Some context on Context

With Go 1.7 the context package was added to Go's standard library, but many still are figuring out what this is actually for and what are the best practices when it comes to adding it to their APIs. This talk will cover the topic in detail, not only explaining how it is to be used. It will also get into the internal details and show some of the most beautiful pieces of code in the context package.

Transcript

2. Is the WiFi working?
3. func pingGoogle(ctx context.Context) error { res, err := http.Get(“https://google.com”) if err != nil { return err } Is the WiFi working?
4. Is the WiFi working? func pingGoogle(ctx context.Context) error { res, err := http.Get(“https://google.com”) if err != nil { return err } if res.StatusCode != http.StatusOK { return fmt.Errorf(“got %s from Google”, res.Status) } return nil }
5. func main() { if err := pingGoogle(); err != nil { log.Printf(“could not ping Google: %v”, err) } } Is the WiFi working?
7. How could we cancel the ping?
8. the context package @francesc
9. Two years ago in Bengaluru … … Dave and I discovered pan
10. the context package - Added to the standard library with go 1.7 in 2016 - golang.org/x/net/context in 2014 - created at Google in 2013 - 492 L.O.C. (369 without comments) - Exposes only two types, one of them just for documentation. How interesting could it be?
11. the Context interface
12. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
13. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
14. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
15. How could we cancel the ping?
16. func pingWithContext(ctx context.Context) error { errc := make(chan error) go func() { errc <- pingGoogle() }() Using the Context interface
17. func pingWithContext(ctx context.Context) error { errc := make(chan error) go func() { errc <- pingGoogle() }() select { case err := <-errc: return err Using the Context interface
18. func pingWithContext(ctx context.Context) error { errc := make(chan error) go func() { errc <- pingGoogle() }() select { case err := <-errc: return err case <-ctx.Done(): return ctx.Err() } } Using the Context interface
19. func main() { ctx := ??? if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
20. creating Context values
21. func Background() Context func TODO() Context func WithCancel(parent Context) (ctx Context, cancel CancelFunc) func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) func WithValue(parent Context, key, val interface{}) Context creating Context values
22. func Background() Context func TODO() Context func WithCancel(parent Context) (ctx Context, cancel CancelFunc) func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) func WithValue(parent Context, key, val interface{}) Context creating Context values
23. func main() { ctx := context.Background() if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
24. func main() { ctx := context.TODO() if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
25. func Background() Context func TODO() Context func WithCancel(parent Context) (ctx Context, cancel CancelFunc) func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) func WithValue(parent Context, key, val interface{}) Context creating Context values
26. func main() { ctx := context.Background() ctx, cancel := context.WithCancel(ctx) time.AfterFunc(1 * time.Second, cancel) if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
28. func Background() Context func TODO() Context func WithCancel(parent Context) (ctx Context, cancel CancelFunc) func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) func WithValue(parent Context, key, val interface{}) Context creating Context values
29. func main() { ctx := context.Background() ctx, cancel := context.WithTimeout(ctx, time.Second) if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
30. func Background() Context func TODO() Context func WithCancel(parent Context) (ctx Context, cancel CancelFunc) func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) func WithValue(parent Context, key, val interface{}) Context creating Context values
31. type key int const userIDKey = 0 func main() { ctx := context.WithValue(userIDKey, 12345) if err := pingWithContext(ctx); err != nil { log.Printf(“could not ping Google: %v”, err) } } Using the Context interface
32. creating Context values Background WithCancel WithValue WithValue WithDeadline WithValue
33. back to the Context interface
34. var Canceled = errors.New("context canceled") errors in context
35. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} errors in context
36. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} errors in context
37. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
38. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
39. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
40. if tmp, ok := err.(interface { Temporary() bool }); ok { if tmp.Temporary() { // retry } else { // report } } errors in context
