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" "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 } // 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 '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 } // 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 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(nextReconnect.Sub(time.Now())) } } func (l *Listener) listenerMain() { l.listenerConnLoop() close(l.Notify) }