Shattered Mirror: An Introduction to Reflect and Unsafe

Go gives developers compile-time safety through its type system, and flexibility through interfaces. Sometimes, interfaces just aren’t flexible enough. Sometimes, you just want to write some code that isn’t safe at all. In this talk, you’ll learn about the outlaw packages of Go: reflect and unsafe. You’ll learn just enough about Go’s type system to understand how reflect and unsafe subvert it. Finally, you’ll see practical examples of using reflect, and some impractical examples of using unsafe.

Transcript

1. @jrbowes Shattered Mirror An introduction to reflect and unsafe
2. @jrbowes
3. @jrbowes
4. @jrbowes
5. @jrbowes
6. @jrbowes
7. You are already using reflect @jrbowes
8. import “encoding/json" @jrbowes
9. import “encoding/json" func Marshal(v interface{}) ([]byte, error) func Unmarshal(data []byte, v interface{}) error @jrbowes
10. type Response struct { Code int `json:"code"` Message string `json:"message"` } @jrbowes
11. You are already using reflect @jrbowes
12. You are already using reflect and unsafe @jrbowes
13. package main func main() { } @jrbowes
14. $> go list -f '{{.Deps}}' @jrbowes
15. $> go list -f '{{.Deps}}' [internal/bytealg internal/cpu runtime runtime/ internal/atomic runtime/internal/sys unsafe] @jrbowes
16. $> go list -f '{{.Deps}}' [internal/bytealg internal/cpu runtime runtime/ internal/atomic runtime/internal/sys unsafe] @jrbowes
17. It starts and ends with types @jrbowes
18. file, err := os.Open("data.txt") @jrbowes
19. file, err := os.Open("data.txt") @jrbowes
20. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes
21. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes
22. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes w := rs.(io.Writer)
23. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes w := rs.(io.Writer) "
24. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes w := rs.(io.Writer) var r io.Reader = w.(io.ReadWriter) "
25. file, err := os.Open("data.txt") var rs io.ReadSeeker = file @jrbowes w := rs.(io.Writer) var r io.Reader = w.(io.ReadWriter) " "
26. @jrbowes os.Open("data.txt")
27. @jrbowes os.Open("data.txt") type File struct {}
28. @jrbowes os.Open("data.txt") type File struct {} type Writer interface {}
29. The Go runtime needs your type information @jrbowes
30. The Go runtime needs your type information …mostly @jrbowes
31. reflect @jrbowes
32. func ValueOf(i interface{}) Value func TypeOf(i interface{}) Type @jrbowes
33. oneThing := another @jrbowes
34. func main() { c := make(chan bool) go func() { v := reflect.ValueOf(c) b := reflect.Zero(v.Type().Elem()) v.Send(b) v.Close() }() for range c { fmt.Println("saw value”) } } @jrbowes
35. func hello(msg string) { fmt.Println(msg) } func main() { f := reflect.ValueOf(hello) args := []reflect.Value{ reflect.ValueOf("hi!"), } f.Call(args) } @jrbowes
36. type A struct { Name string Value string `rename:"val"` } func main() { a := A{Name: "a name", Value: "a value"} v := reflect.ValueOf(a) vt := v.Type() for i := 0; i < v.NumField(); i++ { f := vt.Field(i) name := f.Name if t, ok := f.Tag.Lookup("rename"); ok { name = t } fmt.Printf("%s: %s\n", name, v.Field(i)) } } @jrbowes
37. func main() { st := reflect.StructOf([]reflect.StructField{{ Name: "Message", Type: reflect.TypeOf(""), Tag: `json:"msg"`, }}) s := reflect.New(st) s.Elem().FieldByName("Message").Set(reflect.ValueOf("hi!")) i := s.Interface() o, _ := json.Marshal(i) fmt.Println(string(o)) } @jrbowes
38. @jrbowes
39. @jrbowes
40. @jrbowes reflect.ValueOf() reflect.MakeFunc()
41. unsafe @jrbowes
42. @jrbowes file, err := os.Open(“data.txt”) var rs io.ReadSeeker = file w := rs.(io.Writer) " var r io.Reader = w.(io.ReadWriter) " type Writer interface {} type File struct {} os.Open(“data.txt") oneThing := another
43. @jrbowes file, err := os.Open(“data.txt”) var rs io.ReadSeeker = file w := rs.(io.Writer) " var r io.Reader = w.(io.ReadWriter) " type Writer interface {} type File struct {} os.Open(“data.txt") oneThing := another
44. type A struct { Name string } type B struct { Name, Value string } func main() { b := B{"a thing", "a value”} a := (*A)(&b) } @jrbowes
45. type A struct { Name string } type B struct { Name, Value string } func main() { b := B{"a thing", "a value"} a := (*A)(unsafe.Pointer(&b)) } @jrbowes
46. func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno) @jrbowes
47. reflect @jrbowes
48. reflect @jrbowes needs unsafe
49. package unsafe type ArbitraryType int type Pointer *ArbitraryType func Sizeof(x ArbitraryType) uintptr func Offsetof(x ArbitraryType) uintptr func Alignof(x ArbitraryType) uintptr @jrbowes
50. @jrbowes
51. @jrbowes
52. @jrbowes func billingState( ctx context.Context, billing *client.Billing, op *Operation ) (string, error) next, err := step()
53. @jrbowes func billingState( ctx context.Context, billing *client.Billing, op *Operation ) (string, error) next, err := step()
54. @jrbowes func billingState( ctx context.Context, billing *client.Billing, op *Operation ) (string, error) next, err := step()
55. @jrbowes func billingState( ctx context.Context, billing *client.Billing, op *Operation ) (string, error) next, err := step()
56. Don’t use reflect for runtime dependency injection @jrbowes
57. Keep the flow of control in one direction @jrbowes
58. reflect is best for reading @jrbowes
59. reflect is good for writing data @jrbowes
60. Don’t use unsafe directly ;) @jrbowes
61. FIND ME github.com/jbowes twitter.com/jrbowes manifold.co Thank you! @jrbowes