[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

4 files changed, 126 insertions, 24 deletions

diff --git a/internal/web/assets.go b/internal/web/assets.go
index bc80bc398..ae185354d 100644
--- a/internal/web/assets.go
+++ b/internal/web/assets.go
@@ -21,10 +21,13 @@ import (
 	"fmt"
 	"net/http"
 	"path"
+	"path/filepath"
 	"strings"
 
 	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
 	"github.com/superseriousbusiness/gotosocial/internal/log"
+	"github.com/superseriousbusiness/gotosocial/internal/router"
 )
 
 type fileSystem struct {
@@ -53,7 +56,11 @@ func (fs fileSystem) Open(path string) (http.File, error) {
 // getAssetFileInfo tries to fetch the ETag for the given filePath from the module's
 // assetsETagCache. If it can't be found there, it uses the provided http.FileSystem
 // to generate a new ETag to go in the cache, which it then returns.
-func (m *Module) getAssetETag(filePath string, fs http.FileSystem) (string, error) {
+func getAssetETag(
+	wet withETagCache,
+	filePath string,
+	fs http.FileSystem,
+) (string, error) {
 	file, err := fs.Open(filePath)
 	if err != nil {
 		return "", fmt.Errorf("error opening %s: %s", filePath, err)
@@ -67,7 +74,8 @@ func (m *Module) getAssetETag(filePath string, fs http.FileSystem) (string, erro
 
 	fileLastModified := fileInfo.ModTime()
 
-	if cachedETag, ok := m.eTagCache.Get(filePath); ok && !fileLastModified.After(cachedETag.lastModified) {
+	cache := wet.ETagCache()
+	if cachedETag, ok := cache.Get(filePath); ok && !fileLastModified.After(cachedETag.lastModified) {
 		// only return our cached etag if the file wasn't
 		// modified since last time, otherwise generate a
 		// new one; eat fresh!
@@ -80,7 +88,7 @@ func (m *Module) getAssetETag(filePath string, fs http.FileSystem) (string, erro
 	}
 
 	// put new entry in cache before we return
-	m.eTagCache.Set(filePath, eTagCacheEntry{
+	cache.Set(filePath, eTagCacheEntry{
 		eTag:         eTag,
 		lastModified: fileLastModified,
 	})
@@ -99,7 +107,10 @@ func (m *Module) getAssetETag(filePath string, fs http.FileSystem) (string, erro
 //
 // todo: move this middleware out of the 'web' package and into the 'middleware'
 // package along with the other middlewares
-func (m *Module) assetsCacheControlMiddleware(fs http.FileSystem) gin.HandlerFunc {
+func assetsCacheControlMiddleware(
+	wet withETagCache,
+	fs http.FileSystem,
+) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		// Acquire context from gin request.
 		ctx := c.Request.Context()
@@ -118,7 +129,7 @@ func (m *Module) assetsCacheControlMiddleware(fs http.FileSystem) gin.HandlerFun
 		assetFilePath := strings.TrimPrefix(path.Clean(upath), assetsPathPrefix)
 
 		// either fetch etag from ttlcache or generate it
-		eTag, err := m.getAssetETag(assetFilePath, fs)
+		eTag, err := getAssetETag(wet, assetFilePath, fs)
 		if err != nil {
 			log.Errorf(ctx, "error getting ETag for %s: %s", assetFilePath, err)
 			return
@@ -137,3 +148,23 @@ func (m *Module) assetsCacheControlMiddleware(fs http.FileSystem) gin.HandlerFun
 		// else let the rest of the request be processed normally
 	}
 }
+
+// routeAssets attaches *just* the
+// assets filesystem to the given router.
+func routeAssets(
+	wet withETagCache,
+	r *router.Router,
+	mi ...gin.HandlerFunc,
+) {
+	// Group all static files from assets dir at /assets,
+	// so that they can use the same cache control middleware.
+	webAssetsAbsFilePath, err := filepath.Abs(config.GetWebAssetBaseDir())
+	if err != nil {
+		log.Panicf(nil, "error getting absolute path of assets dir: %s", err)
+	}
+	fs := fileSystem{http.Dir(webAssetsAbsFilePath)}
+	assetsGroup := r.AttachGroup(assetsPathPrefix)
+	assetsGroup.Use(assetsCacheControlMiddleware(wet, fs))
+	assetsGroup.Use(mi...)
+	assetsGroup.StaticFS("/", fs)
+}
diff --git a/internal/web/etag.go b/internal/web/etag.go
index a87d9fa3b..88779871c 100644
--- a/internal/web/etag.go
+++ b/internal/web/etag.go
@@ -29,6 +29,10 @@ import (
 	"codeberg.org/gruf/go-cache/v3"
 )
 
