[feature] Use maintenance router to serve 503 while server is starting/migrating (#3705)

* [feature] Use maintenance router to serve 503 while server is starting/migrating

* love you linter, kissies

internal/web/maintenance.go

@@ -0,0 +1,70 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// 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 <http://www.gnu.org/licenses/>.
+
+package web
+
+import (
+	"net/http"
+	"time"
+
+	"codeberg.org/gruf/go-cache/v3"
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api/health"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/router"
+)
+
+type MaintenanceModule struct {
+	eTagCache cache.Cache[string, eTagCacheEntry]
+}
+
+// NewMaintenance returns a module that routes only
+// static assets, and returns a code 503 maintenance
+// message template to all other requests.
+func NewMaintenance() *MaintenanceModule {
+	return &MaintenanceModule{
+		eTagCache: newETagCache(),
+	}
+}
+
+// ETagCache implements withETagCache.
+func (m *MaintenanceModule) ETagCache() cache.Cache[string, eTagCacheEntry] {
+	return m.eTagCache
+}
+
+func (m *MaintenanceModule) Route(r *router.Router, mi ...gin.HandlerFunc) {
+	// Route static assets.
+	routeAssets(m, r, mi...)
+
+	// Serve OK in response to live
+	// requests, but not ready requests.
+	liveHandler := func(c *gin.Context) {
+		c.Status(http.StatusOK)
+	}
+	r.AttachHandler(http.MethodGet, health.LivePath, liveHandler)
+	r.AttachHandler(http.MethodHead, health.LivePath, liveHandler)
+
+	// For everything else, serve maintenance template.
+	obj := map[string]string{"host": config.GetHost()}
+	r.AttachNoRouteHandler(func(c *gin.Context) {
+		retryAfter := time.Now().Add(120 * time.Second).UTC()
+		c.Writer.Header().Add("Retry-After", "120")
+		c.Writer.Header().Add("Retry-After", retryAfter.Format(http.TimeFormat))
+		c.Header("Cache-Control", "no-store")
+		c.HTML(http.StatusServiceUnavailable, "maintenance.tmpl", obj)
+	})
+}