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Golang並行処理入門part2

8 min read

コレは何?

Golangの並行処理に入門したときのメモであり、以下の記事の続きです。普段はHCLやyamlを中心に扱っているため、久しぶりにGolangのお勉強がてらまとめました。

https://zenn.dev/nameless_gyoza/articles/golang-concurrency-part1

前提

  • 並行処理 is 何?は書いてない
  • Golangの並行処理の一つであるContextについてサラッと書いてる

Context

どんなものか?

goroutineとchannelだけで頑張ろうとするとタイムアウトやgoroutineのcancelハンドリングなどに対する考慮が難しいことが多い。
contextを使うことでこれらの考慮をよりお手軽に対応できるようになる(しかも標準パッケージとして!)。

Package contextでも以下のような説明がある。

Package context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.

The Go Blog - Go Concurrency Patterns: Contextにはこのように書かれている。

At Google, we developed a context package that makes it easy to pass request-scoped values, cancelation signals, and deadlines across API boundaries to all the goroutines involved in handling a request. The package is publicly available as context. This article describes how to use the package and provides a complete working example.

Contextのコアとなるのは Context typeである。
※コードはSource file src/context/context.goから拝借

type Context interface {
    // Deadline returns the time when work done on behalf of this context
    // should be canceled. Deadline returns ok==false when no deadline is
    // set. Successive calls to Deadline return the same results.
    Deadline() (deadline time.Time, ok bool)

    // Done returns a channel that's closed when work done on behalf of this
    // context should be canceled. Done may return nil if this context can
    // never be canceled. Successive calls to Done return the same value.
    // The close of the Done channel may happen asynchronously,
    // after the cancel function returns.
    //
    // WithCancel arranges for Done to be closed when cancel is called;
    // WithDeadline arranges for Done to be closed when the deadline
    // expires; WithTimeout arranges for Done to be closed when the timeout
    // elapses.
    //
    // Done is provided for use in select statements:
    //
    //  // Stream generates values with DoSomething and sends them to out
    //  // until DoSomething returns an error or ctx.Done is closed.
    //  func Stream(ctx context.Context, out chan<- Value) error {
    //  	for {
    //  		v, err := DoSomething(ctx)
    //  		if err != nil {
    //  			return err
    //  		}
    //  		select {
    //  		case <-ctx.Done():
    //  			return ctx.Err()
    //  		case out <- v:
    //  		}
    //  	}
    //  }
    //
    // See https://blog.golang.org/pipelines for more examples of how to use
    // a Done channel for cancellation.
    Done() <-chan struct{}

    // If Done is not yet closed, Err returns nil.
    // If Done is closed, Err returns a non-nil error explaining why:
    // Canceled if the context was canceled
    // or DeadlineExceeded if the context's deadline passed.
    // After Err returns a non-nil error, successive calls to Err return the same error.
    Err() error

    // Value returns the value associated with this context for key, or nil
    // if no value is associated with key. Successive calls to Value with
    // the same key returns the same result.
    //
    // Use context values only for request-scoped data that transits
    // processes and API boundaries, not for passing optional parameters to
    // functions.
    //
    // A key identifies a specific value in a Context. Functions that wish
    // to store values in Context typically allocate a key in a global
    // variable then use that key as the argument to context.WithValue and
    // Context.Value. A key can be any type that supports equality;
    // packages should define keys as an unexported type to avoid
    // collisions.
    //
    // Packages that define a Context key should provide type-safe accessors
    // for the values stored using that key:
    //
    // 	// Package user defines a User type that's stored in Contexts.
    // 	package user
    //
    // 	import "context"
    //
    // 	// User is the type of value stored in the Contexts.
    // 	type User struct {...}
    //
    // 	// key is an unexported type for keys defined in this package.
    // 	// This prevents collisions with keys defined in other packages.
    // 	type key int
    //
    // 	// userKey is the key for user.User values in Contexts. It is
    // 	// unexported; clients use user.NewContext and user.FromContext
    // 	// instead of using this key directly.
    // 	var userKey key
    //
    // 	// NewContext returns a new Context that carries value u.
    // 	func NewContext(ctx context.Context, u *User) context.Context {
    // 		return context.WithValue(ctx, userKey, u)
    // 	}
    //
    // 	// FromContext returns the User value stored in ctx, if any.
    // 	func FromContext(ctx context.Context) (*User, bool) {
    // 		u, ok := ctx.Value(userKey).(*User)
    // 		return u, ok
    // 	}
    Value(key interface{}) interface{}
}

Contextを利用する上でのお作法

Package contextには以下のようなルールが記載されている。

  • Do not store Contexts inside a struct type; instead, pass a Context explicitly to each function that needs it. The Context should be the first parameter, typically named ctx:
// Bad
type User struct{
 ctx    context.Context
 userID string
 ...
}

// Good
func DoSomething(ctx context.Context, arg Arg) error {
// ... use ctx ...
}
  • Do not pass a nil Context, even if a function permits it. Pass context.TODO if you are unsure about which Context to use.
  • Use context Values only for request-scoped data that transits processes and APIs, not for passing optional parameters to functions.
    • 型安全ではないため、APIリクエストに伴って付与される情報に限り利用する

      Good examples of request scoped data include user IDs extracted from headers, authentication tokens tied to cookies or session IDs, distributed tracing IDs, and so on.

// Don't do this.
func MyMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func (w http.ResponseWriter, r *http.Request) {
		// 型安全じゃないから無闇に使うものじゃない
		db := r.Context().Value(DatabaseContextKey).(Database)
		// Use db somehow 
		next.ServeHTTP(w, r) 
	})
}
  • The same Context may be passed to functions running in different goroutines; Contexts are safe for simultaneous use by multiple goroutines.

参考