+type withETagCache interface {
+	ETagCache() cache.Cache[string, eTagCacheEntry]
+}
+
 func newETagCache() cache.TTLCache[string, eTagCacheEntry] {
 	eTagCache := cache.NewTTL[string, eTagCacheEntry](0, 1000, 0)
 	eTagCache.SetTTL(time.Hour, false)
diff --git a/internal/web/maintenance.go b/internal/web/maintenance.go
new file mode 100644
index 000000000..f05fa83dd
--- /dev/null
+++ b/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)
+	})
+}
diff --git a/internal/web/web.go b/internal/web/web.go
index ddf7d53ea..cfadc9283 100644
--- a/internal/web/web.go
+++ b/internal/web/web.go
@@ -21,14 +21,11 @@ import (
 	"context"
 	"net/http"
 	"net/url"
-	"path/filepath"
 
 	"codeberg.org/gruf/go-cache/v3"
 	"github.com/gin-gonic/gin"
 	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
-	"github.com/superseriousbusiness/gotosocial/internal/config"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
-	"github.com/superseriousbusiness/gotosocial/internal/log"
 	"github.com/superseriousbusiness/gotosocial/internal/middleware"
 	"github.com/superseriousbusiness/gotosocial/internal/processing"
 	"github.com/superseriousbusiness/gotosocial/internal/router"
@@ -87,22 +84,22 @@ func New(db db.DB, processor *processing.Processor) *Module {
 	}
 }
 
+// ETagCache implements withETagCache.
+func (m *Module) ETagCache() cache.Cache[string, eTagCacheEntry] {
+	return m.eTagCache
+}
+
+// Route attaches the assets filesystem and profile,
+// status, and other web handlers to the router.
 func (m *Module) Route(r *router.Router, mi ...gin.HandlerFunc) {
-	// Group all static files from assets dir at /assets,
-	// so that they can use the same cache control middleware.
-	webAssetsAbsFilePath, err := filepath.Abs(config.GetWebAssetBaseDir())
-	if err != nil {
-		log.Panicf(nil, "error getting absolute path of assets dir: %s", err)
-	}
-	fs := fileSystem{http.Dir(webAssetsAbsFilePath)}
-	assetsGroup := r.AttachGroup(assetsPathPrefix)
-	assetsGroup.Use(m.assetsCacheControlMiddleware(fs))
-	assetsGroup.Use(mi...)
-	assetsGroup.StaticFS("/", fs)
+	// Route static assets.
+	routeAssets(m, r, mi...)
 
-	// handlers that serve profiles and statuses should use the SignatureCheck
-	// middleware, so that requests with content-type application/activity+json
-	// can still be served
+	// Route all other endpoints + handlers.
+	//
+	// Handlers that serve profiles and statuses should use
+	// the SignatureCheck middleware, so that requests with
+	// content-type application/activity+json can be served
 	profileGroup := r.AttachGroup(profileGroupPath)
 	profileGroup.Use(mi...)
 	profileGroup.Use(middleware.SignatureCheck(m.isURIBlocked), middleware.CacheControl(middleware.CacheControlConfig{
@@ -111,7 +108,7 @@ func (m *Module) Route(r *router.Router, mi ...gin.HandlerFunc) {
 	profileGroup.Handle(http.MethodGet, "", m.profileGETHandler) // use empty path here since it's the base of the group
 	profileGroup.Handle(http.MethodGet, statusPath, m.threadGETHandler)
 
-	// Attach individual web handlers which require no specific middlewares
+	// Individual web handlers requiring no specific middlewares.
 	r.AttachHandler(http.MethodGet, "/", m.indexHandler) // front-page
 	r.AttachHandler(http.MethodGet, settingsPathPrefix, m.SettingsPanelHandler)
 	r.AttachHandler(http.MethodGet, settingsPanelGlob, m.SettingsPanelHandler)
@@ -128,7 +125,7 @@ func (m *Module) Route(r *router.Router, mi ...gin.HandlerFunc) {
 	r.AttachHandler(http.MethodGet, signupPath, m.signupGETHandler)
 	r.AttachHandler(http.MethodPost, signupPath, m.signupPOSTHandler)
 
-	// Attach redirects from old endpoints to current ones for backwards compatibility
+	// Redirects from old endpoints to for back compat.
 	r.AttachHandler(http.MethodGet, "/auth/edit", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })
 	r.AttachHandler(http.MethodGet, "/user", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })
 	r.AttachHandler(http.MethodGet, "/admin", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, adminPanelPath) })