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pubsub.go
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pubsub.go
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package pubsub
import (
"context"
"database/sql"
"errors"
"io"
"net"
"sync"
"time"
"github.com/google/uuid"
"github.com/lib/pq"
"github.com/prometheus/client_golang/prometheus"
"golang.org/x/xerrors"
"cdr.dev/slog"
)
// Listener represents a pubsub handler.
type Listener func(ctx context.Context, message []byte)
// ListenerWithErr represents a pubsub handler that can also receive error
// indications
type ListenerWithErr func(ctx context.Context, message []byte, err error)
// ErrDroppedMessages is sent to ListenerWithErr if messages are dropped or
// might have been dropped.
var ErrDroppedMessages = xerrors.New("dropped messages")
// Pubsub is a generic interface for broadcasting and receiving messages.
// Implementors should assume high-availability with the backing implementation.
type Pubsub interface {
Subscribe(event string, listener Listener) (cancel func(), err error)
SubscribeWithErr(event string, listener ListenerWithErr) (cancel func(), err error)
Publish(event string, message []byte) error
Close() error
}
// msgOrErr either contains a message or an error
type msgOrErr struct {
msg []byte
err error
}
// msgQueue implements a fixed length queue with the ability to replace elements
// after they are queued (but before they are dequeued).
//
// The purpose of this data structure is to build something that works a bit
// like a golang channel, but if the queue is full, then we can replace the
// last element with an error so that the subscriber can get notified that some
// messages were dropped, all without blocking.
type msgQueue struct {
ctx context.Context
cond *sync.Cond
q [BufferSize]msgOrErr
front int
size int
closed bool
l Listener
le ListenerWithErr
}
func newMsgQueue(ctx context.Context, l Listener, le ListenerWithErr) *msgQueue {
if l == nil && le == nil {
panic("l or le must be non-nil")
}
q := &msgQueue{
ctx: ctx,
cond: sync.NewCond(&sync.Mutex{}),
l: l,
le: le,
}
go q.run()
return q
}
func (q *msgQueue) run() {
for {
// wait until there is something on the queue or we are closed
q.cond.L.Lock()
for q.size == 0 && !q.closed {
q.cond.Wait()
}
if q.closed {
q.cond.L.Unlock()
return
}
item := q.q[q.front]
q.front = (q.front + 1) % BufferSize
q.size--
q.cond.L.Unlock()
// process item without holding lock
if item.err == nil {
// real message
if q.l != nil {
q.l(q.ctx, item.msg)
continue
}
if q.le != nil {
q.le(q.ctx, item.msg, nil)
continue
}
// unhittable
continue
}
// if the listener wants errors, send it.
if q.le != nil {
q.le(q.ctx, nil, item.err)
}
}
}
func (q *msgQueue) enqueue(msg []byte) {
q.cond.L.Lock()
defer q.cond.L.Unlock()
if q.size == BufferSize {
// queue is full, so we're going to drop the msg we got called with.
// We also need to record that messages are being dropped, which we
// do at the last message in the queue. This potentially makes us
// lose 2 messages instead of one, but it's more important at this
// point to warn the subscriber that they're losing messages so they
// can do something about it.
back := (q.front + BufferSize - 1) % BufferSize
q.q[back].msg = nil
q.q[back].err = ErrDroppedMessages
return
}
// queue is not full, insert the message
next := (q.front + q.size) % BufferSize
q.q[next].msg = msg
q.q[next].err = nil
q.size++
q.cond.Broadcast()
}
func (q *msgQueue) close() {
q.cond.L.Lock()
defer q.cond.L.Unlock()
defer q.cond.Broadcast()
q.closed = true
}
// dropped records an error in the queue that messages might have been dropped
func (q *msgQueue) dropped() {
q.cond.L.Lock()
defer q.cond.L.Unlock()
if q.size == BufferSize {
// queue is full, but we need to record that messages are being dropped,
// which we do at the last message in the queue. This potentially drops
// another message, but it's more important for the subscriber to know.
back := (q.front + BufferSize - 1) % BufferSize
q.q[back].msg = nil
q.q[back].err = ErrDroppedMessages
return
}
// queue is not full, insert the error
next := (q.front + q.size) % BufferSize
q.q[next].msg = nil
q.q[next].err = ErrDroppedMessages
q.size++
q.cond.Broadcast()
}
// pqListener is an interface that represents a *pq.Listener for testing
type pqListener interface {
io.Closer
Listen(string) error
Unlisten(string) error
NotifyChan() <-chan *pq.Notification
}
type pqListenerShim struct {
*pq.Listener
}
func (l pqListenerShim) NotifyChan() <-chan *pq.Notification {
return l.Notify
}
// PGPubsub is a pubsub implementation using PostgreSQL.
