List of advices and tricks for Go \ʕ◔ϖ◔ʔ/
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README.md

Go-advices

(Some of advices are implemented in go-critic )

中文版

Contents

Code

  • go fmt your code, make everyone happier

  • multiple if statements can be collapsed into switch

  • use chan struct{} to pass signal, chan bool makes it less clear

  • prefer 30 * time.Second instead of time.Duration(30) * time.Second

  • always wrap for-select idiom to a function

  • group const declarations by type and var by logic and/or type

  • every blocking or IO function call should be cancelable or at least timeoutable

  • implement Stringer interface for integers const values

  • check your defer's error

    defer func() {
        err := ocp.Close()
        if err != nil {
            rerr = err
        }
    }()
  • don't use checkErr function which panics or does os.Exit

  • use panic only in very specific situations, you have to handle error

  • don't use alias for enums 'cause this breaks type safety

    package main
    type Status = int
    type Format = int // remove `=` to have type safety
    
    const A Status = 1
    const B Format = 1
    
    func main() {
      println(A == B)
    }
  • if you're going to omit returning params, do it explicitly

    • so prefer this _ = f() to this f()
  • we've a short form for slice initialization a := []T{}

  • iterate over array or slice using range loop

    • instead of for i := 3; i < 7; i++ {...} prefer for _, c := range a[3:7] {...}
  • use backquote(`) for multiline strings

  • skip unused param with _

    func f(a int, _ string) {}
  • If you are comparing timestamps, use time.Before or time.After. Don't use time.Sub to get a duration and then check its value.

  • always pass context as a first param to a func with a ctx name

  • few params of the same type can be defined in a short way

    func f(a int, b int, s string, p string)
    func f(a, b int, s, p string)
  • the zero value of a slice is nil

      var s []int
      fmt.Println(s, len(s), cap(s))
      if s == nil {
        fmt.Println("nil!")
      }
      // Output:
      // [] 0 0
      // nil!
    var a []string
    b := []string{}
    
    fmt.Println(reflect.DeepEqual(a, []string{}))
    fmt.Println(reflect.DeepEqual(b, []string{}))
    // Output:
    // false
    // true
  • do not compare enum types with <, >, <= and >=

    • use explicit values, don't do this:
    value := reflect.ValueOf(object)
    kind := value.Kind()
    if kind >= reflect.Chan && kind <= reflect.Slice {
      // ...
    }
  • use %+v to print data with sufficient details

  • be careful with empty struct struct{}, see issue: https://github.com/golang/go/issues/23440

    func f1() {
      var a, b struct{}
      print(&a, "\n", &b, "\n") // Prints same address
      fmt.Println(&a == &b)     // Comparison returns false
    }
    
    func f2() {
      var a, b struct{}
      fmt.Printf("%p\n%p\n", &a, &b) // Again, same address
      fmt.Println(&a == &b)          // ...but the comparison returns true
    }
  • wrap errors with http://github.com/pkg/errors

    • so: errors.Wrap(err, "additional message to a given error")
  • be careful with range in Go:

  • reading nonexistent key from map will not panic

    • value := map["no_key"] will be zero value
    • value, ok := map["no_key"] is much better
  • do not use raw params for file operation

    • instead of an octal parameter like os.MkdirAll(root, 0700)
    • use predefined constants of this type os.FileMode
  • don't forget to specify a type for iota

    const (
      _ = iota
      testvar         // will be int
    )

    vs

    type myType int
    const (
      _ myType = iota
      testvar         // will be myType
    )
  • use _ = b[7] for early bounds check to guarantee safety of writes below

  • don’t use encoding/gob on structs you don’t own

    • it's not protected from newly added or reordered fields

Concurrency

  • best candidate to make something once in a thread-safe way is sync.Once
    • don't use flags, mutexes, channels or atomics
  • to block forever use select{}, omit channels, waiting for a signal
  • don't close in-channel, this is a responsibility of it's creator
    • writing to a closed channel will cause a panic
  • func NewSource(seed int64) Source in math/rand is not concurrency-safe. The default lockedSource is concurrency-safe, see issue: https://github.com/golang/go/issues/3611

