					db.putConn(ret.conn, ret.err, false)
				}
			}
			return nil, ctx.Err()
		case ret, ok := <-req:
			atomic.AddInt64(&db.waitDuration, int64(time.Since(waitStart)))

			if !ok {
				return nil, errDBClosed
			}
			if ret.err == nil && ret.conn.expired(lifetime) {
				ret.conn.Close()
				return nil, driver.ErrBadConn
			}
			if ret.conn == nil {
				return nil, ret.err
			}
			// Lock around reading lastErr to ensure the session resetter finished.
			ret.conn.Lock()
			err := ret.conn.lastErr
			ret.conn.Unlock()
			if err == driver.ErrBadConn {
				ret.conn.Close()
				return nil, driver.ErrBadConn
			}
			return ret.conn, ret.err
		}
	}

	db.numOpen++ // optimistically
	db.mu.Unlock()
	ci, err := db.connector.Connect(ctx)
	if err != nil {
		db.mu.Lock()
		db.numOpen-- // correct for earlier optimism
		db.maybeOpenNewConnections()
		db.mu.Unlock()
		return nil, err
	}
	db.mu.Lock()
	dc := &driverConn{
		db:        db,
		createdAt: nowFunc(),
		ci:        ci,
		inUse:     true,
	}
	db.addDepLocked(dc, dc)
	db.mu.Unlock()
	return dc, nil
}

// putConnHook is a hook for testing.
var putConnHook func(*DB, *driverConn)

// noteUnusedDriverStatement notes that ds 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, ds *driverStmt) {
	db.mu.Lock()
	defer db.mu.Unlock()
	if c.inUse {
		c.onPut = append(c.onPut, func() {
			ds.Close()
		})
	} else {
		c.Lock()
		fc := c.finalClosed
		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, resetSession bool) {
	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)
	}
	if db.closed {
		// Connections do not need to be reset if they will be closed.
		// Prevents writing to resetterCh after the DB has closed.
		resetSession = false
	}
	if resetSession {
		if _, resetSession = dc.ci.(driver.SessionResetter); resetSession {
			// Lock the driverConn here so it isn't released until
			// the connection is reset.
			// The lock must be taken before the connection is put into
			// the pool to prevent it from being taken out before it is reset.
			dc.Lock()
		}
	}
	added := db.putConnDBLocked(dc, nil)
	db.mu.Unlock()

	if !added {
		if resetSession {
			dc.Unlock()
		}
		dc.Close()
		return
	}
	if !resetSession {
		return
	}
	select {
	default:
		// If the resetterCh is blocking then mark the connection
		// as bad and continue on.
		dc.lastErr = driver.ErrBadConn
		dc.Unlock()
	case db.resetterCh <- dc:
	}
}

// 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 {
		if db.maxIdleConnsLocked() > len(db.freeConn) {
			db.freeConn = append(db.freeConn, dc)
			db.startCleanerLocked()
			return true
		}
		db.maxIdleClosed++
	}
	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, nil, query)
}

// prepareDC prepares a query on the driverConn and calls release before
// returning. When cg == nil it implies that a connection pool is used, and
// when cg != nil only a single driver connection is used.
func (db *DB) prepareDC(ctx context.Context, dc *driverConn, release func(error), cg stmtConnGrabber, query string) (*Stmt, error) {
	var ds *driverStmt
	var err error
	defer func() {
		release(err)
	}()
	withLock(dc, func() {
		ds, err = dc.prepareLocked(ctx, cg, query)
	})
	if err != nil {
		return nil, err
	}
	stmt := &Stmt{
		db:    db,
		query: query,
		cg:    cg,
		cgds:  ds,
	}

	// When cg == nil this statement will need to keep track of various
	// connections they are prepared on and record the stmt dependency on
	// the DB.
	if cg == nil {
		stmt.css = []connStmt{{dc, ds}}
		stmt.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)
	}()
	execerCtx, ok := dc.ci.(driver.ExecerContext)
	var execer driver.Execer
	if !ok {
		execer, ok = dc.ci.(driver.Execer)
	}
	if ok {
		var nvdargs []driver.NamedValue
		var resi driver.Result
		withLock(dc, func() {
			nvdargs, err = driverArgsConnLocked(dc.ci, nil, args)
			if err != nil {
				return
			}
			resi, err = ctxDriverExec(ctx, execerCtx, execer, query, nvdargs)
		})
		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, dc.ci, 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, nil, dc, dc.releaseConn, query, args)
}

// queryDC executes a query on the given connection.
// The connection gets released by the releaseConn function.
// The ctx context is from a query method and the txctx context is from an
// optional transaction context.
func (db *DB) queryDC(ctx, txctx context.Context, dc *driverConn, releaseConn func(error), query string, args []interface{}) (*Rows, error) {
	queryerCtx, ok := dc.ci.(driver.QueryerContext)
	var queryer driver.Queryer
	if !ok {
		queryer, ok = dc.ci.(driver.Queryer)
	}
	if ok {
		var nvdargs []driver.NamedValue
		var rowsi driver.Rows
		var err error
		withLock(dc, func() {
			nvdargs, err = driverArgsConnLocked(dc.ci, nil, args)
			if err != nil {
				return
			}
			rowsi, err = ctxDriverQuery(ctx, queryerCtx, queryer, query, nvdargs)
		})
		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, txctx)
			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, dc.ci, 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, txctx)
	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.
// 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.
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.
// 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.
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.connector.Driver()
}

