fix(migrations): stop endless comment paging when migrating from Gitea (#39419)

`GET /repos/{owner}/{repo}/issues/{index}/comments` ignores `page` and
`limit` and always returns every comment. The Gitea downloader pages it
and stops only on a short page, so migrating from Gitea or Forgejo loops
forever on any issue with at least `MAX_RESPONSE_ITEMS` comments,
without an error.

Paging is kept in case the endpoint gets paginated
(https://github.com/go-gitea/gitea/issues/6132,
https://github.com/go-gitea/gitea/issues/18082). The loop now stops when
a page is longer than the limit or starts with an already seen comment.

Prior art: Forgejo fixed its copy in
https://codeberg.org/forgejo/forgejo/pulls/9274 (report:
https://codeberg.org/Codeberg/Community/issues/1542).

---
AI-assisted: drafted with Claude Code (claude-opus-5-5), reviewed by me.

Please let me know if you have any suggestions or comments, I ran into
this issue myself when I was trying to migrate repositories from Forgejo
-> Gitea.

---------

Co-authored-by: silverwind <me@silverwind.io>

services/migrations/gitea_downloader.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"time"
 
+	"gitea.dev/modules/container"
 	"gitea.dev/modules/log"
 	base "gitea.dev/modules/migration"
 	"gitea.dev/modules/structs"
@@ -472,6 +473,7 @@ func (g *GiteaDownloader) GetIssues(ctx context.Context, page, perPage int) ([]*
 // GetComments returns comments according issueNumber
 func (g *GiteaDownloader) GetComments(ctx context.Context, commentable base.Commentable) ([]*base.Comment, bool, error) {
 	allComments := make([]*base.Comment, 0, g.maxPerPage)
+	seenIDs := container.Set[int64]{}
 
 	for i := 1; ; i++ {
 		// make sure gitea can shutdown gracefully
@@ -488,8 +490,12 @@ func (g *GiteaDownloader) GetComments(ctx context.Context, commentable base.Comm
 		if err != nil {
 			return nil, false, fmt.Errorf("error while listing comments for issue #%d. Error: %w", commentable.GetForeignIndex(), err)
 		}
+		if len(comments) == 0 || seenIDs.Contains(comments[0].ID) {
+			break // the endpoint ignores page and limit, so page 2 repeats page 1
+		}
 
 		for _, comment := range comments {
+			seenIDs.Add(comment.ID)
 			reactions, err := g.getCommentReactions(comment.ID)
 			if err != nil {
 				WarnAndNotice("Unable to load comment reactions during migrating issue #%d for comment %d in %s. Error: %v", commentable.GetForeignIndex(), comment.ID, g, err)
@@ -508,7 +514,7 @@ func (g *GiteaDownloader) GetComments(ctx context.Context, commentable base.Comm
 			})
 		}
 
-		if !g.pagination || len(comments) < g.maxPerPage {
+		if !g.pagination || len(comments) != g.maxPerPage {
 			break
 		}
 	}

services/migrations/gitea_downloader_test.go

@@ -4,10 +4,15 @@
 package migrations
 
 import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"runtime"
 	"sort"
+	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -309,3 +314,49 @@ func TestGiteaDownloadRepo(t *testing.T) {
 		},
 	}, reviews)
 }
+
+func TestGiteaDownloadCommentsPaging(t *testing.T) {
+	for _, tc := range []struct {
+		maxResponseItems, commentCount, requests int
+		paginated                                bool
+	}{
+		{maxResponseItems: 2, commentCount: 2, requests: 2},
+		{maxResponseItems: 2, commentCount: 3, requests: 1},
+		{maxResponseItems: 2, commentCount: 4, requests: 3, paginated: true},
+		{maxResponseItems: 0, commentCount: 0, requests: 1},
+	} {
+		t.Run(strconv.Itoa(tc.commentCount), func(t *testing.T) {
+			commentRequests := 0
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				switch r.URL.Path {
+				case "/api/v1/version":
+					_, _ = w.Write([]byte(`{"version":"1.27.0"}`))
+				case "/api/v1/settings/api":
+					_, _ = fmt.Fprintf(w, `{"max_response_items":%d}`, tc.maxResponseItems)
+				case "/api/v1/repos/o/r/issues/1/comments":
+					commentRequests++
+					comments := make([]string, tc.commentCount)
+					for i := range comments {
+						comments[i] = fmt.Sprintf(`{"id":%d,"user":{}}`, i+1)
+					}
+					if tc.paginated {
+						page, _ := strconv.Atoi(r.URL.Query().Get("page"))
+						comments = comments[(page-1)*tc.maxResponseItems : min(page*tc.maxResponseItems, len(comments))]
+					}
+					_, _ = w.Write([]byte("[" + strings.Join(comments, ",") + "]"))
+				default:
+					_, _ = w.Write([]byte(`[]`))
+				}
+			}))
+			defer server.Close()
+
+			downloader, err := NewGiteaDownloader(t.Context(), server.URL, "o/r", "", "", "")
+			require.NoError(t, err)
+
+			comments, _, err := downloader.GetComments(t.Context(), &base.Issue{Number: 1})
+			require.NoError(t, err)
+			assert.Len(t, comments, tc.commentCount)
+			assert.Equal(t, tc.requests, commentRequests)
+		})
+	}
+}