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Coding in the context era
- 1. Coding in the ContextEra Go Conference 2017 Spring Mar 25, 2017 Daisuke Maki @lestrrat
- 2. • @lestrrat • Perl/Gohacker, author, father • Author of github.com/peco/peco • Organizer for builderscon
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- 4. (we run onKubernetes)
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- 6. Web+DB Press vol98 (Apr 2017)
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- 9. func (t *Obj)Start() { go func() { for { select { case <-t.done: … default: } … }() } } }
- 10. func (t *Obj)Stop() { close(t.done) }
- 11. THIS IS NOTSAFE
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- 13. API is ambiguous •Does it start a new goroutine every time Start() is called? • Does it error from the second one?
- 14. Only start one? •Maybe Start() can return an error if it has already been started • Bonus points: can it be re-entrant? • Requires state synchronization
- 15. func (obj *Obj)Start() error { obj.mu.Lock() if obj.started { obj.mu.Unlock() return errors.New(`already running`) } obj.started = true obj.mu.Unlock() … }
- 16. func (obj *Obj)Start() error { … go func() { defer func() { obj.mu.Lock() obj.started = false obj.mu.Unlock() }() for { … } }() }
- 17. func (obj *Obj)Start() error { … go func() { for { select { case <-obj.done: return default: } } … }() }
- 18. func (obj *Obj)Stop() error { obj.mu.Lock() if obj.started { close(obj.done) obj.mu.Unlock() return errors.New(`already running`) } obj.started = true obj.Unlock() }
- 19. This is stilla simplified version.
- 20. Life is tooshort for manual synchronization of concurrently executed code 人類に並行実行されている コードの手動同期は難しすぎる…！
- 21. Nobody wants todo this
- 22. Root cause: sharedresource • obj.started is shared • There is no way to escape manual locking
- 23. TRUTH YOU WILL MAKEA MISTAKE WHEN MANUALLY SYNCHRONIZING sync.Mutexes
- 24. AVOID sync.Mutex (unless youknow what you are doing)
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- 26. context.Context • abstracts awayshared state for cancellation
- 27. context.Context • Explicit cancellation •Timeouts • Deadlines • It’s a pattern: you can use it anywhere!
- 28. // To savesome typing, please assume the // following for the rest of this talk: ctxbg := context.Background() delay := 5 * time.Second
- 29. func (obj *Obj)Run(ctx context.Context) error { for { select { case <-ctx.Done(): return nil default: } … } }
- 30. ctx, cancel :=context.WithCancel(ctxbg) go obj.Run(ctx) // later in your code cancel()
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- 34. ctx, cancel :=context.WithTimeout(ctxbg, delay) defer cancel() go obj1.Run(ctx) // use a diff. ctx if need be go obj2.Run(ctx) // re-entrant! yay! go obj3.Run(ctx) Explicit semantics
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- 36. Prediction/Recommendation • Almost everythingthat can (1) block or (2) spawn a goroutine will support context.Context very soon. • You should use context.Context for anything that blocks!
- 37. Learn to ♡contexts
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