Follow @wxiaoguang's suggestion

models/repo/release.go

@@ -93,6 +93,10 @@ func init() {
 	db.RegisterModel(new(Release))
 }
 
+// legacyAttachmentMissingRepoIDCutoff marks the date when repo_id started to be written during uploads
+// (2026-01-16T00:00:00Z). Older rows might have repo_id=0 and should be tolerated once.
+const legacyAttachmentMissingRepoIDCutoff timeutil.TimeStamp = 1768521600
+
 func (r *Release) LoadRepo(ctx context.Context) (err error) {
 	if r.Repo != nil {
 		return nil
@@ -186,6 +190,13 @@ func AddReleaseAttachments(ctx context.Context, releaseID int64, attachmentUUIDs
 	}
 
 	for i := range attachments {
+		if attachments[i].RepoID == 0 && attachments[i].CreatedUnix < legacyAttachmentMissingRepoIDCutoff {
+			attachments[i].RepoID = rel.RepoID
+			if _, err = db.GetEngine(ctx).ID(attachments[i].ID).Cols("repo_id").Update(attachments[i]); err != nil {
+				return fmt.Errorf("update attachment repo_id [%d]: %w", attachments[i].ID, err)
+			}
+		}
+
 		if attachments[i].RepoID != rel.RepoID {
 			return util.NewPermissionDeniedErrorf("attachment belongs to different repository")
 		}

models/repo/release_test.go

@@ -6,6 +6,7 @@ package repo
 import (
 	"testing"
 
+	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/util"
 
@@ -51,3 +52,41 @@ func TestAddReleaseAttachmentsRejectsDifferentRepo(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Zero(t, attach.ReleaseID, "attachment should not be linked to release on failure")
 }
+
+func TestAddReleaseAttachmentsAllowsLegacyMissingRepoID(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	legacyUUID := "a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a20" // attachment 10 has repo_id 0
+	err := AddReleaseAttachments(t.Context(), 1, []string{legacyUUID})
+	assert.NoError(t, err)
+
+	attach, err := GetAttachmentByUUID(t.Context(), legacyUUID)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, attach.RepoID)
+	assert.EqualValues(t, 1, attach.ReleaseID)
+}
+
+func TestAddReleaseAttachmentsRejectsRecentZeroRepoID(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	recentUUID := "a0eebc99-9c0b-4ef8-bb6d-6bb9bd3800aa"
+	attachment := &Attachment{
+		UUID:        recentUUID,
+		RepoID:      0,
+		IssueID:     0,
+		ReleaseID:   0,
+		CommentID:   0,
+		Name:        "recent-zero",
+		CreatedUnix: legacyAttachmentMissingRepoIDCutoff + 1,
+	}
+	assert.NoError(t, db.Insert(t.Context(), attachment))
+
+	err := AddReleaseAttachments(t.Context(), 1, []string{recentUUID})
+	assert.Error(t, err)
+	assert.ErrorIs(t, err, util.ErrPermissionDenied)
+
+	attach, err := GetAttachmentByUUID(t.Context(), recentUUID)
+	assert.NoError(t, err)
+	assert.Zero(t, attach.ReleaseID)
+	assert.Zero(t, attach.RepoID)
+}