`null` gets stripped from user-defined language server configurations

[bzoracler]

I'm setting configurations through the user-defined language server UI, and find that properties with values explicitly set as null are being stripped before the server receives them:

[Trace - 15:45:59] Sending request 'initialize - (1)'.
Params: {
  <...>,
  "initializationOptions": {
    "field2": "name",
    "field3": "nullaaaa"
  },

[Trace - 15:50:03] Sending notification 'workspace/didChangeConfiguration'
Params: {
  "settings": {
    "fieldB": "name"
  }
}

When I use the server to request the options/settings from the client, the fields which were set to null are now simply missing.

I expect preservation of fields set to null when sent from the client to the server¹. The configuration object that's being retrieved from the UI² shows that null is actually present in the client configuration object before being sent to the server, so something must be happening after reading the configuration.

---

1. According to the language server specification, the types of initializationOptions and return value of configuration requests are both LSPAny, which is a type union including LSPObject and null. LSPObject is defined as:

   type LSPObject = { [key: string]: LSPAny };

   So, LSPObjects are allowed to have string keys and null values.

2. Using the following modification of com.redhat.devtools.lsp4ij.server.definition.launching.UserDefinedLanguageServerDefinition:

   ...
   + import com.google.gson.JsonElement;
     import com.google.gson.JsonParser;
   ...
     public Object getLanguageServerConfiguration() {
             if (configuration == null && configurationContent != null && !configurationContent.isBlank()) {
                 try {
                     configuration = JsonParser.parseReader(new StringReader(configurationContent));
                 } catch (Exception e) {
                     LOGGER.error("Error while parsing JSON configuration for the language server '" + getId() + "'", e);
                 }
             }
   +         LOGGER.error("CONFIGURATION IS: " + ((JsonElement)configuration).toString());
             return configuration;
     }

   IDE Logs:

   SEVERE - com.redhat.devtools.lsp4ij.server.definition.launching.UserDefinedLanguageServerDefinition - CONFIGURATION IS: {"fieldA":null,"fieldB":"name"}

[angelozerr]

Indeed that's strange. Is there any chance that you investigate more the problem?

[bzoracler]

The problem is in com.redhat.devtools.lsp4ij.console.explorer.TracingMessageConsumer, in the locations where it calls MessageJsonHandler.toString(...).

org.eclipse.lsp4j.jsonrpc.json.MessageJsonHandler#toString uses a default Gson instance which doesn't serialise nulls.

I made a temporary patch using a Gson instance which turns on null serialisation, and the nulls started appearing in the traces.

[angelozerr]

For configuration I undestand that it is better to see null fields, but for other lsp request responses message like completion , showing null fields are not interesting, will grow up the lsp message in the console which will polluate the lsp console.