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 {

backend/internal/zarinhub/client_test.go

@@ -98,6 +98,7 @@ func TestClientConvertSuccess(t *testing.T) {
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{token}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -172,6 +173,7 @@ func TestClientReturnsRejectedForApplicationFailure(
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -264,6 +266,7 @@ func TestClientMapsHTTPStatusErrors(t *testing.T) {
 					},
 					server.URL,
 					&fakeTokenProvider{tokens: []string{"test-token"}},
+					0,
 				)
 
 				result, err := client.Convert(
@@ -322,6 +325,7 @@ func TestClientReturnsInvalidResponseForMalformedJSON(
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -367,6 +371,7 @@ func TestClientReturnsTimeout(t *testing.T) {
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	_, err := client.Convert(
@@ -403,6 +408,7 @@ func TestClientRejectsOversizedResponse(t *testing.T) {
 		&http.Client{Timeout: time.Second},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	_, err := client.Convert(
@@ -442,7 +448,7 @@ func TestClientRefreshesAndRetriesUnauthorizedRequest(t *testing.T) {
 	defer server.Close()
 
 	tokens := &fakeTokenProvider{tokens: []string{"expired-token", "fresh-token"}}
-	client := NewClient(server.Client(), server.URL, tokens)
+	client := NewClient(server.Client(), server.URL, tokens, 0)
 
 	result, err := client.Convert(context.Background(), "6037997512345678")
 	if err != nil {
@@ -478,3 +484,186 @@ func (p *fakeTokenProvider) AccessToken(context.Context) (string, error) {
 func (p *fakeTokenProvider) Invalidate(token string) {
 	p.invalidated = append(p.invalidated, token)
 }
+
+func TestRetryDelay(t *testing.T) {
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{attempt: -1, want: 200 * time.Millisecond},
+		{attempt: 0, want: 200 * time.Millisecond},
+		{attempt: 1, want: 400 * time.Millisecond},
+		{attempt: 2, want: 800 * time.Millisecond},
+		{attempt: 3, want: 1600 * time.Millisecond},
+		{attempt: 4, want: 2 * time.Second},
+		{attempt: 10, want: 2 * time.Second},
+	}
+
+	for _, tt := range tests {
+		got := retryDelay(tt.attempt)
+		if got != tt.want {
+			t.Errorf("retryDelay(%d) = %v, want %v", tt.attempt, got, tt.want)
+		}
+	}
+}
+
+func TestNewClientSanitizesNegativeRetries(t *testing.T) {
+	client := NewClient(nil, "", nil, -1)
+	if client.maxRetries != 0 {
+		t.Errorf("maxRetries = %d, want 0", client.maxRetries)
+	}
+}
+
+func TestClientRetriesOnTransientErrorsUntilSuccess(t *testing.T) {
+	const expectedIBAN = "IR820540102680020817909002"
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		if requestCount == 1 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			fmt.Fprint(w, `{"error":"temporary outage"}`)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprintf(w, `{
+			"data":{"iban":%q},
+			"meta":{"isSuccess":true,"errors":[]}
+		}`, expectedIBAN)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+
+	result, err := client.Convert(context.Background(), "6037997512345678")
+	if err != nil {
+		t.Fatalf("Convert() unexpected error = %v", err)
+	}
+	if result.IBAN != expectedIBAN {
+		t.Errorf("IBAN = %q, want %q", result.IBAN, expectedIBAN)
+	}
+	if requestCount != 2 {
+		t.Errorf("request count = %d, want 2", requestCount)
+	}
+	if len(recordedDelays) != 1 {
+		t.Fatalf("len(recordedDelays) = %d, want 1", len(recordedDelays))
+	}
+	if recordedDelays[0] != 200*time.Millisecond {
+		t.Errorf("delay[0] = %v, want 200ms", recordedDelays[0])
+	}
+}
+
+func TestClientRetriesExhausted(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusGatewayTimeout)
+		fmt.Fprint(w, `{"error":"gateway timeout"}`)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+
+	_, err := client.Convert(context.Background(), "6037997512345678")
+	if !errors.Is(err, ErrUnavailable) {
+		t.Fatalf("error = %v, want ErrUnavailable", err)
+	}
+	if requestCount != 3 {
+		t.Errorf("request count = %d, want 3", requestCount)
+	}
+	if len(recordedDelays) != 2 {
+		t.Fatalf("len(recordedDelays) = %d, want 2", len(recordedDelays))
+	}
+	if recordedDelays[0] != 200*time.Millisecond {
+		t.Errorf("delay[0] = %v, want 200ms", recordedDelays[0])
+	}
+	if recordedDelays[1] != 400*time.Millisecond {
+		t.Errorf("delay[1] = %v, want 400ms", recordedDelays[1])
+	}
+}
+
+func TestClientDoesNotRetryNonTransientErrors(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprint(w, `{"error":"invalid card format"}`)
+	}))
+	defer server.Close()
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		t.Error("retryWait should not be called for non-transient errors")
+		return true
+	}
+
+	_, err := client.Convert(context.Background(), "6037997512345678")
+	if !errors.Is(err, ErrRejected) {
+		t.Fatalf("error = %v, want ErrRejected", err)
+	}
+	if requestCount != 1 {
+		t.Errorf("request count = %d, want 1", requestCount)
+	}
+}
+
+func TestClientStopsRetryOnContextCancellation(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer server.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(c context.Context, delay time.Duration) bool {
+		cancel()
+		return waitForRetry(ctx, delay)
+	}
+
+	_, err := client.Convert(ctx, "6037997512345678")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("error = %v, want context.Canceled", err)
+	}
+	if requestCount != 1 {
+		t.Errorf("request count = %d, want 1", requestCount)
+	}
+}