feat: add rate-limit retry with exponential backoff in Gitea API client

Add automatic retry logic to doRequest for HTTP 429 responses. Uses
Retry-After header when present, otherwise exponential backoff
(1s, 2s, 4s). Respects context cancellation during waits. Defaults
to 3 max retries with 1s base delay. Includes 7 new tests covering
retry success, exhaustion, Retry-After header, context cancellation,
non-429 errors, and backoff calculation.

Closes leeworks-agents/gitea-mobile#132

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/gitea/client.go

@@ -8,8 +8,11 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
+	"math"
 	"net/http"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -27,6 +30,11 @@ type Client struct {
 	maxConcurrent int
 	// cacheTTL controls how long cache entries remain valid.
 	cacheTTL time.Duration
+
+	// maxRetries is the maximum number of retries for rate-limited requests.
+	maxRetries int
+	// baseRetryDelay is the initial backoff delay before the first retry.
+	baseRetryDelay time.Duration
 }
 
 type cacheEntry struct {
@@ -129,39 +137,102 @@ func NewClient(baseURL string) *Client {
 		httpClient: &http.Client{
 			Timeout: 30 * time.Second,
 		},
-		cache:         make(map[string]*cacheEntry),
-		maxConcurrent: 5,
-		cacheTTL:      30 * time.Second,
+		cache:          make(map[string]*cacheEntry),
+		maxConcurrent:  5,
+		cacheTTL:       30 * time.Second,
+		maxRetries:     3,
+		baseRetryDelay: 1 * time.Second,
 	}
 }
 
 // doRequest performs an authenticated HTTP request to the Gitea API.
+// It automatically retries on HTTP 429 (rate limit) responses with
+// exponential backoff, respecting the Retry-After header when present.
 func (c *Client) doRequest(ctx context.Context, token, method, path string, body io.Reader) (*http.Response, error) {
 	url := c.baseURL + "/api/v1" + path
 
-	req, err := http.NewRequestWithContext(ctx, method, url, body)
-	if err != nil {
-		return nil, fmt.Errorf("creating request: %w", err)
-	}
-
-	req.Header.Set("Authorization", "token "+token)
-	req.Header.Set("Accept", "application/json")
+	// Read the body once so we can replay it on retries.
+	var bodyBytes []byte
 	if body != nil {
-		req.Header.Set("Content-Type", "application/json")
+		var err error
+		bodyBytes, err = io.ReadAll(body)
+		if err != nil {
+			return nil, fmt.Errorf("reading request body: %w", err)
+		}
 	}
 
-	resp, err := c.httpClient.Do(req)
-	if err != nil {
-		return nil, fmt.Errorf("executing request: %w", err)
+	var lastErr error
+	for attempt := 0; attempt <= c.maxRetries; attempt++ {
+		// Recreate the body reader for each attempt.
+		var reqBody io.Reader
+		if bodyBytes != nil {
+			reqBody = strings.NewReader(string(bodyBytes))
+		}
+
+		req, err := http.NewRequestWithContext(ctx, method, url, reqBody)
+		if err != nil {
+			return nil, fmt.Errorf("creating request: %w", err)
+		}
+
+		req.Header.Set("Authorization", "token "+token)
+		req.Header.Set("Accept", "application/json")
+		if bodyBytes != nil {
+			req.Header.Set("Content-Type", "application/json")
+		}
+
+		resp, err := c.httpClient.Do(req)
+		if err != nil {
+			return nil, fmt.Errorf("executing request: %w", err)
+		}
+
+		// Not rate-limited: handle normally.
+		if resp.StatusCode != http.StatusTooManyRequests {
+			if resp.StatusCode >= 400 {
+				defer resp.Body.Close()
+				respBody, _ := io.ReadAll(resp.Body)
+				return nil, fmt.Errorf("API error %d: %s", resp.StatusCode, string(respBody))
+			}
+			return resp, nil
+		}
+
+		// Rate-limited (429): close body and compute retry delay.
+		resp.Body.Close()
+
+		if attempt == c.maxRetries {
+			lastErr = fmt.Errorf("API rate limit exceeded after %d retries (429)", c.maxRetries)
+			break
+		}
+
+		delay := c.retryDelay(resp, attempt)
+		slog.Warn("rate limited by Gitea API, retrying",
+			"attempt", attempt+1,
+			"max_retries", c.maxRetries,
+			"delay", delay,
+			"path", path,
+		)
+
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case <-time.After(delay):
+			// Continue to next attempt.
+		}
 	}
 