41. Go Proverb Dave Cheney - GopherCon 2016 Don’t just check errors, handle them gracefully
42. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
43. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
44. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
45. Interface values are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value nil. - Go specification
46. Interface values are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value nil. - Go specification
47. type myLibKey int ctx = WithValue(ctx, myLibKey(42), “one thing”) context key types
48. type myLibKey int ctx = WithValue(ctx, myLibKey(42), “one thing”) type myOtherLibKey int ctx = WithValue(ctx, myOtherLibKey(42), “the other thing”) context key types
49. context key types type myLibKey int ctx = WithValue(ctx, myLibKey(42), “one thing”) type myOtherLibKey int ctx = WithValue(ctx, myOtherLibKey(42), “the other thing”) ctx.Value(myLibKey(42)) ctx.Value(myOtherLibKey(42))
50. myLibKey(42) != myOtherLibKey(42)
51. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
52. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} Cancel() } the Context interface
53. func doAll(ctx context.Context) { ctx, cancel := context.WithCancel(ctx) defer cancel() for i := 0; i < 10; i++ { go doOne(ctx) } … } the Context interface
54. func doOne(ctx context.Context) { ctx.Cancel() } the Context interface
55. the context implementation
56. satisfying the Context interface context Context
57. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } the Context interface
58. type context struct { done <-chan struct{} err error deadline time.Time Values map[interface{}]interface{} } the Context struct?
59. satisfying the Context interface emptyCtx cancelCtx deadlineCtx timerCtx valueCtx Context
60. empty context
61. type emptyCtx struct{} the empty context
62. type emptyCtx struct{} var background = new(emptyCtx) var todo = new(emptyCtx) the empty context
63. the empty context type emptyCtx struct{} var background = new(emptyCtx) var todo = new(emptyCtx) func (e *emptyCtx) String() string { switch e { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" }
64. the empty context type emptyCtx struct{} var background = new(emptyCtx) var todo = new(emptyCtx) func (e *emptyCtx) String() string { switch e { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" }
65. fmt.Println(todo) // background the empty context type emptyCtx struct{} var background = new(emptyCtx) var todo = new(emptyCtx) func (e *emptyCtx) String() string { switch e { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" }
66. the empty context type emptyCtx struct{} var background = new(emptyCtx) // 0x1040a124 var todo = new(emptyCtx) // 0x1040a124 func (e *emptyCtx) String() string { switch e { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" }
67. the “empty” context type emptyCtx int var background = new(emptyCtx) var todo = new(emptyCtx) func (e *emptyCtx) String() string { switch e { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" }
68. cancel context
69. type cancelCtx struct { Context done chan struct{} mu sync.Mutex err error } the cancel context
70. func (c cancelCtx) Done() <-chan struct{} { return c.done } func (c cancelCtx) Err() error { c.mu.Lock() defer c.mu.Unlock() return c.err } the cancel context
71. func (c cancelCtx) Done() <-chan struct{} { return c.done } func (c cancelCtx) Err() error { c.mu.Lock() defer c.mu.Unlock() return c.err } the cancel context
72. type cancelCtx struct { Context // provides and Value, Deadline done chan struct{} mu sync.Mutex err error } the cancel context
73. cancellation cascading Background WithCancel WithCancel WithCancel WithCancel
74. children []Context? children []CancelFunc? cancellation cascading child1 child2 child3 child4 child5 parent
75. func WithCancel(parent Context) (Context, CancelFunc) { child := newCancelCtx(parent) go func() { select { case <-parent.Done(): child.cancel(false, parent.Err()) case <-child.Done(): } }() … } cancellation cascading
76. func WithCancel(parent Context) (Context, CancelFunc) { child := newCancelCtx(parent) go func() { select { case <-parent.Done(): child.cancel(false, parent.Err()) case <-child.Done(): } }() … } cancellation cascading
77. func WithCancel(parent Context) (Context, CancelFunc) { child := newCancelCtx(parent) go func() { select { case <-parent.Done(): child.cancel(false, parent.Err()) case <-child.Done(): } }() … } cancellation cascading
78. one goroutine per context?!
79. cancellation cascading // A canceler is a context type that can be canceled directly. The // implementations are *cancelCtx and *timerCtx. type canceler interface { cancel(removeFromParent bool, err error) Done() <-chan struct{} }
80. children map[canceler]struct{} efficient cancellation cascading child1 child2 child3 child4 child5 parent
81. other context?
82. read more code
83. thanks @francesc