type PGPubsub struct {
logger slog.Logger
listenDone chan struct{}
pgListener pqListener
db *sql.DB
qMu sync.Mutex
queues map[string]map[uuid.UUID]*msgQueue
// making the close state its own mutex domain simplifies closing logic so
// that we don't have to hold the qMu --- which could block processing
// notifications while the pqListener is closing.
closeMu sync.Mutex
closedListener bool
closeListenerErr error
publishesTotal *prometheus.CounterVec
subscribesTotal *prometheus.CounterVec
messagesTotal *prometheus.CounterVec
publishedBytesTotal prometheus.Counter
receivedBytesTotal prometheus.Counter
disconnectionsTotal prometheus.Counter
connected prometheus.Gauge
}
// BufferSize is the maximum number of unhandled messages we will buffer
// for a subscriber before dropping messages.
const BufferSize = 2048
// Subscribe calls the listener when an event matching the name is received.
func (p *PGPubsub) Subscribe(event string, listener Listener) (cancel func(), err error) {
return p.subscribeQueue(event, newMsgQueue(context.Background(), listener, nil))
}
func (p *PGPubsub) SubscribeWithErr(event string, listener ListenerWithErr) (cancel func(), err error) {
return p.subscribeQueue(event, newMsgQueue(context.Background(), nil, listener))
}
func (p *PGPubsub) subscribeQueue(event string, newQ *msgQueue) (cancel func(), err error) {
defer func() {
if err != nil {
// if we hit an error, we need to close the queue so we don't
// leak its goroutine.
newQ.close()
p.subscribesTotal.WithLabelValues("false").Inc()
} else {
p.subscribesTotal.WithLabelValues("true").Inc()
}
}()
// The pgListener waits for the response to `LISTEN` on a mainloop that also dispatches
// notifies. We need to avoid holding the mutex while this happens, since holding the mutex
// blocks reading notifications and can deadlock the pgListener.
// c.f. https://github.com/coder/coder/issues/11950
err = p.pgListener.Listen(event)
if err == nil {
p.logger.Debug(context.Background(), "started listening to event channel", slog.F("event", event))
}
if errors.Is(err, pq.ErrChannelAlreadyOpen) {
// It's ok if it's already open!
err = nil
}
if err != nil {
return nil, xerrors.Errorf("listen: %w", err)
}
p.qMu.Lock()
defer p.qMu.Unlock()
var eventQs map[uuid.UUID]*msgQueue
var ok bool
if eventQs, ok = p.queues[event]; !ok {
eventQs = make(map[uuid.UUID]*msgQueue)
p.queues[event] = eventQs
}
id := uuid.New()
eventQs[id] = newQ
return func() {
p.qMu.Lock()
listeners := p.queues[event]
q := listeners[id]
q.close()
delete(listeners, id)
if len(listeners) == 0 {
delete(p.queues, event)
}
p.qMu.Unlock()
// as above, we must not hold the lock while calling into pgListener
if len(listeners) == 0 {
uErr := p.pgListener.Unlisten(event)
p.closeMu.Lock()
defer p.closeMu.Unlock()
if uErr != nil && !p.closedListener {
p.logger.Warn(context.Background(), "failed to unlisten", slog.Error(uErr), slog.F("event", event))
} else {
p.logger.Debug(context.Background(), "stopped listening to event channel", slog.F("event", event))
}
}
}, nil
}
func (p *PGPubsub) Publish(event string, message []byte) error {
p.logger.Debug(context.Background(), "publish", slog.F("event", event), slog.F("message_len", len(message)))
// This is safe because we are calling pq.QuoteLiteral. pg_notify doesn't
// support the first parameter being a prepared statement.
