native dns: do not write query to buffer if waiting on a previous reply

in a normal situation this would not happen, as events on a resolver would be dealt one at a time, but in a fiber-scheduler environment like async, the first query could be buffered in the 1st fiber switch, the second could then be enqueued on 2nd, then flush buffer and fiber switch on read, and a third query would then enter a corrupt state where, due to the buffer having been flushed on the 2nd fiber, write the 3rd query before receiving the second one, and messing up the pending query bookkeeping, making the fiber then block (due to DNS query never returning back) and expose that as an empty interest registration from some other connection on the selector

this is fixed by using @timer as a proxy for knowing that a given DNS query has been flushed but is still waiting on a response (or an eventual retry)

lib/httpx/resolver/native.rb

@@ -42,6 +42,7 @@ module HTTPX
       @read_buffer = "".b
       @write_buffer = Buffer.new(@resolver_options[:packet_size])
       @state = :idle
+      @timer = nil
     end
 
     def close
@@ -153,6 +154,8 @@ module HTTPX
       @timer = @current_selector.after(timeout) do
         next unless @connections.include?(connection)
 
+        @timer = nil
+
         do_retry(h, connection, timeout)
       end
     end
@@ -270,6 +273,8 @@ module HTTPX
     def parse(buffer)
       @timer.cancel
 
+      @timer = nil
+
       code, result = Resolver.decode_dns_answer(buffer)
 
       case code
@@ -383,7 +388,8 @@ module HTTPX
 
       raise Error, "no URI to resolve" unless connection
 
-      return unless @write_buffer.empty?
+      # do not buffer query if previous is still in the buffer or awaiting reply/retry
+      return unless @write_buffer.empty? && @timer.nil?
 
       hostname ||= @queries.key(connection)
 

sig/resolver/native.rbs

@@ -23,6 +23,7 @@ module HTTPX
       @large_packet: Buffer?
       @io: UDP | TCP
       @name: String?
+      @timer: Timers::Timer?
 
       attr_reader state: Symbol