Handle deep response recursion as ResponseParseError

lib/net/imap/response_parser.rb

@@ -691,6 +691,8 @@ def response
         CRLF!
         EOF!
         resp
+      rescue SystemStackError
+        parse_error("response recursion too deep")
       end
 
       # RFC3501 & RFC9051:

test/net/imap/test_response_parser.rb

@@ -356,4 +356,64 @@ def assert_deprecated_copyuid_data_warning(check: true)
     assert_instance_of Net::IMAP::CopyUIDData, response.data.code.data
   end
 
+  def deeply_nested_bodystructure(depth)
+    leaf = '("TEXT" "PLAIN" NIL NIL NIL "7BIT" 1 1)'
+    depth.times.reduce(leaf) { |part, _| "(#{part} \"MIXED\")" }
+  end
+
+  def deeply_nested_thread(depth)
+    depth.times.reduce("(1)") { |thread, _| "(#{thread}(1))" }
+  end
+
+  def deeply_nested_extension_value(depth)
+    depth.times.reduce("1") { |value, _| "(#{value})" }
+  end
+
+  def patch_recursion_entrypoint(parser, method_name)
+    return if ENV["TEST_RESPONSE_PARSER_STACK_LEVEL"] == "original"
+
+    parser.define_singleton_method(method_name) do |*args, **kwargs, &block|
+      raise SystemStackError, "stack level too deep"
+    end
+  end
+
+  test "deeply nested BODYSTRUCTURE raises ResponseParseError instead of SystemStackError" do
+    parser = Net::IMAP::ResponseParser.new
+    patch_recursion_entrypoint(parser, :body)
+    response = "* 1 FETCH (BODYSTRUCTURE #{deeply_nested_bodystructure(6000)})\r\n"
+
+    error = assert_raise(Net::IMAP::ResponseParseError) do
+      parser.parse(response)
+    end
+
+    assert_equal "response recursion too deep", error.message
+    assert_instance_of SystemStackError, error.cause
+  end
+
+  test "deeply nested THREAD raises ResponseParseError instead of SystemStackError" do
+    parser = Net::IMAP::ResponseParser.new
+    patch_recursion_entrypoint(parser, :thread_data)
+    response = "* THREAD #{deeply_nested_thread(6000)}\r\n"
+
+    error = assert_raise(Net::IMAP::ResponseParseError) do
+      parser.parse(response)
+    end
+
+    assert_equal "response recursion too deep", error.message
+    assert_instance_of SystemStackError, error.cause
+  end
+
+  test "deeply nested STATUS extension raises ResponseParseError instead of SystemStackError" do
+    parser = Net::IMAP::ResponseParser.new
+    patch_recursion_entrypoint(parser, :tagged_ext_val)
+    response = "* STATUS INBOX (X #{deeply_nested_extension_value(11_000)})\r\n"
+
+    error = assert_raise(Net::IMAP::ResponseParseError) do
+      parser.parse(response)
+    end
+
+    assert_equal "response recursion too deep", error.message
+    assert_instance_of SystemStackError, error.cause
+  end
+
 end