Implement engine_process_connections timer

The previous code did not have an implementation to set timers for processing
connections. This leads to an occasional deadlock where process_connections
must be called to continue. This change implements this timer modelled after
the lsquic reference implementation.

Reference: https://github.com/litespeedtech/lsquic/blob/master/bin/prog.c#L535

src/lsquic/client.cr

@@ -145,6 +145,7 @@ module QUIC
     @read_timeout : Float64?
     @socket : UDPSocket?
     @stream_ctx : StreamCtx?
+    @process_fiber : Fiber
 
     def initialize(@host : String, port = nil, tls : Bool | OpenSSL::SSL::Context::Client = false)
       check_host_only(@host)
@@ -162,6 +163,7 @@ module QUIC
       @stream_channel = Channel(StreamCtx?).new(20)
       @stream_ctx = nil
       @engine_open = false
+      @process_fiber = Fiber.new { puts "process_fiber started before run_engine" }
     end
 
     def run_engine
@@ -209,7 +211,7 @@ module QUIC
         while stream_ctx = @stream_channel.receive
           LibLsquic.conn_set_ctx(conn, Box.box(stream_ctx))
           LibLsquic.conn_make_stream(conn)
-          LibLsquic.engine_process_conns(engine)
+          client_process_conns(engine)
         end
         @engine_open = false
         LibLsquic.engine_destroy(engine)
@@ -217,13 +219,25 @@ module QUIC
         @socket = nil
       end
 
+      @process_fiber = spawn do
+        loop do
+          sleep
+          LibLsquic.engine_process_conns(engine)
+          diff = 0
+          # check advisory time
+          if LibLsquic.engine_earliest_adv_tick(engine, pointerof(diff)) != 0
+            Thread.current.scheduler.@current.resume_event.add(diff.microseconds)
+          end
+        end
+      end
+
       begin
         buffer = Bytes.new(0x600)
         loop do
           bytes_read = socket.read buffer
           break if !@engine_open
           LibLsquic.engine_packet_in(engine, buffer[0, bytes_read], bytes_read, socket.local_address, socket.remote_address, Box.box(socket), 0) if bytes_read != 0
-          LibLsquic.engine_process_conns(engine)
+          client_process_conns(engine)
         end
         @socket.try &.close
         @socket = nil
@@ -232,6 +246,10 @@ module QUIC
       end
     end
 
+    def client_process_conns(engine)
+      Crystal::Scheduler.yield @process_fiber
+    end
+
     def socket : UDPSocket
       socket = @socket
       return socket.not_nil! if @socket