What is context.Context and why is it the first parameter in so many Go functions?

// The context.Context interface:
// Done()     <-chan struct{}  — closed when cancelled or deadline passed
// Err()      error           — nil, Canceled, or DeadlineExceeded
// Deadline() (time.Time, bool)
// Value(key) any

// Root contexts
ctx := context.Background()  // top-level: no deadline, no cancel
ctx  = context.TODO()        // placeholder when ctx not yet determined

// Derived contexts — ALWAYS defer cancel()
ctx1, cancel1 := context.WithCancel(context.Background())
defer cancel1()  // prevents goroutine leak inside context machinery

ctx2, cancel2 := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel2()

// Use in a worker goroutine
func fetchData(ctx context.Context, url string) ([]byte, error) {
    req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
    resp, err := http.DefaultClient.Do(req)
    if err != nil { return nil, err }  // err contains context.DeadlineExceeded
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}

// Checking cancellation in a loop
func processItems(ctx context.Context, items []Item) error {
    for _, item := range items {
        select {
        case <-ctx.Done():   // cancelled — stop early
            return ctx.Err()
        default:            // continue
        }
        process(item)
    }
    return nil
}