Email delivery via HTTP provider with context, timeout, and idempotency

                  package mail

import (
  "context"
  "bytes"
  "encoding/json"
  "errors"
  "net/http"
  "time"
)

type Payload struct {
  From    string `json:"from"`
  To      string `json:"to"`
  Subject string `json:"subject"`
  Text    string `json:"text"`
}

func Send(ctx context.Context, client *http.Client, endpoint, apiKey, idemKey string, p Payload) error {
  b, err := json.Marshal(p)
  if err != nil {
    return err
  }

  cctx, cancel := context.WithTimeout(ctx, 3*time.Second)
  defer cancel()

  req, err := http.NewRequestWithContext(cctx, http.MethodPost, endpoint, bytes.NewReader(b))
  if err != nil {
    return err
  }
  req.Header.Set("Content-Type", "application/json")
  req.Header.Set("Authorization", "Bearer "+apiKey)
  req.Header.Set("Idempotency-Key", idemKey)

  resp, err := client.Do(req)
  if err != nil {
    return err
  }
  defer resp.Body.Close()
  if resp.StatusCode < 200 || resp.StatusCode >= 300 {
    return errors.New("email provider returned non-2xx")
  }
  return nil
}