feat(client): implement exponential backoff retry for network errors

backend/internal/zarinhub/client.go

@@ -9,16 +9,24 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"time"
 
 	"card-to-iban/internal/service"
 )
 
-const maxResponseBodySize = 1 << 20
+const (
+	maxResponseBodySize = 1 << 20
+
+	retryBaseDelay = 200 * time.Millisecond
+	retryMaxDelay  = 2 * time.Second
+)
 
 type Client struct {
 	httpClient    *http.Client
 	endpointURL   string
 	tokenProvider TokenProvider
+	maxRetries    int
+	retryWait     func(ctx context.Context, delay time.Duration) bool
 }
 
 var _ service.CardToIBANClient = (*Client)(nil)
@@ -32,17 +40,114 @@ func NewClient(
 	httpClient *http.Client,
 	endpointURL string,
 	tokenProvider TokenProvider,
+	maxRetries int,
 ) *Client {
+	if maxRetries < 0 {
+		maxRetries = 0
+	}
+
 	return &Client{
 		httpClient:    httpClient,
 		endpointURL:   endpointURL,
 		tokenProvider: tokenProvider,
+		maxRetries:    maxRetries,
+		retryWait:     waitForRetry,
+	}
+}
+
+func isRetryable(err error) bool {
+	return errors.Is(err, ErrTimeout) ||
+		errors.Is(err, ErrUnavailable)
+}
+
+func retryDelay(attempt int) time.Duration {
+	delay := retryBaseDelay
+
+	for i := 0; i < attempt; i++ {
+		delay *= 2
+
+		if delay >= retryMaxDelay {
+			return retryMaxDelay
+		}
+	}
+
+	return delay
+}
+
+func waitForRetry(
+	ctx context.Context,
+	delay time.Duration,
+) bool {
+	timer := time.NewTimer(delay)
+	defer timer.Stop()
+
+	select {
+	case <-ctx.Done():
+		return false
+
+	case <-timer.C:
+		return true
 	}
 }
 
 func (c *Client) Convert(
 	ctx context.Context,
 	cardNumber string,
+) (service.ConversionResult, error) {
+	var (
+		result service.ConversionResult
+		err    error
+	)
+
+	waitFn := c.retryWait
+	if waitFn == nil {
+		waitFn = waitForRetry
+	}
+
+	for attempt := 0; attempt <= c.maxRetries; attempt++ {
+		result, err = c.convertOnce(
+			ctx,
+			cardNumber,
+		)
+
+		if err == nil {
+			return result, nil
+		}
+
+		// Don't retry permanent/non-transient errors.
+		if !isRetryable(err) {
+			return result, err
+		}
+
+		// We already used all configured retries.
+		if attempt == c.maxRetries {
+			return result, err
+		}
+
+		// If the caller canceled the request, stop immediately.
+		if ctx.Err() != nil {
+			if errors.Is(ctx.Err(), context.Canceled) {
+				return result, fmt.Errorf("zarinhub request canceled: %w", ctx.Err())
+			}
+			return result, err
+		}
+
+		delay := retryDelay(attempt)
+
+		if !waitFn(ctx, delay) {
+			if errors.Is(ctx.Err(), context.Canceled) {
+				return result, fmt.Errorf("zarinhub request canceled: %w", ctx.Err())
+			}
+			return result, err
+		}
+	}
+
+	return result, err
+}
+
+func (c *Client) convertOnce(
+	ctx context.Context,
+	cardNumber string,
 ) (service.ConversionResult, error) {
 	accessToken, err := c.tokenProvider.AccessToken(ctx)
 	if err != nil {