ames: minimum viable decrement underflow hotfix (#375)

We were incrementing the queue size counter conditionally, only for
packets we want to forward. But for all packets, we were issuing a scry,
and the callback would unconditionally decrement the queue size counter,
leading to underflows. That, in turn, would result in vere thinking the
queue was full when it wasn't.

Here, we make the least invasive change that resolves that particular
bug. There may or may not still be other things wrong with the
logic/factoring around the packet queue and lane scries.

f9e8542 changed the factoring here, making normal packet sending and
forwarding take the same code path. ~~But also, even before that change,
we were always sending the head of the packet queue to whatever lane
scry result we got, which feels like it might not be right, _especially
given that we don't even put the packet in queue when not forwarding_.~~
(edit: i think that was an incorrect reading of the code, we pass a
pointer to the specific packet, not the whole queue.) I suspect we just
don't want to scry for the lane in the normal sending codepath. We
didn't do that before recent changes either. (edit: as currently
written, we keep packets we want to send in memory until the scry gets
back to us, without tracking them anywhere.) But I don't know the full
context of the recent changes, so I'm only solving the specific
symptom/bug here.

Since this bug prevents forwarding from working reliable under certain
circumstances, this should go out as a hotfix.

pkg/vere/io/ames.c

@@ -1393,8 +1393,6 @@ _ames_lane_scry_cb(void* vod_p, u3_noun nun)
   u3_ames* sam_u = pac_u->sam_u;
   u3_weak    las = u3r_at(7, nun);
 
-  sam_u->sat_u.foq_d--;
-
   //  if scry fails, remember we can't scry, and just inject the packet
   //
   if ( u3_none == las ) {
@@ -1425,6 +1423,19 @@ _ames_lane_scry_cb(void* vod_p, u3_noun nun)
   u3z(nun);
 }
 
+/*  _ames_lane_scry_forward_cb(): learn lanes to forward packet on
+ */
+static void
+_ames_lane_scry_forward_cb(void *vod_p, u3_noun nun)
+{
+  u3_panc *pan_u = vod_p;
+  u3_ames *sam_u = pan_u->pac_u->sam_u;
+
+  sam_u->sat_u.foq_d--;
+
+  _ames_lane_scry_cb(vod_p, nun);
+}
+
 /* _ames_try_send(): try to send a packet to a ship and its sponsors
 */
 static void
@@ -1488,7 +1499,9 @@ _ames_try_send(u3_pact* pac_u, c3_o for_o)
     pan_u->pac_u = pac_u;
     pan_u->for_o = for_o;
 
-    //  if forwarding, enqueue
+    u3_noun pax = _lane_scry_path(u3i_chubs(2, pac_u->pre_u.rec_d));
+
+    //  if forwarding, enqueue the packet and scry for the lane
     //
     if ( c3y == for_o ) {
       if ( 0 != sam_u->pan_u ) {
@@ -1497,14 +1510,16 @@ _ames_try_send(u3_pact* pac_u, c3_o for_o)
       }
       sam_u->pan_u = pan_u;
       sam_u->sat_u.foq_d++;
-    }
 
-    //  scry the lane out of ames
+      u3_pier_peek_last(sam_u->pir_u, u3_nul, c3__ax,
+                        u3_nul, pax, pan_u, _ames_lane_scry_forward_cb);
+    }
+    //  otherwise, just scry for the lane
     //
-    u3_noun pax = _lane_scry_path(u3i_chubs(2, pac_u->pre_u.rec_d));
-
-    u3_pier_peek_last(sam_u->pir_u, u3_nul, c3__ax,
-                      u3_nul, pax, pan_u, _ames_lane_scry_cb);
+    else {
+      u3_pier_peek_last(sam_u->pir_u, u3_nul, c3__ax,
+                        u3_nul, pax, pan_u, _ames_lane_scry_cb);
+    }
   }
 }