Use correct error response for cancellations

lib/ruby_lsp/base_server.rb

@@ -91,7 +91,8 @@ def start
         # The following requests need to be executed in the main thread directly to avoid concurrency issues. Everything
         # else is pushed into the incoming queue
         case method
-        when "initialize", "initialized", "textDocument/didOpen", "textDocument/didClose", "textDocument/didChange"
+        when "initialize", "initialized", "textDocument/didOpen", "textDocument/didClose", "textDocument/didChange",
+             "$/cancelRequest"
           process_message(message)
         when "shutdown"
           send_log_message("Shutting down Ruby LSP...")
@@ -133,6 +134,12 @@ def pop_response
       @outgoing_queue.pop
     end
 
+    # This method is only intended to be used in tests! Pushes a message to the incoming queue directly
+    sig { params(message: T::Hash[Symbol, T.untyped]).void }
+    def push_message(message)
+      @incoming_queue << message
+    end
+
     sig { abstract.params(message: T::Hash[Symbol, T.untyped]).void }
     def process_message(message); end
 
@@ -154,7 +161,11 @@ def new_worker
           # Check if the request was cancelled before trying to process it
           @mutex.synchronize do
             if id && @cancelled_requests.include?(id)
-              send_message(Result.new(id: id, response: nil))
+              send_message(Error.new(
+                id: id,
+                code: Constant::ErrorCodes::REQUEST_CANCELLED,
+                message: "Request #{id} was cancelled",
+              ))
               @cancelled_requests.delete(id)
               next
             end

lib/ruby_lsp/utils.rb

@@ -173,6 +173,9 @@ class Error
     sig { returns(String) }
     attr_reader :message
 
+    sig { returns(Integer) }
+    attr_reader :code
+
     sig { params(id: Integer, code: Integer, message: String, data: T.nilable(T::Hash[Symbol, T.untyped])).void }
     def initialize(id:, code:, message:, data: nil)
       @id = id

test/server_test.rb

@@ -744,6 +744,51 @@ def test_requests_to_a_non_existing_position_return_error
     assert_match("Request textDocument/completion failed to find the target position.", error.message)
   end
 
+  def test_cancelling_requests_returns_expected_error_code
+    uri = URI("file:///foo.rb")
+
+    @server.process_message({
+      method: "textDocument/didOpen",
+      params: {
+        textDocument: {
+          uri: uri,
+          text: "class Foo\nend",
+          version: 1,
+          languageId: "ruby",
+        },
+      },
+    })
+
+    mutex = Mutex.new
+    mutex.lock
+
+    # Use a mutex to lock the request in the middle so that we can cancel it before it finishes
+    @server.stubs(:text_document_definition) do |_message|
+      mutex.synchronize { 123 }
+    end
+
+    thread = Thread.new do
+      @server.push_message({
+        id: 1,
+        method: "textDocument/definition",
+        params: {
+          textDocument: {
+            uri: uri,
+          },
+          position: { line: 0, character: 6 },
+        },
+      })
+    end
+
+    @server.process_message({ method: "$/cancelRequest", params: { id: 1 } })
+    mutex.unlock
+    thread.join
+
+    error = find_message(RubyLsp::Error)
+    assert_equal(RubyLsp::Constant::ErrorCodes::REQUEST_CANCELLED, error.code)
+    assert_equal("Request 1 was cancelled", error.message)
+  end
+
   private
 
   def with_uninstalled_rubocop(&block)