[MM-56792] Clarify large plugin upload error (#26271)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/api4/plugin.go

@@ -20,7 +20,8 @@ import (
 )
 
 const (
-	MaximumPluginFileSize = 50 * 1024 * 1024
+	// MaxPluginMemory is the maximum number of bytes to hold in memory when reading a plugin bundle.
+	MaxPluginMemory = 50 * 1024 * 1024
 )
 
 func (api *API) InitPlugin() {
@@ -57,7 +58,11 @@ func uploadPlugin(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := r.ParseMultipartForm(MaximumPluginFileSize); err != nil {
+	if err := r.ParseMultipartForm(MaxPluginMemory); err != nil {
+		if err.Error() == "http: request body too large" {
+			c.Err = model.NewAppError("uploadPlugin", "api.plugin.upload.file_too_large.app_error", nil, "", http.StatusRequestEntityTooLarge)
+			return
+		}
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}

server/channels/api4/plugin_test.go

@@ -126,6 +126,23 @@ func TestPlugin(t *testing.T) {
 		require.Error(t, err)
 		CheckBadRequestStatus(t, resp)
 
+		plugin_sz := int64(111 * 1024 * 1024)
+		fd, err := os.Create(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+		_, err = fd.Seek(plugin_sz-1, 0)
+		require.NoError(t, err)
+		_, err = fd.Write([]byte{0})
+		require.NoError(t, err)
+		err = fd.Close()
+		require.NoError(t, err)
+		bigData, err := os.ReadFile(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+		_, resp, err = client.UploadPlugin(context.Background(), bytes.NewReader(bigData))
+		require.Error(t, err)
+		CheckRequestEntityTooLargeStatus(t, resp)
+		err = os.Remove(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+
 		th.App.UpdateConfig(func(cfg *model.Config) { *cfg.PluginSettings.Enable = false })
 		_, resp, err = client.UploadPlugin(context.Background(), bytes.NewReader(tarData))
 		require.Error(t, err)

server/channels/testlib/resources.go

@@ -83,6 +83,7 @@ func getTestResourcesToSetup() []testResourceDetails {
 		{"templates", "templates", resourceTypeFolder, actionSymlink},
 		{"tests", "tests", resourceTypeFolder, actionSymlink},
 		{"fonts", "fonts", resourceTypeFolder, actionSymlink},
+		{"channels/app/plugin_api_tests", "channels/app/plugin_api_tests", resourceTypeFolder, actionSymlink},
 		{"channels/utils/policies-roles-mapping.json", "channels/utils/policies-roles-mapping.json", resourceTypeFile, actionSymlink},
 	}
 

server/i18n/en.json

@@ -2448,6 +2448,10 @@
     "id": "api.plugin.upload.file.app_error",
     "translation": "Unable to open file in multipart/form request."
   },
+  {
+    "id": "api.plugin.upload.file_too_large.app_error",
+    "translation": "Uploaded plugin size exceeds limit. This limit can be changed in the System Console via File Storage > Maximum File Size"
+  },
   {
     "id": "api.plugin.upload.no_file.app_error",
     "translation": "Missing file in multipart/form request."