Restore METAR-based runway selection.

As part of this, add the ability to distinguish default vs explicit
airport selection via a new /sim/presets/airport-requested flag. This
enables us to more cleanly handle different combinations of startup,
especially the case where the user requests an airport but no runway
(wants auto selection), ensuring we don’t look for the default airport’s
runway (from location-presets.xml) in that case.

src/Main/options.cxx

@@ -1352,6 +1352,14 @@ fgOptScenario( const char *arg )
     return FG_OPTIONS_OK;
 }
 
+static int
+fgOptAirport( const char *arg )
+{
+    fgSetString("/sim/presets/airport-id", arg );
+    fgSetBool("/sim/presets/airport-requested", true );
+    return FG_OPTIONS_OK;
+}
+
 static int
 fgOptRunway( const char *arg )
 {
@@ -1584,7 +1592,7 @@ struct OptionDesc {
     {"disable-sound",                false, OPTION_BOOL,   "/sim/sound/working", false, "", 0 },
     {"enable-sound",                 false, OPTION_BOOL,   "/sim/sound/working", true, "", 0 },
     {"sound-device",                 true,  OPTION_STRING, "/sim/sound/device-name", false, "", 0 },
-    {"airport",                      true,  OPTION_STRING, "/sim/presets/airport-id", false, "", 0 },
+    {"airport",                      true,  OPTION_FUNC,   "", false, "", fgOptAirport },
     {"runway",                       true,  OPTION_FUNC,   "", false, "", fgOptRunway },
     {"vor",                          true,  OPTION_FUNC,   "", false, "", fgOptVOR },
     {"vor-frequency",                true,  OPTION_DOUBLE, "/sim/presets/vor-freq", false, "", fgOptVOR },

src/Main/positioninit.cxx

@@ -548,6 +548,7 @@ bool initPosition()
   }
   
   string apt = fgGetString("/sim/presets/airport-id");
+  const bool apt_req = fgGetBool("/sim/presets/airport-requested");
   string rwy_no = fgGetString("/sim/presets/runway");
   bool rwy_req = fgGetBool("/sim/presets/runway-requested");
   string vor = fgGetString("/sim/presets/vor-id");
@@ -581,6 +582,14 @@ bool initPosition()
   if (hdg > 9990.0)
     hdg = fgGetDouble("/environment/config/boundary/entry/wind-from-heading-deg", 270);
 
+  if (apt_req && !rwy_req) {
+      // ensure that if the users asks for a specific airport, but not a runway,
+      // presumably because they want automatic selection, we do not look
+      // for the default runway (from $FGDATA/location-preset.xml) which is
+      // likely missing.
+      rwy_no.clear();
+  }
+
   if ( !set_pos && !apt.empty() && !parkpos.empty() ) {
     // An airport + parking position is requested
     // since this depends on parking, which is part of dynamics, and hence
@@ -695,13 +704,17 @@ bool finalizeMetar()
   
   double hdg = fgGetDouble( "/environment/metar/base-wind-dir-deg", 9999.0 );
   string apt = fgGetString("/sim/presets/airport-id");
-  string rwy = fgGetString("/sim/presets/runway");
   double strthdg = fgGetDouble( "/sim/startup/options/heading-deg", 9999.0 );
   string parkpos = fgGetString( "/sim/presets/parkpos" );
   bool onground = fgGetBool( "/sim/presets/onground", false );
+  const bool rwy_req = fgGetBool("/sim/presets/runway-requested");
   // this logic is taken from former startup.nas
-  bool needMetar = (hdg < 360.0) && !apt.empty() && (strthdg > 360.0) &&
+  bool needMetar = (hdg < 360.0) &&
-  rwy.empty() && onground && parkpos.empty();
+    !apt.empty() &&
+    (strthdg > 360.0) &&
+    !rwy_req &&
+    onground &&
+    parkpos.empty();
   
   if (needMetar) {
     // timeout so we don't spin forever if the network is down