//nolint:gosec
_, err := p.db.ExecContext(context.Background(), `select pg_notify(`+pq.QuoteLiteral(event)+`, $1)`, message)
if err != nil {
p.publishesTotal.WithLabelValues("false").Inc()
return xerrors.Errorf("exec pg_notify: %w", err)
}
p.publishesTotal.WithLabelValues("true").Inc()
p.publishedBytesTotal.Add(float64(len(message)))
return nil
}
// Close closes the pubsub instance.
func (p *PGPubsub) Close() error {
p.logger.Info(context.Background(), "pubsub is closing")
err := p.closeListener()
<-p.listenDone
p.logger.Debug(context.Background(), "pubsub closed")
return err
}
// closeListener closes the pgListener, unless it has already been closed.
func (p *PGPubsub) closeListener() error {
p.closeMu.Lock()
defer p.closeMu.Unlock()
if p.closedListener {
return p.closeListenerErr
}
p.closedListener = true
p.closeListenerErr = p.pgListener.Close()
return p.closeListenerErr
}
// listen begins receiving messages on the pq listener.
func (p *PGPubsub) listen() {
defer func() {
p.logger.Info(context.Background(), "pubsub listen stopped receiving notify")
close(p.listenDone)
}()
notify := p.pgListener.NotifyChan()
for notif := range notify {
// A nil notification can be dispatched on reconnect.
if notif == nil {
p.logger.Debug(context.Background(), "notifying subscribers of a reconnection")
p.recordReconnect()
continue
}
p.listenReceive(notif)
}
}
func (p *PGPubsub) listenReceive(notif *pq.Notification) {
sizeLabel := messageSizeNormal
if len(notif.Extra) >= colossalThreshold {
sizeLabel = messageSizeColossal
}
p.messagesTotal.WithLabelValues(sizeLabel).Inc()
p.receivedBytesTotal.Add(float64(len(notif.Extra)))
p.qMu.Lock()
defer p.qMu.Unlock()
queues, ok := p.queues[notif.Channel]
if !ok {
return
}
extra := []byte(notif.Extra)
for _, q := range queues {
q.enqueue(extra)
}
}
func (p *PGPubsub) recordReconnect() {
p.qMu.Lock()
defer p.qMu.Unlock()
for _, listeners := range p.queues {
for _, q := range listeners {
q.dropped()
}
}
}
// logDialer is a pq.Dialer and pq.DialerContext that logs when it starts
// connecting and when the TCP connection is established.
type logDialer struct {
logger slog.Logger
d net.Dialer
}
var (
_ pq.Dialer = logDialer{}
_ pq.DialerContext = logDialer{}
)
func (d logDialer) Dial(network, address string) (net.Conn, error) {
ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
defer cancel()
return d.DialContext(ctx, network, address)
}
func (d logDialer) DialTimeout(network, address string, timeout time.Duration) (net.Conn, error) {
ctx, cancel := context.WithTimeout(context.Background(), timeout)
defer cancel()
return d.DialContext(ctx, network, address)
}
func (d logDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
deadline, hasDeadline := ctx.Deadline()
timeoutMS := 0
if hasDeadline {
timeoutMS = int(time.Until(deadline) / time.Millisecond)
}
logger := d.logger.With(slog.F("network", network), slog.F("address", address), slog.F("timeout_ms", timeoutMS))
logger.Info(ctx, "pubsub dialing postgres")
start := time.Now()
conn, err := d.d.DialContext(ctx, network, address)
if err != nil {
logger.Error(ctx, "pubsub failed to dial postgres")
return nil, err
}
elapsed := time.Since(start)
logger.Info(ctx, "pubsub postgres TCP connection established", slog.F("elapsed_ms", elapsed.Milliseconds()))
return conn, nil
}
func (p *PGPubsub) startListener(ctx context.Context, connectURL string) error {
p.connected.Set(0)
// Creates a new listener using pq.
var (
errCh = make(chan error)
dialer = logDialer{
logger: p.logger,
// pq.defaultDialer uses a zero net.Dialer as well.
d: net.Dialer{},
}
)
p.pgListener = pqListenerShim{
Listener: pq.NewDialListener(dialer, connectURL, time.Second, time.Minute, func(t pq.ListenerEventType, err error) {
switch t {
case pq.ListenerEventConnected:
p.logger.Info(ctx, "pubsub connected to postgres")
p.connected.Set(1.0)
case pq.ListenerEventDisconnected:
p.logger.Error(ctx, "pubsub disconnected from postgres", slog.Error(err))
p.connected.Set(0)
case pq.ListenerEventReconnected:
p.logger.Info(ctx, "pubsub reconnected to postgres")
p.connected.Set(1)
case pq.ListenerEventConnectionAttemptFailed:
p.logger.Error(ctx, "pubsub failed to connect to postgres", slog.Error(err))
}
// This callback gets events whenever the connection state changes.
