notify.go

package pq

// Package pq is a pure Go Postgres driver for the database/sql package.
// This module contains support for Postgres LISTEN/NOTIFY.

import (
	"errors"
	"fmt"
	"io"
	"sync"
	"sync/atomic"
	"time"
)

// Notification represents a single notification from the database.
type Notification struct {
	// Process ID (PID) of the notifying postgres backend.
	BePid int
	// Name of the channel the notification was sent on.
	Channel string
	// Payload, or the empty string if unspecified.
	Extra string
}

func recvNotification(r *readBuf) *Notification {
	bePid := r.int32()
	channel := r.string()
	extra := r.string()

	return &Notification{bePid, channel, extra}
}

const (
	connStateIdle int32 = iota
	connStateExpectResponse
	connStateExpectReadyForQuery
)

type message struct {
	typ byte
	err error
}

var errListenerConnClosed = errors.New("pq: ListenerConn has been closed")

// ListenerConn is a low-level interface for waiting for notifications.  You
// should use Listener instead.
type ListenerConn struct {
	// guards cn and err
	connectionLock sync.Mutex
	cn             *conn
	err            error

	connState int32

	// the sending goroutine will be holding this lock
	senderLock sync.Mutex

	notificationChan chan<- *Notification

	replyChan chan message
}

// Creates a new ListenerConn.  Use NewListener instead.
func NewListenerConn(name string, notificationChan chan<- *Notification) (*ListenerConn, error) {
	cn, err := Open(name)
	if err != nil {
		return nil, err
	}

	l := &ListenerConn{
		cn:               cn.(*conn),
		notificationChan: notificationChan,
		connState:        connStateIdle,
		replyChan:        make(chan message, 2),
	}

	go l.listenerConnMain()

	return l, nil
}

// We can only allow one goroutine at a time to be running a query on the
// connection for various reasons, so the goroutine sending on the connection
// must be holding senderLock.
//
// Returns an error if an unrecoverable error has occurred and the ListenerConn
// should be abandoned.
func (l *ListenerConn) acquireSenderLock() error {
	l.connectionLock.Lock()
	defer l.connectionLock.Unlock()
	if l.err != nil {
		return l.err
	}
	l.senderLock.Lock()
	return nil
}

func (l *ListenerConn) releaseSenderLock() {
	l.senderLock.Unlock()
}

// setState advances the protocol state to newState.  Returns false if moving
// to that state from the current state is not allowed.
func (l *ListenerConn) setState(newState int32) bool {
	var expectedState int32

	switch newState {
	case connStateIdle:
		expectedState = connStateExpectReadyForQuery
	case connStateExpectResponse:
		expectedState = connStateIdle
	case connStateExpectReadyForQuery:
		expectedState = connStateExpectResponse
	default:
		panic(fmt.Sprintf("unexpected listenerConnState %d", newState))
	}

	return atomic.CompareAndSwapInt32(&l.connState, expectedState, newState)
}

// Main logic is here: receive messages from the postgres backend, forward
// notifications and query replies and keep the internal state in sync with the
// protocol state.  Returns when the connection has been lost, is about to go
// away or should be discarded because we couldn't agree on the state with the
// server backend.
func (l *ListenerConn) listenerConnLoop() (err error) {
	defer l.cn.errRecover(&err)

	r := &readBuf{}
	for {
		t, err := l.cn.recvMessage(r)
		if err != nil {
			return err
		}

		switch t {
		case 'A':
			// recvNotification copies all the data so we don't need to worry
			// about the scratch buffer being overwritten.
			l.notificationChan <- recvNotification(r)

		case 'E':
			// We might receive an ErrorResponse even when not in a query; it
			// is expected that the server will close the connection after
			// that, but we should make sure that the error we display is the
			// one from the stray ErrorResponse, not io.ErrUnexpectedEOF.
			if !l.setState(connStateExpectReadyForQuery) {
				return parseError(r)
			}
			l.replyChan <- message{t, parseError(r)}

		case 'C', 'I':
			if !l.setState(connStateExpectReadyForQuery) {
				// protocol out of sync
				return fmt.Errorf("unexpected CommandComplete")
			}
			// ExecSimpleQuery doesn't need to know about this message

		case 'Z':
			if !l.setState(connStateIdle) {
				// protocol out of sync
				return fmt.Errorf("unexpected ReadyForQuery")
			}
			l.replyChan <- message{t, nil}

		case 'N', 'S':
			// ignore
		default:
			return fmt.Errorf("unexpected message %q from server in listenerConnLoop", t)
		}
	}
}

