Avoid false alarm in assertion that involves floating point arithmetic

The FP arithmetic that leads to the branch with the assertion and the
assertion's FP arithmetic are now in monotonic relationship.

Author: nfrisby

simulation/src/Chan/TCP.hs

@@ -143,15 +143,15 @@ transport tracer tcpprops sendbuf recvbuf = do
     -- if the connection was idle too long, reset the window size
     let tcpstate' :: TcpState
         tcpstate'
-          | now' `diffTime` now <= tcpIdleResetTime tcpprops =
-              tcpstate
-          | otherwise =
+          | tcpIdleResetTime tcpprops now <= now' =
               let allAcksArrived =
                     PQ.foldrWithKeyU
-                      (\t _ ok -> t < now' && ok)
+                      (\t _ ok -> t <= now' && ok)
                       True
                       (tcpAcknowledgements tcpstate)
                in assert allAcksArrived initTcpState
+          | otherwise =
+              tcpstate
 
     -- send it
     let msgsize = messageSizeBytes msg
@@ -178,8 +178,10 @@ transport tracer tcpprops sendbuf recvbuf = do
 -- We could do that, but the algorithm also uses a minimum of 1s, which appears
 -- to be the limit in practice. It converges to 1 RTT if there's not much
 -- jitter. So we just use the max of the RTT and 1s.
-tcpIdleResetTime :: TcpConnProps -> DiffTime
-tcpIdleResetTime TcpConnProps{tcpLatency} =
-  max 1 rtt
- where
-  rtt = tcpLatency * 2
+--
+-- The signature and order of operations is awkward here because it needs to
+-- exactly match an assertion near the use case, since these types are
+-- represented as 'Double's.
+tcpIdleResetTime :: TcpConnProps -> Time -> Time
+tcpIdleResetTime TcpConnProps{tcpLatency} t =
+  max (1 `addTime` t) (tcpLatency `addTime` (tcpLatency `addTime` t))