partial fix for timeout handling

1) "Unknown transfer id" message was because the local variable "tid"
is not a transfer id, it is a sequence number  - so the check was
actually comparing expected vs actual acknowledged sequence number,
not TID.  It's still a problem if we get the wrong one, but it
indicates a lost packet (so we should resend) not a packet that was
sent from somewhere else.

2) if the ACK packet has not been received, our retry should involve
_resending_ it, not just trying to wait for it again.

3) I have removed the timeout condition for terminating the resend
loop, because in practice (assuming both ends have the same timeout
setting) all it did was ensure that the loop only ran once. The
timeout is supposed to regulate how long we wait for before retrying
(it doesn't do this, we wait indefinitely), not how long we wait for
before giving up.

pkgs/tufted/tftp.lua

@@ -195,19 +195,22 @@ function tftp:handle_RRQ(socket, host, port, source, options)
                 ]]
                 yield(false, true)
             end
-            socket:sendto(self.DATA(data, tid), host, port)
-            --[[  Now check for an ACK.
-                RFC1350 requires that for every packet sent, an ACK is received
-                before the next packet can be sent.
-            ]]
+
             local acked
             local retried = 0
             local timeout = time() + timeout_secs
             local timedout = false
             repeat
+                socket:sendto(self.DATA(data, tid), host, port)
+                --[[  Now check for an ACK.
+                   RFC1350 requires that for every packet sent, an ACK is received
+                   before the next packet can be sent.
+                ]]
                 yield(true, false) -- we need to wait until the socket is readable again
                 local ack, ackhost, ackport = socket:recvfrom(ACKSIZE)
-                if ackhost ~= host or ackport ~= port or self.parse_ACK(ack) ~= tid then
+                local ack_sequence = self.parse_ACK(ack)
+
+                if ackhost ~= host or ackport ~= port then
                    --[[https://tools.ietf.org/html/rfc1350#page-5
                        "If a source TID does not match, the packet should be
                        discarded as erroneously sent from somewhere else.
@@ -216,13 +219,21 @@ function tftp:handle_RRQ(socket, host, port, source, options)
                    ]]
                    socket:sendto(err(ERR_UNKNOWN_ID), ackhost, ackport)
                    yield(true, false)
-                else
-                    acked = true
-                end
+		elseif ack_sequence ~= tid then
+		   -- this looks confusing, but the local variable
+		   -- "tid" here is actually block number (aka
+		   -- sequence number), not tid at all.
+		   log(("ack received for old block %d (expecting %d)"):format(ack_sequence, tid))
+		   acked = false
+		else
+		   acked = true
+		end
+
+		if not acked then log("resending") end
                 retried = retried + 1
                 timedout = time() > timeout
-            until acked or retried > ACK_RETRIES or timedout
-            if timedout or retried > ACK_RETRIES then
+            until acked or retried > ACK_RETRIES
+            if retried > ACK_RETRIES then
                 --There doesn't seem to be a standard error for timeout.
                 socket:sendto(err("Ack timeout"), host, port)
                 error("Timeout waiting for ACK")