fix(migration): add retry to migration request helper

Resolves https://github.com/go-vikunja/vikunja/issues/1788

pkg/modules/migration/helpers.go

@@ -19,9 +19,13 @@ package migration
 import (
 	"bytes"
 	"context"
+	"crypto/rand"
+	"math"
+	"math/big"
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 )
 
 // DownloadFile downloads a file and returns its contents
@@ -61,17 +65,48 @@ func DoPost(url string, form url.Values) (resp *http.Response, err error) {
 
 // DoPostWithHeaders does an api request and allows to pass in arbitrary headers
 func DoPostWithHeaders(url string, form url.Values, headers map[string]string) (resp *http.Response, err error) {
-	req, err := http.NewRequestWithContext(context.Background(), http.MethodPost, url, strings.NewReader(form.Encode()))
-	if err != nil {
-		return
-	}
-
-	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
-
-	for key, value := range headers {
-		req.Header.Add(key, value)
-	}
+	const maxRetries = 3
+	const baseDelay = 100 * time.Millisecond
 
 	hc := http.Client{}
-	return hc.Do(req)
+
+	for attempt := 0; attempt < maxRetries; attempt++ {
+		req, err := http.NewRequestWithContext(context.Background(), http.MethodPost, url, strings.NewReader(form.Encode()))
+		if err != nil {
+			return nil, err
+		}
+
+		req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+
+		for key, value := range headers {
+			req.Header.Add(key, value)
+		}
+
+		resp, err = hc.Do(req)
+		if err != nil {
+			return nil, err
+		}
+
+		// Don't retry on non-5xx status codes
+		if resp.StatusCode < 500 {
+			return resp, nil
+		}
+
+		// Don't retry on last attempt
+		if attempt == maxRetries-1 {
+			return resp, nil
+		}
+
+		// Close the body before retrying
+		resp.Body.Close()
+
+		// Exponential backoff with jitter
+		delay := baseDelay * time.Duration(math.Pow(2, float64(attempt)))
+		maxJitter := int64(delay / 2)
+		jitterBig, _ := rand.Int(rand.Reader, big.NewInt(maxJitter))
+		jitter := time.Duration(jitterBig.Int64())
+		time.Sleep(delay + jitter)
+	}
+
+	return resp, nil
 }

pkg/modules/migration/helpers_test.go

@@ -0,0 +1,98 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package migration
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"sync/atomic"
+	"testing"
+)
+
+func TestDoPostWithHeaders_RetriesOn500(t *testing.T) {
+	var attempts atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		count := attempts.Add(1)
+		if count < 3 {
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	form := url.Values{"key": {"value"}}
+	resp, err := DoPostWithHeaders(server.URL, form, map[string]string{})
+
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("expected status 200, got %d", resp.StatusCode)
+	}
+	if attempts.Load() != 3 {
+		t.Errorf("expected 3 attempts, got %d", attempts.Load())
+	}
+}
+
+func TestDoPostWithHeaders_GivesUpAfter3Retries(t *testing.T) {
+	var attempts atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		attempts.Add(1)
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer server.Close()
+
+	form := url.Values{"key": {"value"}}
+	resp, err := DoPostWithHeaders(server.URL, form, map[string]string{})
+
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusInternalServerError {
+		t.Errorf("expected status 500, got %d", resp.StatusCode)
+	}
+	if attempts.Load() != 3 {
+		t.Errorf("expected 3 attempts, got %d", attempts.Load())
+	}
+}
+
+func TestDoPostWithHeaders_DoesNotRetryOn4xx(t *testing.T) {
+	var attempts atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		attempts.Add(1)
+		w.WriteHeader(http.StatusBadRequest)
+	}))
+	defer server.Close()
+
+	form := url.Values{"key": {"value"}}
+	resp, err := DoPostWithHeaders(server.URL, form, map[string]string{})
+
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("expected status 400, got %d", resp.StatusCode)
+	}
+	if attempts.Load() != 1 {
+		t.Errorf("expected 1 attempt (no retries on 4xx), got %d", attempts.Load())
+	}
+}