// This is the main routine for the goroutine receiving on the database
// connection.  Most of the main logic is in listenerConnLoop.
func (l *ListenerConn) listenerConnMain() {
	err := l.listenerConnLoop()

	// listenerConnLoop terminated; we're done, but we still have to clean up.
	// Make sure nobody tries to start any new queries by making sure the err
	// pointer is set.  It is important that we do not overwrite its value; a
	// connection could be closed by either this goroutine or one sending on
	// the connection -- whoever closes the connection is assumed to have the
	// more meaningful error message (as the other one will probably get
	// net.errClosed), so that goroutine sets the error we expose while the
	// other error is discarded.  If the connection is lost while two
	// goroutines are operating on the socket, it probably doesn't matter which
	// error we expose so we don't try to do anything more complex.
	l.connectionLock.Lock()
	if l.err == nil {
		l.err = err
	}
	l.cn.Close()
	l.connectionLock.Unlock()

	// There might be a query in-flight; make sure nobody's waiting for a
	// response to it, since there's not going to be one.
	close(l.replyChan)

	// let the listener know we're done
	close(l.notificationChan)

	// this ListenerConn is done
}

// Send a LISTEN query to the server.  See ExecSimpleQuery.
func (l *ListenerConn) Listen(channel string) (bool, error) {
	return l.ExecSimpleQuery("LISTEN " + QuoteIdentifier(channel))
}

// Send an UNLISTEN query to the server.  See ExecSimpleQuery.
func (l *ListenerConn) Unlisten(channel string) (bool, error) {
	return l.ExecSimpleQuery("UNLISTEN " + QuoteIdentifier(channel))
}

// Send `UNLISTEN *` to the server.  See ExecSimpleQuery.
func (l *ListenerConn) UnlistenAll() (bool, error) {
	return l.ExecSimpleQuery("UNLISTEN *")
}

// Ping the remote server to make sure it's alive.  Non-nil error means the
// connection has failed and should be abandoned.
func (l *ListenerConn) Ping() error {
	sent, err := l.ExecSimpleQuery("")
	if !sent {
		return err
	}
	if err != nil {
		// shouldn't happen
		panic(err)
	}
	return nil
}

// Attempt to send a query on the connection.  Returns an error if sending the
// query failed, and the caller should initiate closure of this connection.
// The caller must be holding senderLock (see acquireSenderLock and
// releaseSenderLock).
func (l *ListenerConn) sendSimpleQuery(q string) (err error) {
	defer l.cn.errRecover(&err)

	// must set connection state before sending the query
	if !l.setState(connStateExpectResponse) {
		panic("two queries running at the same time")
	}

	// Can't use l.cn.writeBuf here because it uses the scratch buffer which
	// might get overwritten by listenerConnLoop.
	data := writeBuf([]byte("Q\x00\x00\x00\x00"))
	b := &data
	b.string(q)
	l.cn.send(b)

	return nil
}

// Execute a "simple query" (i.e. one with no bindable parameters) on the
// connection.  The possible return values are:
//   1) "executed" is true; the query was executed to completion on the
//      database server.  If the query failed, err will be set to the error
//      returned by the database, otherwise err will be nil.
//   2) If "executed" is false, the query could not be executed on the remote
//      server.  err will be non-nil.
//
// After a call to ExecSimpleQuery has returned an executed=false value, the
// connection has either been closed or will be closed shortly thereafter, and
// all subsequently executed queries will return an error.
func (l *ListenerConn) ExecSimpleQuery(q string) (executed bool, err error) {
	if err = l.acquireSenderLock(); err != nil {
		return false, err
	}
	defer l.releaseSenderLock()

	err = l.sendSimpleQuery(q)
	if err != nil {
		// We can't know what state the protocol is in, so we need to abandon
		// this connection.
		l.connectionLock.Lock()
		defer l.connectionLock.Unlock()
		// Set the error pointer if it hasn't been set already; see
		// listenerConnMain.
		if l.err == nil {
			l.err = err
		}
		l.cn.Close()
		return false, err
	}

