package pq
// Package pq is a pure Go Postgres driver for the database/sql package.
// This module contains support for Postgres LISTEN/NOTIFY.
import (
"context"
"database/sql/driver"
"errors"
"fmt"
"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}
}
// SetNotificationHandler sets the given notification handler on the given
// connection. A runtime panic occurs if c is not a pq connection. A nil handler
// may be used to unset it.
//
// Note: Notification handlers are executed synchronously by pq meaning commands
// won't continue to be processed until the handler returns.
func SetNotificationHandler(c driver.Conn, handler func(*Notification)) {
c.(*conn).notificationHandler = handler
}
// NotificationHandlerConnector wraps a regular connector and sets a notification handler
// on it.
type NotificationHandlerConnector struct {
driver.Connector
notificationHandler func(*Notification)
}
// Connect calls the underlying connector's connect method and then sets the
// notification handler.
func (n *NotificationHandlerConnector) Connect(ctx context.Context) (driver.Conn, error) {
c, err := n.Connector.Connect(ctx)
if err == nil {
SetNotificationHandler(c, n.notificationHandler)
}
return c, err
}
// ConnectorNotificationHandler returns the currently set notification handler, if any. If
// the given connector is not a result of ConnectorWithNotificationHandler, nil is
// returned.
func ConnectorNotificationHandler(c driver.Connector) func(*Notification) {
if c, ok := c.(*NotificationHandlerConnector); ok {
return c.notificationHandler
}
return nil
}
// ConnectorWithNotificationHandler creates or sets the given handler for the given
// connector. If the given connector is a result of calling this function
// previously, it is simply set on the given connector and returned. Otherwise,
// this returns a new connector wrapping the given one and setting the notification
// handler. A nil notification handler may be used to unset it.
//
// The returned connector is intended to be used with database/sql.OpenDB.
//
// Note: Notification handlers are executed synchronously by pq meaning commands
// won't continue to be processed until the handler returns.
func ConnectorWithNotificationHandler(c driver.Connector, handler func(*Notification)) *NotificationHandlerConnector {
if c, ok := c.(*NotificationHandlerConnector); ok {
c.notificationHandler = handler
return c
}
return &NotificationHandlerConnector{Connector: c, notificationHandler: handler}
}
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
}
// NewListenerConn creates a new ListenerConn. Use NewListener instead.
func NewListenerConn(name string, notificationChan chan<- *Notification) (*ListenerConn, error) {
return newDialListenerConn(defaultDialer{}, name, notificationChan)
}
func newDialListenerConn(d Dialer, name string, c chan<- *Notification) (*ListenerConn, error) {
cn, err := DialOpen(d, name)
if err != nil {
return nil, err
}
l := &ListenerConn{
cn: cn.(*conn),
notificationChan: c,
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 {
// we must acquire senderLock first to avoid deadlocks; see ExecSimpleQuery
l.senderLock.Lock()
l.connectionLock.Lock()
err := l.err
l.connectionLock.Unlock()
if err != nil {
l.senderLock.Unlock()
return err
}
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 errRecoverNoErrBadConn(&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 'T', 'D':
// only used by tests; ignore
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 'S':
// ignore
case 'N':
if n := l.cn.noticeHandler; n != nil {
n(parseError(r))
}
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
}
// Listen sends a LISTEN query to the server. See ExecSimpleQuery.
func (l *ListenerConn) Listen(channel string) (bool, error) {
return l.ExecSimpleQuery("LISTEN " + QuoteIdentifier(channel))
}
// Unlisten sends an UNLISTEN query to the server. See ExecSimpleQuery.
func (l *ListenerConn) Unlisten(channel string) (bool, error) {
return l.ExecSimpleQuery("UNLISTEN " + QuoteIdentifier(channel))
}
// UnlistenAll sends an `UNLISTEN *` query 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 errRecoverNoErrBadConn(&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.
b := &writeBuf{
buf: []byte("Q\x00\x00\x00\x00"),
pos: 1,
}
b.string(q)
l.cn.send(b)
return nil
}
// ExecSimpleQuery executes 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()
// Set the error pointer if it hasn't been set already; see
// listenerConnMain.
if l.err == nil {
l.err = err
}
l.connectionLock.Unlock()
l.cn.c.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. err should have been set already.
l.connectionLock.Lock()
err := l.err
l.connectionLock.Unlock()
return false, err
}
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)
}
}
}
// Close closes the connection.
func (l *ListenerConn) Close() error {
l.connectionLock.Lock()
if l.err != nil {
l.connectionLock.Unlock()
return errListenerConnClosed
}
l.err = errListenerConnClosed
l.connectionLock.Unlock()
// We can't send anything on the connection without holding senderLock.
// Simply close the net.Conn to wake up everyone operating on it.
return l.cn.c.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")
// ErrChannelAlreadyOpen is returned from Listen when a channel is already
// open.
var ErrChannelAlreadyOpen = errors.New("pq: channel is already open")
// ErrChannelNotOpen is returned from Unlisten when a channel is not open.
var ErrChannelNotOpen = errors.New("pq: channel is not open")
// ListenerEventType is an enumeration of listener event types.
type ListenerEventType int
const (
// ListenerEventConnected is emitted only when the database connection
// has been initially initialized. The err argument of the callback
// will always be nil.
ListenerEventConnected ListenerEventType = iota
// ListenerEventDisconnected is emitted after a database connection has
// been lost, either because of an error or because Close has been
// called. The err argument will be set to the reason the database
// connection was lost.
ListenerEventDisconnected
// ListenerEventReconnected is emitted after a database connection has
// been re-established after connection loss. The err argument of the
// callback will always be nil. After this event has been emitted, a
// nil pq.Notification is sent on the Listener.Notify channel.
ListenerEventReconnected
// ListenerEventConnectionAttemptFailed is emitted after a connection
// to the database was attempted, but failed. The err argument will be
// set to an error describing why the connection attempt did not
// succeed.
ListenerEventConnectionAttemptFailed
)
// EventCallbackType is the event callback type. See also ListenerEventType
// constants' documentation.
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
dialer Dialer
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 {
return NewDialListener(defaultDialer{}, name, minReconnectInterval, maxReconnectInterval, eventCallback)
}
// NewDialListener is like NewListener but it takes a Dialer.
func NewDialListener(d Dialer,
name string,
minReconnectInterval time.Duration,
maxReconnectInterval time.Duration,
eventCallback EventCallbackType) *Listener {
l := &Listener{
name: name,
minReconnectInterval: minReconnectInterval,
maxReconnectInterval: maxReconnectInterval,
dialer: d,
eventCallback: eventCallback,
channels: make(map[string]struct{}),
Notify: make(chan *Notification, 32),
}
l.reconnectCond = sync.NewCond(&l.lock)
go l.listenerMain()
return l
}
// NotificationChannel 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(notificationChan <-chan *Notification) {
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
}(notificationChan)
// 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 := newDialListenerConn(l.dialer, 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
// Unblock calls to Listen()
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
}
if l.closed() {
return
}
l.emitEvent(ListenerEventConnectionAttemptFailed, err)
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()
if l.closed() {
return
}
l.emitEvent(ListenerEventDisconnected, err)
time.Sleep(time.Until(nextReconnect))
}
}
func (l *Listener) listenerMain() {
l.listenerConnLoop()
close(l.Notify)
}