-	if resp.StatusCode >= 400 {
-		defer resp.Body.Close()
-		respBody, _ := io.ReadAll(resp.Body)
-		return nil, fmt.Errorf("API error %d: %s", resp.StatusCode, string(respBody))
-	}
+	return nil, lastErr
+}
 
-	return resp, nil
+// retryDelay computes the delay before the next retry attempt. It uses the
+// Retry-After header value (in seconds) if present, otherwise falls back to
+// exponential backoff: baseRetryDelay * 2^attempt.
+func (c *Client) retryDelay(resp *http.Response, attempt int) time.Duration {
+	if ra := resp.Header.Get("Retry-After"); ra != "" {
+		if seconds, err := strconv.Atoi(ra); err == nil && seconds > 0 {
+			return time.Duration(seconds) * time.Second
+		}
+	}
+	// Exponential backoff: 1s, 2s, 4s, ...
+	return c.baseRetryDelay * time.Duration(math.Pow(2, float64(attempt)))
 }
 
 // getFromCache returns cached data if still valid.

internal/gitea/client_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 )
@@ -893,6 +894,140 @@ func TestListAllPullRequests_StateFilter(t *testing.T) {
 	}
 }
 
+// --- Issue #127: Tests for ApplyLabel and SubmitReview ---
+
+func TestApplyLabel(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			t.Errorf("expected POST, got %s", r.Method)
+		}
+		if r.URL.Path != "/api/v1/repos/owner1/repo1/issues/42/labels" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		if r.Header.Get("Authorization") != "token test-token" {
+			t.Error("missing or wrong Authorization header")
+		}
+
+		var body map[string]interface{}
+		if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+			t.Fatalf("failed to decode body: %v", err)
+		}
+		labels, ok := body["labels"].([]interface{})
+		if !ok {
+			t.Fatalf("expected labels array, got %T", body["labels"])
+		}
+		if len(labels) != 2 {
+			t.Errorf("expected 2 label IDs, got %d", len(labels))
+		}
+		// Verify the label IDs are correct (JSON numbers are float64).
+		if labels[0].(float64) != 10 || labels[1].(float64) != 20 {
+			t.Errorf("expected label IDs [10, 20], got %v", labels)
+		}
+
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode([]map[string]interface{}{
+			{"id": 10, "name": "bug"},
+			{"id": 20, "name": "enhancement"},
+		})
+	}))
+	defer server.Close()
+
+	c := NewClient(server.URL)
+	c.setCache("issues-org1", "should-be-invalidated")
+
+	err := c.ApplyLabel(context.Background(), "test-token", "owner1", "repo1", 42, []int64{10, 20})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// Verify cache was invalidated.
+	_, ok := c.getFromCache("issues-org1")
+	if ok {
+		t.Error("expected cache to be invalidated after ApplyLabel")
+	}
+}
+
+func TestApplyLabel_Error(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		fmt.Fprintln(w, `{"message":"issue not found"}`)
+	}))
+	defer server.Close()
+
+	c := NewClient(server.URL)
+	err := c.ApplyLabel(context.Background(), "test-token", "owner1", "repo1", 999, []int64{10})
+	if err == nil {
+		t.Fatal("expected error for 404 response, got nil")
+	}
+	if !strings.Contains(err.Error(), "404") {
+		t.Errorf("error should contain status code 404, got: %v", err)
+	}
+}
+
+func TestSubmitReview(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			t.Errorf("expected POST, got %s", r.Method)
+		}
+		if r.URL.Path != "/api/v1/repos/owner1/repo1/pulls/7/reviews" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		if r.Header.Get("Authorization") != "token test-token" {
+			t.Error("missing or wrong Authorization header")
+		}
+
+		var body map[string]string
+		if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+			t.Fatalf("failed to decode body: %v", err)
+		}
+		if body["event"] != "APPROVED" {
+			t.Errorf("expected event=APPROVED, got %q", body["event"])
+		}
+		if body["body"] != "Looks good!" {
+			t.Errorf("expected body='Looks good!', got %q", body["body"])
+		}
+
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode(map[string]interface{}{
+			"id":    1,
+			"state": "APPROVED",
+			"body":  body["body"],
+		})
+	}))
+	defer server.Close()
+
+	c := NewClient(server.URL)
+	c.setCache("pulls-org1", "should-be-invalidated")
+
+	err := c.SubmitReview(context.Background(), "test-token", "owner1", "repo1", 7, "APPROVED", "Looks good!")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// Verify cache was invalidated.
+	_, ok := c.getFromCache("pulls-org1")
+	if ok {
+		t.Error("expected cache to be invalidated after SubmitReview")
+	}
+}
+
+func TestSubmitReview_Error(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnprocessableEntity)
+		fmt.Fprintln(w, `{"message":"validation failed"}`)
+	}))
+	defer server.Close()
+
+	c := NewClient(server.URL)
+	err := c.SubmitReview(context.Background(), "test-token", "owner1", "repo1", 7, "INVALID", "")
+	if err == nil {
+		t.Fatal("expected error for 422 response, got nil")
+	}
+	if !strings.Contains(err.Error(), "422") {
+		t.Errorf("error should contain status code 422, got: %v", err)
+	}
+}
+
 func TestListAllPullRequests_Pagination(t *testing.T) {
 	now := time.Date(2026, 3, 28, 12, 0, 0, 0, time.UTC)
 
