Go Context パッケージ 2026年完全ガイド：キャンセル、タイムアウト、面接対策

type Context interface {
    // Deadline returns the time when work should be canceled
    Deadline() (deadline time.Time, ok bool)
    
    // Done returns a channel that closes when the context is canceled
    Done() <-chan struct{}
    
    // Err returns the reason why Done was closed
    Err() error
    
    // Value returns the value associated with key, or nil
    Value(key any) any
}

package main

import (
    "context"
    "log"
    "net/http"
)

func main() {
    // Background: the root context for your application
    ctx := context.Background()
    
    // Use it as parent for derived contexts
    server := &http.Server{Addr: ":8080"}
    
    // TODO: placeholder when context source is unclear
    // Static analysis tools can flag context.TODO() for review
    processLegacyData(context.TODO())
}

func processLegacyData(ctx context.Context) {
    // Context parameter enables future cancellation support
    log.Println("Processing data...")
}

package main

import (
    "context"
    "errors"
    "fmt"
    "time"
)

func worker(ctx context.Context, id int, results chan<- int) {
    for {
        select {
        case <-ctx.Done():
            // Check why cancellation occurred
            if cause := context.Cause(ctx); cause != nil {
                fmt.Printf("Worker %d stopped: %v\n", id, cause)
            }
            return
        default:
            // Simulate work
            time.Sleep(100 * time.Millisecond)
            results <- id * 10
        }
    }
}

func main() {
    // WithCancelCause provides error context
    ctx, cancel := context.WithCancelCause(context.Background())
    results := make(chan int, 10)
    
    // Start 3 workers
    for i := 1; i <= 3; i++ {
        go worker(ctx, i, results)
    }
    
    // Collect some results
    for i := 0; i < 5; i++ {
        fmt.Println("Result:", <-results)
    }
    
    // Cancel with a specific reason
    cancel(errors.New("shutdown requested by user"))
    
    // context.Cause retrieves the cancellation reason
    time.Sleep(50 * time.Millisecond)
    fmt.Println("Cause:", context.Cause(ctx))
}

package main

import (
    "context"
    "fmt"
    "time"
)

// fetchFromAPI simulates an HTTP call
func fetchFromAPI(ctx context.Context, endpoint string) (string, error) {
    // Simulate variable response time
    responseTime := time.Duration(100+endpoint[0]%150) * time.Millisecond
    
    select {
    case <-time.After(responseTime):
        return fmt.Sprintf("Response from %s", endpoint), nil
    case <-ctx.Done():
        return "", ctx.Err()
    }
}

func main() {
    // WithTimeout: relative duration
    ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
    defer cancel() // Always call cancel to release resources
    
    result, err := fetchFromAPI(ctx, "api.example.com/users")
    if err != nil {
        if err == context.DeadlineExceeded {
            fmt.Println("Request timed out")
        } else {
            fmt.Println("Request canceled")
        }
        return
    }
    fmt.Println(result)
    
    // WithDeadline: absolute time
    deadline := time.Now().Add(500 * time.Millisecond)
    ctx2, cancel2 := context.WithDeadline(context.Background(), deadline)
    defer cancel2()
    
    result2, _ := fetchFromAPI(ctx2, "api.example.com/orders")
    fmt.Println(result2)
}

package main

import (
    "context"
    "fmt"
    "log"
    "net/http"
    "time"
)

// Define custom key types to avoid collisions
type contextKey string

const (
    requestIDKey contextKey = "requestID"
    userIDKey    contextKey = "userID"
)

// middleware adds request ID to context
func requestIDMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        requestID := r.Header.Get("X-Request-ID")
        if requestID == "" {
            requestID = generateRequestID()
        }
        
        // Create new context with request ID
        ctx := context.WithValue(r.Context(), requestIDKey, requestID)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

// handler retrieves values from context
func handler(w http.ResponseWriter, r *http.Request) {
    requestID, ok := r.Context().Value(requestIDKey).(string)
    if !ok {
        requestID = "unknown"
    }
    
    log.Printf("[%s] Processing request", requestID)
    fmt.Fprintf(w, "Request ID: %s", requestID)
}

func generateRequestID() string {
    return fmt.Sprintf("req-%d", time.Now().UnixNano())
}

func main() {
    mux := http.NewServeMux()
    mux.HandleFunc("/", handler)
    
    http.ListenAndServe(":8080", requestIDMiddleware(mux))
}

package main

import (
    "context"
    "fmt"
    "log"
    "time"
)

func saveAuditLog(ctx context.Context, message string) error {
    // Use WithoutCancel: audit logs must complete even if request canceled
    cleanCtx := context.WithoutCancel(ctx)
    
    // Simulate database write
    select {
    case <-time.After(50 * time.Millisecond):
        log.Printf("Audit: %s", message)
        return nil
    case <-cleanCtx.Done():
        // This branch never executes: WithoutCancel context is never canceled
        return cleanCtx.Err()
    }
}

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
    
    // AfterFunc: register cleanup when context ends
    stop := context.AfterFunc(ctx, func() {
        fmt.Println("Context ended, running cleanup")
    })
    
    // Perform operation
    time.Sleep(50 * time.Millisecond)
    
    // Cancel stops the AfterFunc from running if called before context ends
    if stop() {
        fmt.Println("Cleanup prevented")
    }
    
    cancel()
    
    // Audit log completes despite parent cancellation
    saveAuditLog(ctx, "Operation completed")
}

// BAD: context stored in struct
type Service struct {
    ctx context.Context // Never do this
}

// GOOD: pass context to each method
type Service struct{}

func (s *Service) Process(ctx context.Context, data []byte) error {
    // Context flows through the call chain
    return s.save(ctx, data)
}

// Correct pattern for long-running goroutines
func processStream(ctx context.Context, stream <-chan Data) {
    for {
        select {
        case <-ctx.Done():
            log.Println("Shutting down processor")
            return
        case data, ok := <-stream:
            if !ok {
                return
            }
            handle(data)
        }
    }
}

package main

import (
    "context"
    "time"
)

// MISTAKE 1: Ignoring context cancellation
func badWorker(ctx context.Context) {
    for {
        // Missing select on ctx.Done()
        doExpensiveWork() // Never stops when context canceled
    }
}

// MISTAKE 2: Not calling cancel
func leakyTimeout() {
    ctx, _ := context.WithTimeout(context.Background(), time.Second)
    // Timer goroutine leaks until timeout expires
    _ = ctx
}

// MISTAKE 3: Using string keys for values
func collisionProne(ctx context.Context) context.Context {
    // Different packages might use same string key
    return context.WithValue(ctx, "userID", 123) // Bad
}

// CORRECT versions
func goodWorker(ctx context.Context) {
    for {
        select {
        case <-ctx.Done():
            return
        default:
            doExpensiveWork()
        }
    }
}

func properTimeout() {
    ctx, cancel := context.WithTimeout(context.Background(), time.Second)
    defer cancel() // Always release resources
    _ = ctx
}

type userIDKey struct{}

func collisionSafe(ctx context.Context) context.Context {
    return context.WithValue(ctx, userIDKey{}, 123) // Good
}

func doExpensiveWork() {}