fix(api): set default pagination and Link header for repoListTags (#9201)

- Set default pagination, so the API allows cases like `?limit=1`.
- Set the Link header when there are more items, but not shown because of pagination.
- Resolves forgejo/forgejo#8828

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9201
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: deadkittens <montage_inches78@icloud.com>
Co-committed-by: deadkittens <montage_inches78@icloud.com>

routers/api/v1/repo/tag.go

@@ -55,6 +55,7 @@ func ListTags(ctx *context.APIContext) {
 	//     "$ref": "#/responses/notFound"
 
 	listOpts := utils.GetListOptions(ctx)
+	listOpts.SetDefaultValues()
 
 	tags, total, err := ctx.Repo.GitRepo.GetTagInfos(listOpts.Page, listOpts.PageSize)
 	if err != nil {
@@ -72,7 +73,7 @@ func ListTags(ctx *context.APIContext) {
 
 		apiTags[i] = convert.ToTag(ctx.Repo.Repository, tags[i])
 	}
-
+	ctx.SetLinkHeader(total, listOpts.PageSize)
 	ctx.SetTotalCountHeader(int64(total))
 	ctx.JSON(http.StatusOK, &apiTags)
 }

routers/api/v1/repo/tag_test.go

@@ -0,0 +1,39 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"testing"
+
+	"forgejo.org/models/unittest"
+	"forgejo.org/services/contexttest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestListTagsSetsLinkHeader(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+
+	// limit=1 so that any repo with >=2 tags will paginate
+	ctx, resp := contexttest.MockAPIContext(t, "GET /api/v1/repos/user2/repo1/tags?limit=1")
+	contexttest.LoadRepo(t, ctx, 1)
+	contexttest.LoadUser(t, ctx, 2)
+	contexttest.LoadGitRepo(t, ctx)
+
+	// Ensure at least two tags exist for pagination
+	commit, err := ctx.Repo.GitRepo.GetBranchCommit("master")
+	require.NoError(t, err)
+	_ = ctx.Repo.GitRepo.CreateTag("listtags-linkheader-a", commit.ID.String())
+	_ = ctx.Repo.GitRepo.CreateTag("listtags-linkheader-b", commit.ID.String())
+
+	ListTags(ctx)
+
+	assert.Equal(t, 200, ctx.Resp.Status())
+
+	link := resp.Header().Get("Link")
+	assert.NotEmpty(t, link, "Link header should be set for paginated responses")
+	assert.Contains(t, link, "rel=\"next\"")
+	assert.Contains(t, link, "page=2")
+}