// Don't send if the errChannel has already been closed.
select {
case <-errCh:
return
default:
errCh <- err
close(errCh)
}
}),
}
select {
case err := <-errCh:
if err != nil {
_ = p.pgListener.Close()
return xerrors.Errorf("create pq listener: %w", err)
}
case <-ctx.Done():
_ = p.pgListener.Close()
return ctx.Err()
}
return nil
}
// these are the metrics we compute implicitly from our existing data structures
var (
currentSubscribersDesc = prometheus.NewDesc(
"coder_pubsub_current_subscribers",
"The current number of active pubsub subscribers",
nil, nil,
)
currentEventsDesc = prometheus.NewDesc(
"coder_pubsub_current_events",
"The current number of pubsub event channels listened for",
nil, nil,
)
)
// We'll track messages as size "normal" and "colossal", where the
// latter are messages larger than 7600 bytes, or 95% of the postgres
// notify limit. If we see a lot of colossal packets that's an indication that
// we might be trying to send too much data over the pubsub and are in danger of
// failing to publish.
const (
colossalThreshold = 7600
messageSizeNormal = "normal"
messageSizeColossal = "colossal"
)
// Describe implements, along with Collect, the prometheus.Collector interface
// for metrics.
func (p *PGPubsub) Describe(descs chan<- *prometheus.Desc) {
// explicit metrics
p.publishesTotal.Describe(descs)
p.subscribesTotal.Describe(descs)
p.messagesTotal.Describe(descs)
p.publishedBytesTotal.Describe(descs)
p.receivedBytesTotal.Describe(descs)
p.disconnectionsTotal.Describe(descs)
p.connected.Describe(descs)
// implicit metrics
descs <- currentSubscribersDesc
descs <- currentEventsDesc
}
// Collect implements, along with Describe, the prometheus.Collector interface
// for metrics
func (p *PGPubsub) Collect(metrics chan<- prometheus.Metric) {
// explicit metrics
p.publishesTotal.Collect(metrics)
p.subscribesTotal.Collect(metrics)
p.messagesTotal.Collect(metrics)
p.publishedBytesTotal.Collect(metrics)
p.receivedBytesTotal.Collect(metrics)
p.disconnectionsTotal.Collect(metrics)
p.connected.Collect(metrics)
// implicit metrics
p.qMu.Lock()
events := len(p.queues)
subs := 0
for _, subscriberMap := range p.queues {
subs += len(subscriberMap)
}
p.qMu.Unlock()
metrics <- prometheus.MustNewConstMetric(currentSubscribersDesc, prometheus.GaugeValue, float64(subs))
metrics <- prometheus.MustNewConstMetric(currentEventsDesc, prometheus.GaugeValue, float64(events))
}
// New creates a new Pubsub implementation using a PostgreSQL connection.
func New(startCtx context.Context, logger slog.Logger, database *sql.DB, connectURL string) (*PGPubsub, error) {
p := newWithoutListener(logger, database)
if err := p.startListener(startCtx, connectURL); err != nil {
return nil, err
}
go p.listen()
logger.Info(startCtx, "pubsub has started")
return p, nil
}
// newWithoutListener creates a new PGPubsub without creating the pqListener.
func newWithoutListener(logger slog.Logger, database *sql.DB) *PGPubsub {
return &PGPubsub{
logger: logger,
listenDone: make(chan struct{}),
db: database,
queues: make(map[string]map[uuid.UUID]*msgQueue),
publishesTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "publishes_total",
Help: "Total number of calls to Publish",
}, []string{"success"}),
subscribesTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "subscribes_total",
Help: "Total number of calls to Subscribe/SubscribeWithErr",
}, []string{"success"}),
messagesTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "messages_total",
Help: "Total number of messages received from postgres",
}, []string{"size"}),
publishedBytesTotal: prometheus.NewCounter(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "published_bytes_total",
Help: "Total number of bytes successfully published across all publishes",
}),
receivedBytesTotal: prometheus.NewCounter(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "received_bytes_total",
Help: "Total number of bytes received across all messages",
}),
disconnectionsTotal: prometheus.NewCounter(prometheus.CounterOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "disconnections_total",
Help: "Total number of times we disconnected unexpectedly from postgres",
}),
connected: prometheus.NewGauge(prometheus.GaugeOpts{
Namespace: "coder",
Subsystem: "pubsub",
Name: "connected",
Help: "Whether we are connected (1) or not connected (0) to postgres",
}),
}
}