fix(actions): make runner list pagination order deterministic (#38313)

The runner-listing API endpoints (`admin`, `org`, `user`, `repo`, and
`shared`) return runners in a non-deterministic order across pages. When
paging through runners, some runners from page 1 could reappear on page
2 (and others get skipped entirely).

The cause is in `FindRunnerOptions.ToOrders()`. Most sort modes order by
a **non-unique** column only:

- default / `online` / `offline` → `last_online`
- `alphabetically` / `reversealphabetically` → `name`

When multiple runners tie on the sort key (e.g. every offline runner
shares `last_online = 0`, or two runners have the same name), the
database is free to return the tied rows in any order between separate
queries. Combined with `LIMIT`/`OFFSET` pagination, this means the same
runner can land on more than one page.

## Fix

Append the unique primary key `id` as a stable tiebreaker to each
non-unique sort order:

The `newest`/`oldest` modes already sort by the unique `id`, so they are
left unchanged.

models/actions/runner.go

@@ -251,21 +251,24 @@ func (opts FindRunnerOptions) ToConds() builder.Cond {
 }
 
 func (opts FindRunnerOptions) ToOrders() string {
+	// A unique tiebreaker (id) is appended so that runners sharing the same
+	// last_online or name keep a deterministic order across paginated queries,
+	// otherwise the same runner may appear on more than one page.
 	switch opts.Sort {
 	case "online":
-		return "last_online DESC"
+		return "last_online DESC, id ASC"
 	case "offline":
-		return "last_online ASC"
+		return "last_online ASC, id ASC"
 	case "alphabetically":
-		return "name ASC"
+		return "name ASC, id ASC"
 	case "reversealphabetically":
-		return "name DESC"
+		return "name DESC, id ASC"
 	case "newest":
 		return "id DESC"
 	case "oldest":
 		return "id ASC"
 	}
-	return "last_online DESC"
+	return "last_online DESC, id ASC"
 }
 
 // GetRunnerByUUID returns a runner via uuid

models/actions/runner_test.go

@@ -0,0 +1,77 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"fmt"
+	"testing"
+
+	"gitea.dev/models/db"
+	"gitea.dev/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFindRunnerOptions_ToOrders_StableTiebreaker(t *testing.T) {
+	// Sorts on a non-unique column must end with the unique id tiebreaker so
+	// pagination is deterministic; without it, runners sharing the same
+	// last_online or name can appear on more than one page. Sorts already on
+	// the unique id need no tiebreaker.
+	expected := map[string]string{
+		"":                      "last_online DESC, id ASC",
+		"online":                "last_online DESC, id ASC",
+		"offline":               "last_online ASC, id ASC",
+		"alphabetically":        "name ASC, id ASC",
+		"reversealphabetically": "name DESC, id ASC",
+		"newest":                "id DESC",
+		"oldest":                "id ASC",
+	}
+	for sort, want := range expected {
+		assert.Equal(t, want, FindRunnerOptions{Sort: sort}.ToOrders(), "sort %q", sort)
+	}
+}
+
+func TestFindRunners_PaginationNoDuplicates(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	ctx := t.Context()
+
+	// Create several runners that all share the same last_online value so the
+	// primary sort key (last_online) is tied for all of them.
+	const ownerID = 1000
+	const count = 6
+	for i := range count {
+		runner := &ActionRunner{
+			Name:       "paginated-runner",
+			UUID:       fmt.Sprintf("PAGINATE-TEST-0000-0000-00000000000%d", i),
+			TokenHash:  fmt.Sprintf("paginate-test-token-hash-%d", i),
+			OwnerID:    ownerID,
+			RepoID:     0,
+			LastOnline: 42,
+		}
+		require.NoError(t, db.Insert(ctx, runner))
+	}
+
+	// Page through the runners and ensure every id is returned exactly once.
+	seen := make(map[int64]int)
+	const pageSize = 2
+	for page := 1; ; page++ {
+		runners, err := db.Find[ActionRunner](ctx, FindRunnerOptions{
+			ListOptions: db.ListOptions{Page: page, PageSize: pageSize},
+			OwnerID:     ownerID,
+		})
+		require.NoError(t, err)
+		if len(runners) == 0 {
+			break
+		}
+		for _, r := range runners {
+			seen[r.ID]++
+		}
+	}
+
+	assert.Len(t, seen, count, "each runner should be returned exactly once across all pages")
+	for id, n := range seen {
+		assert.Equal(t, 1, n, "runner %d appeared on %d pages", id, n)
+	}
+}