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observer.go
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observer.go
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// Package observer provides a facility for publishing progress updates and
// state changes from parts of the daemon, an a SSE http handler for consumers
// of these events.
package observer
import (
"context"
"encoding/json"
"errors"
"fmt"
"log"
"net/http"
)
// EventType represents all possible types of events that can be observed.
type EventType string
// All values for EventType
const (
Progress EventType = "progress"
Started EventType = "started"
Finished EventType = "finished"
Errored EventType = "errored"
Aborted EventType = "aborted"
)
type event struct {
ID string `json:"id"`
Type EventType `json:"-"` // type is included in the SSE
Message string `json:"message"`
Completed uint `json:"completed"`
Total uint `json:"total"`
}
type notification struct {
Type EventType
Message string
Increment bool
}
// Observer recieves events via Notify, and publishes them as SSEs via its
// ServeHTTP function.
type Observer struct {
notify chan *event
closed chan int
observers map[chan []byte]bool
newObservers chan chan []byte
closedObservers chan chan []byte
}
type transaction struct {
requestID string
total uint
current uint
events chan<- *event
totalUpdates <-chan uint
notifications <-chan *notification
observerClosed <-chan int
ctxDone <-chan struct{}
}
// Notifier belongs to a transactions and represents one segment in a series of
// actions. A Notifier can send many messages.
type Notifier struct {
total uint
current uint
transaction *transaction
totalUpdates chan<- uint
notifications chan<- *notification
observerClosed <-chan int
ctxDone <-chan struct{}
}
func (t *transaction) start() {
go func() {
for {
select {
case notification := <-t.notifications:
if notification.Increment {
t.current++
}
evt := &event{
ID: t.requestID,
Type: notification.Type,
Message: notification.Message,
Completed: t.current,
Total: t.total,
}
t.events <- evt
case size := <-t.totalUpdates:
t.total += size
case <-t.ctxDone: // transaction has completed
return
case <-t.observerClosed: // observer has shutdown
return
}
}
}()
}
// Notifier creates a child notifier to this Notifier
func (n *Notifier) Notifier(total uint) *Notifier {
notifier := &Notifier{
total: total,
current: 0,
transaction: nil,
totalUpdates: n.totalUpdates,
notifications: n.notifications,
observerClosed: n.observerClosed,
ctxDone: n.ctxDone,
}
n.totalUpdates <- total
return notifier
}
// Notify publishes an event to all SSE observers.
func (n *Notifier) Notify(eventType EventType, message string, increment bool) {
notif := ¬ification{
Type: eventType,
Message: message,
Increment: increment,
}
if increment {
n.current++
}
if n.current > n.total {
panic(fmt.Sprintf(
"notifications exceed maximum %d/%d", n.current, n.total))
}
select {
case n.notifications <- notif:
return
case <-n.observerClosed:
return
case <-n.ctxDone:
return
}
}
// New returns a new initialized Observer
func New() *Observer {
return &Observer{
notify: make(chan *event),
closed: make(chan int),
observers: make(map[chan []byte]bool),
newObservers: make(chan chan []byte),
closedObservers: make(chan chan []byte),
}
}
// Notifier creates a new transaction for sending notifications
func (o *Observer) Notifier(ctx context.Context, total uint) (*Notifier, error) {
if ctx == nil {
return nil, errors.New("Context must be provided")
}
id, ok := ctx.Value("id").(string)
if !ok {
return nil, errors.New("Missing 'id' property in Context")
}
totalUpdates := make(chan uint)
notifications := make(chan *notification)
t := &transaction{
requestID: id,
total: total,
current: 0,
totalUpdates: totalUpdates,
events: o.notify,
notifications: notifications,
observerClosed: o.closed,
ctxDone: ctx.Done(),
}
t.start()
n := &Notifier{
total: total,
current: 0,
transaction: t,
totalUpdates: totalUpdates,
notifications: notifications,
observerClosed: t.observerClosed,
ctxDone: t.ctxDone,
}
return n, nil
}
// ServeHTTP implements the http.Handler interface for providing server-sent
// events of observed notifications.
func (o *Observer) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
rwf := rw.(http.Flusher)
closed := rw.(http.CloseNotifier).CloseNotify()
notify := make(chan []byte)
o.newObservers <- notify
defer func() {
o.closedObservers <- notify
}()
rw.Header().Set("Content-Type", "text/event-stream")
rw.Header().Set("Connection", "keep-alive")
rwf.Flush()
for {
select {
case evt := <-notify:
// Write the event to the client. we ignore errors here, and
// let the close channel tell us when to stop writing.
rw.Write(evt)
rwf.Flush()
case <-closed: // client has disconnected. Exit.
return
case <-o.closed: // The Observer is shutting down. Exit.
return
}
}
}
// Start begins listening for notifications of observable events. It returns
// after stop has been called.
func (o *Observer) Start() {
for {
select {
case evt := <-o.notify: // We have an event to observe
if len(o.observers) == 0 {
continue
}
evtb, err := json.Marshal(evt)
if err != nil {
log.Printf("Error marshaling event: %s", err)
continue
}
sse := []byte("event: " + evt.Type + "\ndata: ")
sse = append(sse, append(evtb, []byte("\n\n")...)...)
for n := range o.observers {
n <- sse
}
case n := <-o.newObservers:
o.observers[n] = true
case n := <-o.closedObservers:
delete(o.observers, n)
case <-o.closed: // The Observer has been closed.
return
}
}
}
// Stop terminates propagation of events through the observer
func (o *Observer) Stop() {
close(o.closed)
}