If we squelch the first message from an aircraft, emit it when we see a second message.

This is possible now that the SBS output doesn't rely on the global block timestamp;
the output will look like this:

MSG,8,111,11111,4AC954,111111,2015/02/08,17:57:53.917,2015/02/08,17:57:53.936,,,,,,,,,,,,0
MSG,7,111,11111,392AEB,111111,2015/02/08,17:57:53.744,2015/02/08,17:57:53.936,,15375,,,,,,,,,,0
MSG,8,111,11111,392AEB,111111,2015/02/08,17:57:53.917,2015/02/08,17:57:53.936,,,,,,,,,,,,0
MSG,6,111,11111,800387,111111,2015/02/08,17:57:53.919,2015/02/08,17:57:53.936,,,,,,,,4745,0,0,0,0

where the "receive timestamp" (first time column) goes backwards to reflect the original reception
time of the delayed message, but the "forwarded timestamp" (second time column) reflects the actual
forwarding time.

dump1090.h

@@ -218,7 +218,6 @@
 #include "stats.h"
 #include "cpr.h"
 #include "icao_filter.h"
-#include "track.h"
 
 //======================== structure declarations =========================
 
@@ -406,6 +405,9 @@ struct modesMessage {
     int  bFlags;                // Flags related to fields in this structure
 };
 
+// This one needs modesMessage:
+#include "track.h"
+
 // ======================== function declarations =========================
 
 #ifdef __cplusplus

mode_s.c

@@ -1212,10 +1212,20 @@ void useModesMessage(struct modesMessage *mm) {
     // see a second message?
 
     if (Modes.net) {
-        if (Modes.net_verbatim || a->messages > 1)
+        if (Modes.net_verbatim || a->messages > 1) {
+            // If this is the second message, and we
+            // squelched the first message, then re-emit the
+            // first message now.
+            if (!Modes.net_verbatim && a->messages == 2) {
+                fprintf(stderr, "reemit first message for %06x\n", a->addr);
+                displayModesMessage(&a->first_message);
+                modesQueueOutput(&a->first_message);
+            }
+
             modesQueueOutput(mm);
         }
     }
+}
 
 //
 // ===================== Mode S detection and decoding  ===================

track.c

@@ -80,6 +80,10 @@ struct aircraft *trackCreateAircraft(struct modesMessage *mm) {
             mm->bFlags  |= MODES_ACFLAGS_ALTITUDE_VALID;
         }
     }
+
+    // Copy the first message so we can emit it later when a second message arrives.
+    a->first_message = *mm;
+
     return (a);
 }
 

track.h

@@ -90,6 +90,8 @@ struct aircraft {
     double        lat, lon;       // Coordinated obtained from CPR encoded data
     int           bFlags;         // Flags related to valid fields in this structure
     struct aircraft *next;        // Next aircraft in our linked list
+
+    struct modesMessage first_message;  // A copy of the first message we received for this aircraft.
 };