	// now we just wait for a reply..
	for {
		m, ok := <-l.replyChan
		if !ok {
			// We lost the connection to server, don't bother waiting for a
			// a response.
			return false, io.EOF
		}
		switch m.typ {
		case 'Z':
			// sanity check
			if m.err != nil {
				panic("m.err != nil")
			}
			// done; err might or might not be set
			return true, err

		case 'E':
			// sanity check
			if m.err == nil {
				panic("m.err == nil")
			}
			// server responded with an error; ReadyForQuery to follow
			err = m.err

		default:
			return false, fmt.Errorf("unknown response for simple query: %q", m.typ)
		}
	}
}

func (l *ListenerConn) Close() error {
	l.connectionLock.Lock()
	defer l.connectionLock.Unlock()
	if l.err != nil {
		return errListenerConnClosed
	}
	l.err = errListenerConnClosed
	return l.cn.Close()
}

// Err() returns the reason the connection was closed.  It is not safe to call
// this function until l.Notify has been closed.
func (l *ListenerConn) Err() error {
	return l.err
}

var errListenerClosed = errors.New("pq: Listener has been closed")

var ErrChannelAlreadyOpen = errors.New("pq: channel is already open")
var ErrChannelNotOpen = errors.New("pq: channel is not open")

type ListenerEventType int

const (
	// Emitted only when the database connection has been initially
	// initialized.  err will always be nil.
	ListenerEventConnected ListenerEventType = iota

	// Emitted after a database connection has been lost, either because of an
	// error or because Close has been called.  err will be set to the reason
	// the database connection was lost.
	ListenerEventDisconnected

	// Emitted after a database connection has been re-established after
	// connection loss.  err will always be nil.  After this event has been
	// emitted, a nil pq.Notification is sent on the Listener.Notify channel.
	ListenerEventReconnected

	// Emitted after a connection to the database was attempted, but failed.
	// err will be set to an error describing why the connection attempt did
	// not succeed.
	ListenerEventConnectionAttemptFailed
)

type EventCallbackType func(event ListenerEventType, err error)

// Listener provides an interface for listening to notifications from a
// PostgreSQL database.  For general usage information, see section
// "Notifications".
//
// Listener can safely be used from concurrently running goroutines.
type Listener struct {
	// Channel for receiving notifications from the database.  In some cases a
	// nil value will be sent.  See section "Notifications" above.
	Notify chan *Notification

	name                 string
	minReconnectInterval time.Duration
	maxReconnectInterval time.Duration
	eventCallback        EventCallbackType

	lock                 sync.Mutex
	isClosed             bool
	reconnectCond        *sync.Cond
	cn                   *ListenerConn
	connNotificationChan <-chan *Notification
	channels             map[string]struct{}
}

// NewListener creates a new database connection dedicated to LISTEN / NOTIFY.
//
// name should be set to a connection string to be used to establish the
// database connection (see section "Connection String Parameters" above).
//
// minReconnectInterval controls the duration to wait before trying to
// re-establish the database connection after connection loss.  After each
// consecutive failure this interval is doubled, until maxReconnectInterval is
// reached.  Successfully completing the connection establishment procedure
// resets the interval back to minReconnectInterval.
//
// The last parameter eventCallback can be set to a function which will be
// called by the Listener when the state of the underlying database connection
// changes.  This callback will be called by the goroutine which dispatches the
// notifications over the Notify channel, so you should try to avoid doing
// potentially time-consuming operations from the callback.
func NewListener(name string,
	minReconnectInterval time.Duration,
	maxReconnectInterval time.Duration,
	eventCallback EventCallbackType) *Listener {
	l := &Listener{
		name:                 name,
		minReconnectInterval: minReconnectInterval,
		maxReconnectInterval: maxReconnectInterval,
		eventCallback:        eventCallback,

		channels: make(map[string]struct{}),

		Notify: make(chan *Notification, 32),
	}
	l.reconnectCond = sync.NewCond(&l.lock)

	go l.listenerMain()

	return l
}

// Returns the notification channel for this listener.  This is the same
// channel as Notify, and will not be recreated during the life time of the
// Listener.
func (l *Listener) NotificationChannel() <-chan *Notification {
	return l.Notify
}

