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supervisor.go
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supervisor.go
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package supervisor
import (
"context"
"fmt"
"runtime/debug"
"sync"
"time"
"github.com/pkg/errors"
)
// A supervisor provides an abstraction for managing a group of
// related goroutines, and provides:
//
// - startup and shutdown ordering based on dependencies
// - both graceful and hard shutdown
// - error propagation
// - retry
// - logging
//
type Supervisor struct {
mutex *sync.Mutex
changed *sync.Cond // used to signal when a worker is ready or done
context context.Context
wantsShutdown bool // signals we are in shutdown mode
names []string // list of worker names in order added
workers map[string]*Worker // keyed by worker name
errors []error
Logger Logger
}
// centralize a bit of lock management
func (s *Supervisor) change(f func()) {
s.mutex.Lock()
defer s.mutex.Unlock()
f()
s.changed.Broadcast()
}
func WithContext(ctx context.Context) *Supervisor {
mu := &sync.Mutex{}
return &Supervisor{
mutex: mu,
changed: sync.NewCond(mu),
context: ctx,
workers: make(map[string]*Worker),
Logger: &DefaultLogger{},
}
}
// Supervise adds a Worker to be run as a Process when s.Run() is
// called.
func (s *Supervisor) Supervise(worker *Worker) {
s.change(func() {
_, exists := s.workers[worker.Name]
if exists {
panic(fmt.Sprintf("worker already exists: %s", worker.Name))
}
worker.supervisor = s
s.add(worker)
})
}
func (s *Supervisor) add(worker *Worker) {
s.workers[worker.Name] = worker
s.names = append(s.names, worker.Name)
}
// this assumes that s.mutex is already held
func (s *Supervisor) remove(worker *Worker) {
delete(s.workers, worker.Name)
var newNames []string
for _, name := range s.names {
if name == worker.Name {
continue
} else {
newNames = append(newNames, name)
}
}
s.names = newNames
}
// A supervisor will run until all its workers exit. There are
// multiple ways workers can exit:
//
// - normally (returning a non-nil error)
// - error (either via return result or panic)
// - graceful shutdown request
// - canceled context
//
// A normal exit does not trigger any special action other than
// causing Run to return if it is the last worker.
//
// If a worker exits with an error, the behavior depends on the value
// of the Retry flag on the worker. If Retry is true, the worker will
// be restarted. If not the supervisor shutdown sequence is triggerd.
//
// The supervisor shutdown sequence can be deliberately triggered by
// invoking supervisor.Shutdown(). This can be done from any goroutine
// including workers.
//
// The graceful shutdown sequence shuts down workers in an order that
// respects worker dependencies.
func (s *Supervisor) Run() []error {
s.mutex.Lock()
defer s.mutex.Unlock()
// we make cancel trigger shutdown so that simple cases only
// need to worry about shutdown
go func() {
ticker := time.NewTicker(1 * time.Second)
for {
select {
case <-s.context.Done():
s.Shutdown()
ticker.Stop()
return
case <-ticker.C:
s.changed.Broadcast()
}
}
}()
// reconcile may delete workers
s.reconcile()
for len(s.workers) > 0 {
s.changed.Wait()
s.reconcile()
}
return s.errors
}
// Triggers a graceful shutdown sequence. This can be invoked from any
// goroutine.
func (s *Supervisor) Shutdown() {
s.change(func() {
s.wantsShutdown = true
})
}
// Gets the worker with the specified name. Will return nil if no such
// worker exists.
func (s *Supervisor) Get(name string) *Worker {
s.mutex.Lock()
defer s.mutex.Unlock()
return s.workers[name]
}
func (s *Supervisor) dependents(worker *Worker) (result []*Worker) {
for _, n := range s.names {
w := s.workers[n]
for _, r := range w.Requires {
if r == worker.Name {
result = append(result, w)
break
}
}
}
return
}
// make sure anything that would like to be running is actually
// running
func (s *Supervisor) reconcile() {
var cleanup []string
for _, n := range s.names {
w := s.workers[n]
remove := w.reconcile()
if remove {
cleanup = append(cleanup, w.Name)
}
}
for _, n := range cleanup {
w := s.workers[n]
s.remove(w)
}
}
func (s *Supervisor) launch(worker *Worker) {
process := &Process{
supervisor: s,
worker: worker,
shutdown: make(chan struct{}),
}
worker.process = process
go func() {
var err error
func() {
defer func() {
if r := recover(); r != nil {
stack := string(debug.Stack())
err = errors.Errorf("WORKER PANICKED: %v\n%s", r, stack)
}
}()
time.Sleep(worker.retryDelay)
err = worker.Work(process)
}()
s.mutex.Lock()
defer s.mutex.Unlock()
worker.process = nil
if err != nil {
process.Logf("ERROR: %v", err)
if worker.Retry {
if worker.shuttingDown() {
s.remove(worker)
worker.done = true
} else {
worker.retryDelay = nextDelay(worker.retryDelay)
process.Logf("retrying after %s...", worker.retryDelay.String())
}
} else {
s.remove(worker)
worker.error = err
s.errors = append(s.errors, worker)
s.wantsShutdown = true
worker.done = true
}
} else {
process.Logf("exited")
s.remove(worker)
worker.done = true
}
s.changed.Broadcast()
}()
}