Allow cmd line & add-ons to supply FGData extensions

Allow add-ons to define an FGData subdir, and support multiple —data=
command line arguments, to avoid the need to modify FG_ROOT (which might
be read-only). Allow additional data paths to be either higher or lower
priority than FG_ROOT, so that add-ons cannot replace files in FG_ROOT.

src/Add-ons/AddonManager.cxx

@@ -204,9 +204,16 @@ AddonManager::registerAddon(const SGPath& addonPath)
   string msg = "Registered add-on '" + addon->getName() + "' (" + addonId +
                ") version " + addonVersion(addonId)->str() + "; "
                "base path is '" + addonRealPath.utf8Str() + "'";
-  // This preserves the registration order
-  _registeredAddons.push_back(std::move(addon));
-  SG_LOG(SG_GENERAL, SG_INFO, msg);
+
+  auto dataPath = addonRealPath / "FGData";
+  if (dataPath.exists()) {
+      SG_LOG(SG_GENERAL, SG_INFO, "Registering data path for add-on: " << addon->getName());
+      globals->append_data_path(dataPath, true /* after FG_ROOT */);
+  }
+    
+    // This preserves the registration order
+    _registeredAddons.push_back(addon);
+    SG_LOG(SG_GENERAL, SG_INFO, msg);
 
   return addonId;
 }

src/Main/globals.cxx

@@ -302,6 +302,7 @@ PathList FGGlobals::get_data_paths() const
 {
     PathList r(additional_data_paths);
     r.push_back(fg_root);
+    r.insert(r.end(), _dataPathsAfterFGRoot.begin(), _dataPathsAfterFGRoot.end());
     return r;
 }
 
@@ -318,13 +319,23 @@ PathList FGGlobals::get_data_paths(const std::string& suffix) const
     return r;
 }
 
-void FGGlobals::append_data_path(const SGPath& path)
+void FGGlobals::append_data_path(const SGPath& path, bool afterFGRoot)
 {
     if (!path.exists()) {
         SG_LOG(SG_GENERAL, SG_WARN, "adding non-existant data path:" << path);
     }
 
-    additional_data_paths.push_back(path);
+    using RM = simgear::ResourceManager;
+    auto resManager = RM::instance();
+    if (afterFGRoot) {
+        _dataPathsAfterFGRoot.push_back(path);
+        // after FG_ROOT
+        resManager->addBasePath(path, static_cast<RM::Priority>(RM::PRIORITY_DEFAULT - 10));
+    } else {
+        additional_data_paths.push_back(path);
+        // after NORMAL prioirty, but ahead of FG_ROOT
+        resManager->addBasePath(path, static_cast<RM::Priority>(RM::PRIORITY_DEFAULT + 10));
+    }
 }
 
 SGPath FGGlobals::find_data_dir(const std::string& pathSuffix) const
@@ -342,8 +353,15 @@ SGPath FGGlobals::find_data_dir(const std::string& pathSuffix) const
         return rootPath;
     }
 
-    SG_LOG(SG_GENERAL, SG_WARN, "dir not found in any data path:" << pathSuffix);
-    return SGPath();
+    for (SGPath p : _dataPathsAfterFGRoot) {
+        p.append(pathSuffix);
+        if (p.exists()) {
+            return p;
+        }
+    }
+
+    SG_LOG(SG_IO, SG_WARN, "not found in any data path: '" << pathSuffix << "'");
+    return SGPath{};
 }
 
 void FGGlobals::append_fg_scenery (const PathList &paths)

src/Main/globals.hxx

@@ -101,6 +101,8 @@ private:
      */
     PathList additional_data_paths;
 
+    PathList _dataPathsAfterFGRoot; ///< paths with a /lower/ prioirty than FGRoot
+
     // Users home directory for data
     SGPath fg_home;
 
@@ -216,7 +218,7 @@ public:
      */
     PathList get_data_paths(const std::string& suffix) const;
 
-    void append_data_path(const SGPath& path);
+    void append_data_path(const SGPath& path, bool afterFGRoot = false);
 
     /**
      * Given a path suffix (eg 'Textures' or 'AI/Traffic'), find the

src/Main/main.cxx

@@ -173,7 +173,7 @@ static void initTerrasync()
     // add the terrasync root as a data path so data can be retrieved from it
     // (even if we are in read-only mode)
     SGPath terraSyncDir(globals->get_terrasync_dir());
-    globals->append_data_path(terraSyncDir);
+    globals->append_data_path(terraSyncDir, false /* = ahead of FG_ROOT */);
 
     if (fgGetBool("/sim/fghome-readonly", false)) {
         return;

src/Main/options.cxx

@@ -727,6 +727,20 @@ fgOptAddon(const char *arg)
   return FG_OPTIONS_OK;
 }
 
+static int
+fgOptAdditionalDataDir(const char* arg)
+{
+    const SGPath dataPath = SGPath::fromUtf8(arg);
+    if (!dataPath.exists()) {
+        SG_LOG(SG_GENERAL, SG_ALERT, "--data path not found:'" << dataPath << "'");
+        flightgear::fatalMessageBoxWithoutExit("FlightGear", "Data path not found");
+        return FG_OPTIONS_EXIT;
+    }
+
+    globals->append_data_path(dataPath, false /* = before FG_ROOT */);
+    return FG_OPTIONS_OK;
+}
+
 static int
 fgOptVOR( const char * arg )
 {
@@ -1821,6 +1835,7 @@ struct OptionDesc {
     {"flight-plan",                  true,  OPTION_STRING,   "/autopilot/route-manager/file-path", false, "", NULL },
     {"config",                       true,  OPTION_IGNORE | OPTION_MULTI,   "", false, "", 0 },
     {"addon",                        true,  OPTION_FUNC | OPTION_MULTI, "", false, "", fgOptAddon },
+    {"data",                         true,  OPTION_FUNC | OPTION_MULTI, "", false, "", fgOptAdditionalDataDir },
     {"aircraft",                     true,  OPTION_STRING, "/sim/aircraft", false, "", 0 },
     {"vehicle",                      true,  OPTION_STRING, "/sim/aircraft", false, "", 0 },
     {"failure",                      true,  OPTION_FUNC | OPTION_MULTI,   "", false, "", fgOptFailure },

src/Network/generic.cxx

@@ -753,7 +753,7 @@ bool FGGeneric::close() {
 void
 FGGeneric::reinit()
 {
-    SGPath path = globals->resolve_maybe_aircraft_path("Protocol/" + file_name);
+    SGPath path = globals->find_data_dir("Protocol/" + file_name);
     if (!path.exists()) {
         SG_LOG(SG_NETWORK, SG_WARN, "Couldn't find protocol file for '" << file_name << "'");
         return;