// Listen starts listening for notifications on a channel.  Calls to this
// function will block until an acknowledgement has been received from the
// server.  Note that Listener automatically re-establishes the connection
// after connection loss, so this function may block indefinitely if the
// connection can not be re-established.
//
// Listen will only fail in three conditions:
//   1) The channel is already open.  The returned error will be
//      ErrChannelAlreadyOpen.
//   2) The query was executed on the remote server, but PostgreSQL returned an
//      error message in response to the query.  The returned error will be a
//      pq.Error containing the information the server supplied.
//   3) Close is called on the Listener before the request could be completed.
//
// The channel name is case-sensitive.
func (l *Listener) Listen(channel string) error {
	l.lock.Lock()
	defer l.lock.Unlock()

	if l.isClosed {
		return errListenerClosed
	}

	// The server allows you to issue a LISTEN on a channel which is already
	// open, but it seems useful to be able to detect this case to spot for
	// mistakes in application logic.  If the application genuinely does't
	// care, it can check the exported error and ignore it.
	_, exists := l.channels[channel]
	if exists {
		return ErrChannelAlreadyOpen
	}

	if l.cn != nil {
		// If gotResponse is true but error is set, the query was executed on
		// the remote server, but resulted in an error.  This should be
		// relatively rare, so it's fine if we just pass the error to our
		// caller.  However, if gotResponse is false, we could not complete the
		// query on the remote server and our underlying connection is about
		// to go away, so we only add relname to l.channels, and wait for
		// resync() to take care of the rest.
		gotResponse, err := l.cn.Listen(channel)
		if gotResponse && err != nil {
			return err
		}
	}

	l.channels[channel] = struct{}{}
	for l.cn == nil {
		l.reconnectCond.Wait()
		// we let go of the mutex for a while
		if l.isClosed {
			return errListenerClosed
		}
	}

	return nil
}

// Unlisten removes a channel from the Listener's channel list.  Returns
// ErrChannelNotOpen if the Listener is not listening on the specified channel.
// Returns immediately with no error if there is no connection.  Note that you
// might still get notifications for this channel even after Unlisten has
// returned.
//
// The channel name is case-sensitive.
func (l *Listener) Unlisten(channel string) error {
	l.lock.Lock()
	defer l.lock.Unlock()

	if l.isClosed {
		return errListenerClosed
	}

	// Similarly to LISTEN, this is not an error in Postgres, but it seems
	// useful to distinguish from the normal conditions.
	_, exists := l.channels[channel]
	if !exists {
		return ErrChannelNotOpen
	}

	if l.cn != nil {
		// Similarly to Listen (see comment in that function), the caller
		// should only be bothered with an error if it came from the backend as
		// a response to our query.
		gotResponse, err := l.cn.Unlisten(channel)
		if gotResponse && err != nil {
			return err
		}
	}

	// Don't bother waiting for resync if there's no connection.
	delete(l.channels, channel)
	return nil
}

// UnlistenAll removes all channels from the Listener's channel list.  Returns
// immediately with no error if there is no connection.  Note that you might
// still get notifications for any of the deleted channels even after
// UnlistenAll has returned.
func (l *Listener) UnlistenAll() error {
	l.lock.Lock()
	defer l.lock.Unlock()

	if l.isClosed {
		return errListenerClosed
	}

	if l.cn != nil {
		// Similarly to Listen (see comment in that function), the caller
		// should only be bothered with an error if it came from the backend as
		// a response to our query.
		gotResponse, err := l.cn.UnlistenAll()
		if gotResponse && err != nil {
			return err
		}
	}

	// Don't bother waiting for resync if there's no connection.
	l.channels = make(map[string]struct{})
	return nil
}

// Ping the remote server to make sure it's alive.  Non-nil return value means
// that there is no active connection.
func (l *Listener) Ping() error {
	l.lock.Lock()
	defer l.lock.Unlock()

	if l.isClosed {
		return errListenerClosed
	}
	if l.cn == nil {
		return errors.New("no connection")
	}

	return l.cn.Ping()
}

// Clean up after losing the server connection.  Returns l.cn.Err(), which
// should have the reason the connection was lost.
func (l *Listener) disconnectCleanup() error {
	l.lock.Lock()
	defer l.lock.Unlock()