Performance

  • do not omit defer
    • 200ns speedup is negligible in most cases
  • always close http body aka defer r.Body.Close()
    • unless you need leaked goroutine
  • filtering without allocating
      b := a[:0]
      for _, x := range a {
      	if f(x) {
      	    b = append(b, x)
      	}
      }
  • time.Time has pointer field time.Location and this is bad for go GC
    • it's relevant only for big number of time.Time, use timestamp instead
  • prefer regexp.MustCompile instead of regexp.Compile
    • in most cases your regex is immutable, so init it in func init
  • do not overuse fmt.Sprintf in your hot path. It is costly due to maintaining the buffer pool and dynamic dispatches for interfaces.
    • if you are doing fmt.Sprintf("%s%s", var1, var2), consider simple string concatenation.
    • if you are doing fmt.Sprintf("%x", var), consider using hex.EncodeToString or strconv.FormatInt(var, 16)
  • always discard body e.g. io.Copy(ioutil.Discard, resp.Body) if you don't use it
    • HTTP client's Transport will not reuse connections unless the body is read to completion and closed
      res, _ := client.Do(req)
      io.Copy(ioutil.Discard, res.Body)
      defer res.Body.Close()
  • don't use defer in a loop or you'll get a small memory leak
    • 'cause defers will grow your stack without the reason
  • don't forget to stop ticker, unless you need a leaked channel
    ticker := time.NewTicker(1 * time.Second)
    defer ticker.Stop()
  • use custom marshaler to speed up marshaling
    func (entry Entry) MarshalJSON() ([]byte, error) {
      buffer := bytes.NewBufferString("{")
      first := true
      for key, value := range entry {
      	jsonValue, err := json.Marshal(value)
      	if err != nil {
      		return nil, err
      	}
      	if !first {
      		buffer.WriteString(",")
      	}
      	first = false
      	buffer.WriteString(key + ":" + string(jsonValue))
      }
      buffer.WriteString("}")
      return buffer.Bytes(), nil
    }
  • sync.Map isn't a silver bullet, do not use it without a strong reasons
  • storing non-pointer values in sync.Pool allocates memory
  • regular expressions are mutexed
    • to avoid performance degradation in concurrent programs make a copy:
    re, err := regexp.Compile(pattern)
    re2 := re.Copy()
  • to hide a pointer from escape analysis you might carefully(!!!) use this func:
    // noescape hides a pointer from escape analysis.  noescape is
    // the identity function but escape analysis doesn't think the
    // output depends on the input. noescape is inlined and currently
    // compiles down to zero instructions.
    func noescape(p unsafe.Pointer) unsafe.Pointer {
    	x := uintptr(p)
    	return unsafe.Pointer(x ^ 0)
    }
  • for fastest atomic swap you might use this m := (*map[int]int)(atomic.LoadPointer(&ptr))
  • use buffered I/O if you do many sequential reads or writes
    • to reduce number of syscalls

Build

Testing

  • prefer package_test name for tests, rather than package
  • go test -short allows to reduce set of tests to be runned
    func TestSomething(t *testing.T) {
      if testing.Short() {
        t.Skip("skipping test in short mode.")
      }
    }
  • skip test depending on architecture
    if runtime.GOARM == "arm" {
      t.Skip("this doesn't work under ARM")
    }
  • track your allocations with testing.AllocsPerRun
  • run your benchmarks multiple times, to get rid of noise
    • go test -test.bench=. -count=20

Tools

  • quick replace gofmt -w -l -r "panic(err) -> log.Error(err)" .
  • go list allows to find all direct and transitive dependencies
    • go list -f '{{ .Imports }}' package
    • go list -f '{{ .Deps }}' package
  • for fast benchmark comparison we've a benchstat tool
  • go-critic linter enforces several advices from this document

Misc

  • dump goroutines https://stackoverflow.com/a/27398062/433041
    go func() {
      sigs := make(chan os.Signal, 1)
      signal.Notify(sigs, syscall.SIGQUIT)
      buf := make([]byte, 1<<20)
      for {
        <-sigs
        stacklen := runtime.Stack(buf, true)
        log.Printf("=== received SIGQUIT ===\n*** goroutine dump...\n%s\n*** end\n", buf[:stacklen])
      }
    }()
  • check interface implementation during compilation
    var _ io.Reader = (*MyFastReader)(nil)
  • if a param of len is nil then it's zero
  • anonymous structs are cool
    var hits struct {
      sync.Mutex
      n int
    }
    hits.Lock()
    hits.n++
    hits.Unlock()
  • httputil.DumpRequest is very useful thing, don't create your own
  • to get call stack we've runtime.Caller https://golang.org/pkg/runtime/#Caller
  • to marshal arbitrary JSON you can marshal to map[string]interface{}{}
  • configure your CDPATH so you can do cd github.com/golang/go from any directore
    • add this line to your bashrc(or analogue) export CDPATH=$CDPATH:$GOPATH/src