Fix panic in storageHandler (#27446) (#27478)

Backport #27446 by @sryze

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 (?)

Co-authored-by: Sergey Zolotarev <sryze@outlook.com>

routers/web/base.go

@@ -19,19 +19,19 @@ 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)
+				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)
@@ -43,7 +43,7 @@ func storageHandler(storageSetting *setting.Storage, prefix string, objStore sto
 			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 getting URL for %s %s. Error: %v", prefix, rPath, err)
@@ -57,12 +57,12 @@ func storageHandler(storageSetting *setting.Storage, prefix string, objStore sto
 
 	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)
@@ -70,7 +70,7 @@ func storageHandler(storageSetting *setting.Storage, prefix string, objStore sto
 		rPath := strings.TrimPrefix(req.URL.Path, "/"+prefix+"/")
 		rPath = util.PathJoinRelX(rPath)
 		if rPath == "" || rPath == "." {
-				http.Error(w, "file not found", http.StatusNotFound)
+			http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
 			return
 		}
 
@@ -78,7 +78,7 @@ func storageHandler(storageSetting *setting.Storage, prefix string, objStore sto
 		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)
@@ -96,4 +96,3 @@ func storageHandler(storageSetting *setting.Storage, prefix string, objStore sto
 		httpcache.ServeContentWithCacheControl(w, req, path.Base(rPath), fi.ModTime(), fr)
 	})
 }
-}