	// sanity check; can't look at Err() until the channel has been closed
	select {
	case _, ok := <-l.connNotificationChan:
		if ok {
			panic("connNotificationChan not closed")
		}
	default:
		panic("connNotificationChan not closed")
	}

	err := l.cn.Err()
	l.cn.Close()
	l.cn = nil
	return err
}

// Synchronize the list of channels we want to be listening on with the server
// after the connection has been established.
func (l *Listener) resync(cn *ListenerConn, notificationChan <-chan *Notification) error {
	doneChan := make(chan error)
	go func() {
		for channel := range l.channels {
			// If we got a response, return that error to our caller as it's
			// going to be more descriptive than cn.Err().
			gotResponse, err := cn.Listen(channel)
			if gotResponse && err != nil {
				doneChan <- err
				return
			}

			// If we couldn't reach the server, wait for notificationChan to
			// close and then return the error message from the connection, as
			// per ListenerConn's interface.
			if err != nil {
				for _ = range notificationChan {
				}
				doneChan <- cn.Err()
				return
			}
		}
		doneChan <- nil
	}()

	// Ignore notifications while synchronization is going on to avoid
	// deadlocks.  We have to send a nil notification over Notify anyway as
	// we can't possibly know which notifications (if any) were lost while
	// the connection was down, so there's no reason to try and process
	// these messages at all.
	for {
		select {
		case _, ok := <-notificationChan:
			if !ok {
				notificationChan = nil
			}

		case err := <-doneChan:
			return err
		}
	}
}

// caller should NOT be holding l.lock
func (l *Listener) closed() bool {
	l.lock.Lock()
	defer l.lock.Unlock()

	return l.isClosed
}

func (l *Listener) connect() error {
	notificationChan := make(chan *Notification, 32)
	cn, err := NewListenerConn(l.name, notificationChan)
	if err != nil {
		return err
	}

	l.lock.Lock()
	defer l.lock.Unlock()

	err = l.resync(cn, notificationChan)
	if err != nil {
		cn.Close()
		return err
	}

	l.cn = cn
	l.connNotificationChan = notificationChan
	l.reconnectCond.Broadcast()

	return nil
}

// Close disconnects the Listener from the database and shuts it down.
// Subsequent calls to its methods will return an error.  Close returns an
// error if the connection has already been closed.
func (l *Listener) Close() error {
	l.lock.Lock()
	defer l.lock.Unlock()

	if l.isClosed {
		return errListenerClosed
	}

	if l.cn != nil {
		l.cn.Close()
	}
	l.isClosed = true
	l.reconnectCond.Broadcast()

	return nil
}

func (l *Listener) emitEvent(event ListenerEventType, err error) {
	if l.eventCallback != nil {
		l.eventCallback(event, err)
	}
}

// Main logic here: maintain a connection to the server when possible, wait
// for notifications and emit events.
func (l *Listener) listenerConnLoop() {
	var nextReconnect time.Time

	reconnectInterval := l.minReconnectInterval
	for {
		for {
			err := l.connect()
			if err == nil {
				break
			}

			l.emitEvent(ListenerEventConnectionAttemptFailed, err)

			if l.closed() {
				return
			}

			time.Sleep(reconnectInterval)
			reconnectInterval *= 2
			if reconnectInterval > l.maxReconnectInterval {
				reconnectInterval = l.maxReconnectInterval
			}
		}

		if nextReconnect.IsZero() {
			l.emitEvent(ListenerEventConnected, nil)
		} else {
			l.emitEvent(ListenerEventReconnected, nil)
			l.Notify <- nil
		}

		reconnectInterval = l.minReconnectInterval
		nextReconnect = time.Now().Add(reconnectInterval)

		for {
			notification, ok := <-l.connNotificationChan
			if !ok {
				// lost connection, loop again
				break
			}
			l.Notify <- notification
		}

		err := l.disconnectCleanup()

		l.emitEvent(ListenerEventDisconnected, err)

		if l.closed() {
			return
		}

		time.Sleep(nextReconnect.Sub(time.Now()))
	}
}

func (l *Listener) listenerMain() {
	l.listenerConnLoop()
	close(l.Notify)
}