// ErrConnDone is returned by any operation that is performed on a connection
// that has already been returned to the connection pool.
var ErrConnDone = errors.New("sql: connection is already closed")

// Conn returns a single connection by either opening a new connection
// or returning an existing connection from the connection pool. Conn will
// block until either a connection is returned or ctx is canceled.
// Queries run on the same Conn will be run in the same database session.
//
// Every Conn must be returned to the database pool after use by
// calling Conn.Close.
func (db *DB) Conn(ctx context.Context) (*Conn, error) {
	var dc *driverConn
	var err error
	for i := 0; i < maxBadConnRetries; i++ {
		dc, err = db.conn(ctx, cachedOrNewConn)
		if err != driver.ErrBadConn {
			break
		}
	}
	if err == driver.ErrBadConn {
		dc, err = db.conn(ctx, alwaysNewConn)
	}
	if err != nil {
		return nil, err
	}

	conn := &Conn{
		db: db,
		dc: dc,
	}
	return conn, nil
}

type releaseConn func(error)

// Conn represents a single database connection rather than a pool of database
// connections. Prefer running queries from DB unless there is a specific
// need for a continuous single database connection.
//
// A Conn must call Close to return the connection to the database pool
// and may do so concurrently with a running query.
//
// After a call to Close, all operations on the
// connection fail with ErrConnDone.
type Conn struct {
	db *DB

	// closemu prevents the connection 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 until close, at which point
	// it's returned to the connection pool.
	dc *driverConn

	// done transitions from 0 to 1 exactly once, on close.
	// Once done, all operations fail with ErrConnDone.
	// Use atomic operations on value when checking value.
	done int32
}

func (c *Conn) grabConn(context.Context) (*driverConn, releaseConn, error) {
	if atomic.LoadInt32(&c.done) != 0 {
		return nil, nil, ErrConnDone
	}
	c.closemu.RLock()
	return c.dc, c.closemuRUnlockCondReleaseConn, nil
}

// PingContext verifies the connection to the database is still alive.
func (c *Conn) PingContext(ctx context.Context) error {
	dc, release, err := c.grabConn(ctx)
	if err != nil {
		return err
	}
	return c.db.pingDC(ctx, dc, release)
}

// ExecContext executes a query without returning any rows.
// The args are for any placeholder parameters in the query.
func (c *Conn) ExecContext(ctx context.Context, query string, args ...interface{}) (Result, error) {
	dc, release, err := c.grabConn(ctx)
	if err != nil {
		return nil, err
	}
	return c.db.execDC(ctx, dc, release, query, args)
}

// QueryContext executes a query that returns rows, typically a SELECT.
// The args are for any placeholder parameters in the query.
func (c *Conn) QueryContext(ctx context.Context, query string, args ...interface{}) (*Rows, error) {
	dc, release, err := c.grabConn(ctx)
	if err != nil {
		return nil, err
	}
	return c.db.queryDC(ctx, nil, dc, release, 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.
// 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.
func (c *Conn) QueryRowContext(ctx context.Context, query string, args ...interface{}) *Row {
	rows, err := c.QueryContext(ctx, query, args...)
	return &Row{rows: rows, err: err}
}

// 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 (c *Conn) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
	dc, release, err := c.grabConn(ctx)
	if err != nil {
		return nil, err
	}
	return c.db.prepareDC(ctx, dc, release, c, query)
}

// 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 (c *Conn) BeginTx(ctx context.Context, opts *TxOptions) (*Tx, error) {
	dc, release, err := c.grabConn(ctx)
	if err != nil {
		return nil, err
	}
	return c.db.beginDC(ctx, dc, release, opts)
}

// closemuRUnlockCondReleaseConn read unlocks closemu
// as the sql operation is done with the dc.
func (c *Conn) closemuRUnlockCondReleaseConn(err error) {
	c.closemu.RUnlock()
	if err == driver.ErrBadConn {
		c.close(err)
	}
}

func (c *Conn) txCtx() context.Context {
	return nil
}

func (c *Conn) close(err error) error {
	if !atomic.CompareAndSwapInt32(&c.done, 0, 1) {
		return ErrConnDone
	}

	// Lock around releasing the driver connection
	// to ensure all queries have been stopped before doing so.
	c.closemu.Lock()
	defer c.closemu.Unlock()

	c.dc.releaseConn(err)
	c.dc = nil
	c.db = nil
	return err
}

// Close returns the connection to the connection pool.
// All operations after a Close will return with ErrConnDone.
// Close is safe to call concurrently with other operations and will
// block until all other operations finish. It may be useful to first
// cancel any used context and then call close directly after.
func (c *Conn) Close() error {
	return c.close(nil)
}

// 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.cancel()

	tx.closemu.Lock()
	defer tx.closemu.Unlock()

	tx.releaseConn(err)
	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, releaseConn, error) {
	select {
	default:
	case <-ctx.Done():
		return nil, nil, ctx.Err()
	}

	// closeme.RLock must come before the check for isDone to prevent the Tx from
	// closing while a query is executing.
	tx.closemu.RLock()
	if tx.isDone() {
		tx.closemu.RUnlock()
		return nil, nil, ErrTxDone
	}
	if hookTxGrabConn != nil { // test hook
		hookTxGrabConn()
	}
	return tx.dc, tx.closemuRUnlockRelease, nil
}

func (tx *Tx) txCtx() context.Context {
	return tx.ctx
}

// closemuRUnlockRelease is used as a func(error) method value in
// ExecContext and QueryContext. Unlocking in the releaseConn keeps