Add assert_threads_result helper and threads test

CONTRIBUTING.md

@@ -372,6 +372,23 @@ An variable entry looks like this: `{ name: "bar", value: "nil", type: "NilClass
 
 Please note that both `value` and `type` need to be strings.
 
+- assert_threads_result(expected)
+
+Passes if both conditions are true:
+
+1. The number of expected patterns matches the number of threads.
+2. Every pattern matches a thread name. Notice that the order of threads info is not guaranteed.
+
+Example:
+
+```
+assert_threads_result(
+  [
+    /\.rb:\d:in `<main>'/,
+    /\.rb:\d:in `block in foo'/
+  ]
+)
+```
 
 ## To Update README
 

test/protocol/threads_test.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require_relative '../support/test_case'
+
+module DEBUGGER__
+  class ThreadsTest < TestCase
+    PROGRAM = <<~RUBY
+       1| def foo
+       2|   Thread.new { sleep 30 }
+       3| end
+       4|
+       5| foo
+       6| sleep 0.1 # make sure the thread stops
+       7| binding.b
+    RUBY
+
+    def test_reponse_returns_correct_threads_info
+      run_protocol_scenario PROGRAM, cdp: false do
+        req_continue
+
+        assert_threads_result(
+          [
+            /\.rb:\d:in `<main>'/,
+            /\.rb:\d:in `block in foo'/
+          ]
+        )
+
+        req_terminate_debuggee
+      end
+    end
+  end
+end
+

test/support/protocol_utils.rb

@@ -230,6 +230,29 @@ def assert_locals_result expected, frame_idx: 0
       end
     end
 
+    def assert_threads_result(expected_names)
+      case ENV['RUBY_DEBUG_TEST_UI']
+      when 'vscode'
+        res = send_dap_request 'threads'
+        failure_msg = FailureMessage.new{create_protocol_message "result:\n#{JSON.pretty_generate res}."}
+
+        threads = res.dig(:body, :threads)
+
+        assert_equal expected_names.count, threads.count, failure_msg
+
+        thread_names = threads.map { |t| t[:name] }
+
+        expected_names.each do |expected|
+          thread_names.reject! do |name|
+            name.match?(expected)
+          end
+        end
+
+        failure_msg = FailureMessage.new{create_protocol_message "result:\n#{JSON.pretty_generate res}.\nExpect all thread names to be matched. Unmatched threads:"}
+        assert_equal [], thread_names, failure_msg
+      end
+    end
+
     def assert_hover_result expected,  expression: nil, frame_idx: 0
       case ENV['RUBY_DEBUG_TEST_UI']
       when 'vscode'