Fix panic in storageHandler (#27446)

storageHandler() is written as a middleware but is used as an endpoint
handler, and thus `next` is actually `nil`, which causes a null pointer
dereference when a request URL does not match the pattern (where it
calls `next.ServerHTTP()`).

Example CURL command to trigger the panic:

```
curl -I "http://yourhost/gitea//avatars/a"
```

Fixes #27409

---

Note: the diff looks big but it's actually a small change - all I did
was to remove the outer closure (and one level of indentation) ~and
removed the HTTP method and pattern checks as they seem redundant
because go-chi already does those checks~. You might want to check "Hide
whitespace" when reviewing it.

Alternative solution (a bit simpler): append `, misc.DummyOK` to the
route declarations that utilize `storageHandler()` - this makes it
return an empty response when the URL is invalid. I've tested this one
and it works too. Or maybe it would be better to return a 400 error in
that case (?)

routers/web/base.go

@@ -19,81 +19,80 @@ import (
 	"code.gitea.io/gitea/modules/web/routing"
 )
 
-func storageHandler(storageSetting *setting.Storage, prefix string, objStore storage.ObjectStorage) func(next http.Handler) http.Handler {
+func storageHandler(storageSetting *setting.Storage, prefix string, objStore storage.ObjectStorage) http.HandlerFunc {
 	prefix = strings.Trim(prefix, "/")
 	funcInfo := routing.GetFuncInfo(storageHandler, prefix)
-	return func(next http.Handler) http.Handler {
-		if storageSetting.MinioConfig.ServeDirect {
-			return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
-				if req.Method != "GET" && req.Method != "HEAD" {
-					next.ServeHTTP(w, req)
-					return
-				}
-
-				if !strings.HasPrefix(req.URL.Path, "/"+prefix+"/") {
-					next.ServeHTTP(w, req)
-					return
-				}
-				routing.UpdateFuncInfo(req.Context(), funcInfo)
-
-				rPath := strings.TrimPrefix(req.URL.Path, "/"+prefix+"/")
-				rPath = util.PathJoinRelX(rPath)
-
-				u, err := objStore.URL(rPath, path.Base(rPath))
-				if err != nil {
-					if os.IsNotExist(err) || errors.Is(err, os.ErrNotExist) {
-						log.Warn("Unable to find %s %s", prefix, rPath)
-						http.Error(w, "file not found", http.StatusNotFound)
-						return
-					}
-					log.Error("Error whilst getting URL for %s %s. Error: %v", prefix, rPath, err)
-					http.Error(w, fmt.Sprintf("Error whilst getting URL for %s %s", prefix, rPath), http.StatusInternalServerError)
-					return
-				}
-
-				http.Redirect(w, req, u.String(), http.StatusTemporaryRedirect)
-			})
-		}
 
+	if storageSetting.MinioConfig.ServeDirect {
 		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 			if req.Method != "GET" && req.Method != "HEAD" {
-				next.ServeHTTP(w, req)
+				http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
 				return
 			}
 
 			if !strings.HasPrefix(req.URL.Path, "/"+prefix+"/") {
-				next.ServeHTTP(w, req)
+				http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
 				return
 			}
 			routing.UpdateFuncInfo(req.Context(), funcInfo)
 
 			rPath := strings.TrimPrefix(req.URL.Path, "/"+prefix+"/")
 			rPath = util.PathJoinRelX(rPath)
-			if rPath == "" || rPath == "." {
-				http.Error(w, "file not found", http.StatusNotFound)
-				return
-			}
 
-			fi, err := objStore.Stat(rPath)
+			u, err := objStore.URL(rPath, path.Base(rPath))
 			if err != nil {
 				if os.IsNotExist(err) || errors.Is(err, os.ErrNotExist) {
 					log.Warn("Unable to find %s %s", prefix, rPath)
-					http.Error(w, "file not found", http.StatusNotFound)
+					http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
 					return
 				}
-				log.Error("Error whilst opening %s %s. Error: %v", prefix, rPath, err)
-				http.Error(w, fmt.Sprintf("Error whilst opening %s %s", prefix, rPath), http.StatusInternalServerError)
+				log.Error("Error whilst getting URL for %s %s. Error: %v", prefix, rPath, err)
+				http.Error(w, fmt.Sprintf("Error whilst getting URL for %s %s", prefix, rPath), http.StatusInternalServerError)
 				return
 			}
 
-			fr, err := objStore.Open(rPath)
-			if err != nil {
-				log.Error("Error whilst opening %s %s. Error: %v", prefix, rPath, err)
-				http.Error(w, fmt.Sprintf("Error whilst opening %s %s", prefix, rPath), http.StatusInternalServerError)
-				return
-			}
-			defer fr.Close()
-			httpcache.ServeContentWithCacheControl(w, req, path.Base(rPath), fi.ModTime(), fr)
+			http.Redirect(w, req, u.String(), http.StatusTemporaryRedirect)
 		})
 	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if req.Method != "GET" && req.Method != "HEAD" {
+			http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
+			return
+		}
+
+		if !strings.HasPrefix(req.URL.Path, "/"+prefix+"/") {
+			http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+			return
+		}
+		routing.UpdateFuncInfo(req.Context(), funcInfo)
+
+		rPath := strings.TrimPrefix(req.URL.Path, "/"+prefix+"/")
+		rPath = util.PathJoinRelX(rPath)
+		if rPath == "" || rPath == "." {
+			http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+			return
+		}
+
+		fi, err := objStore.Stat(rPath)
+		if err != nil {
+			if os.IsNotExist(err) || errors.Is(err, os.ErrNotExist) {
+				log.Warn("Unable to find %s %s", prefix, rPath)
+				http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+				return
+			}
+			log.Error("Error whilst opening %s %s. Error: %v", prefix, rPath, err)
+			http.Error(w, fmt.Sprintf("Error whilst opening %s %s", prefix, rPath), http.StatusInternalServerError)
+			return
+		}
+
+		fr, err := objStore.Open(rPath)
+		if err != nil {
+			log.Error("Error whilst opening %s %s. Error: %v", prefix, rPath, err)
+			http.Error(w, fmt.Sprintf("Error whilst opening %s %s", prefix, rPath), http.StatusInternalServerError)
+			return
+		}
+		defer fr.Close()
+		httpcache.ServeContentWithCacheControl(w, req, path.Base(rPath), fi.ModTime(), fr)
+	})
 }