@@ -952,3 +1087,161 @@ func TestListAllPullRequests_Pagination(t *testing.T) {
 		t.Error("page 2: HasMore should be false")
 	}
 }
+
+func TestDoRequest_RateLimitRetry(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts <= 2 {
+			w.Header().Set("Retry-After", "0")
+			w.WriteHeader(http.StatusTooManyRequests)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `[{"username":"test-org"}]`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond // Fast for tests.
+
+	resp, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err != nil {
+		t.Fatalf("expected success after retries, got: %v", err)
+	}
+	resp.Body.Close()
+
+	if attempts != 3 {
+		t.Errorf("expected 3 attempts, got %d", attempts)
+	}
+}
+
+func TestDoRequest_RateLimitExhausted(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTooManyRequests)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 2
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	_, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error after exhausting retries")
+	}
+	if !strings.Contains(err.Error(), "rate limit exceeded") {
+		t.Errorf("expected rate limit error, got: %v", err)
+	}
+}
+
+func TestDoRequest_RateLimitWithRetryAfterHeader(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", "1")
+			w.WriteHeader(http.StatusTooManyRequests)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `[]`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	start := time.Now()
+	resp, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	elapsed := time.Since(start)
+	if err != nil {
+		t.Fatalf("expected success, got: %v", err)
+	}
+	resp.Body.Close()
+
+	// Retry-After: 1 means 1 second delay.
+	if elapsed < 900*time.Millisecond {
+		t.Errorf("expected at least ~1s delay from Retry-After header, got %v", elapsed)
+	}
+}
+
+func TestDoRequest_RateLimitCancelledContext(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Retry-After", "60")
+		w.WriteHeader(http.StatusTooManyRequests)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+
+	_, err := c.doRequest(ctx, "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error from cancelled context")
+	}
+}
+
+func TestDoRequest_NonRateLimitErrorNotRetried(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		w.WriteHeader(http.StatusForbidden)
+		fmt.Fprint(w, `{"message":"forbidden"}`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	_, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error for 403")
+	}
+	if attempts != 1 {
+		t.Errorf("expected only 1 attempt for non-429 error, got %d", attempts)
+	}
+}
+
+func TestRetryDelay_WithRetryAfterHeader(t *testing.T) {
+	c := NewClient("https://example.com")
+	c.baseRetryDelay = 1 * time.Second
+
+	resp := &http.Response{Header: http.Header{}}
+	resp.Header.Set("Retry-After", "5")
+
+	delay := c.retryDelay(resp, 0)
+	if delay != 5*time.Second {
+		t.Errorf("expected 5s from Retry-After, got %v", delay)
+	}
+}
+
+func TestRetryDelay_ExponentialBackoff(t *testing.T) {
+	c := NewClient("https://example.com")
+	c.baseRetryDelay = 1 * time.Second
+
+	resp := &http.Response{Header: http.Header{}}
+
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{0, 1 * time.Second},
+		{1, 2 * time.Second},
+		{2, 4 * time.Second},
+	}
+
+	for _, tt := range tests {
+		delay := c.retryDelay(resp, tt.attempt)
+		if delay != tt.want {
+			t.Errorf("attempt %d: got %v, want %v", tt.attempt, delay, tt.want)
+		}
+	}
+}