context

Code

package main

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

func operation(ctx context.Context) error {
    select {
    case <-time.After(2 * time.Second):
        return fmt.Errorf("Operation completed")
    case <-ctx.Done():
        return ctx.Err()
    }
}

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
    defer cancel()

    if err := operation(ctx); err != nil {
        fmt.Println("Error:", err)
    }

    // Context with value
    ctx2 := context.WithValue(context.Background(), "userID", 42)
    fmt.Println(ctx2.Value("userID"))
}

Explanation

context passes cancellation signals, timeouts, and values between goroutines.

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