Add FGGlobals::autosaveFilePath(); improve code related to {load,save}UserSettings()

Apart from providing a public method giving a path to the autosave file,
the main idea of this commit is to reduce redundancy where
globals->get_fg_home() was so far used in every place where the autosave
file is needed or saved. Use an optional argument for
FGGlobals::loadUserSettings() and FGGlobals::saveUserSettings()[1],
since it should be exceptional to access an autosave file in another
location than $FG_HOME.

Also add comments explaining how to avoid security pitfalls with
saveUserSettings() (cf. discussion around
<https://sourceforge.net/p/flightgear/mailman/message/35461636/>).

[1] Argument *added* to this method, for consistency with
    FGGlobals::loadUserSettings().

src/Main/globals.cxx

@@ -681,17 +681,31 @@ static std::string autosaveName()
     return os.str();
 }
 
-// Load user settings from autosave.xml
-void
-FGGlobals::loadUserSettings(const SGPath& dataPath)
+SGPath FGGlobals::autosaveFilePath(SGPath userDataPath) const
 {
-    // remember that we have (tried) to load any existing autsave.xml
+    if (userDataPath.isNull()) {
+        userDataPath = get_fg_home();
+    }
+
+    return simgear::Dir(userDataPath).file(autosaveName());
+}
+
+// Load user settings from the autosave file (normally in $FG_HOME)
+void
+FGGlobals::loadUserSettings(SGPath userDataPath)
+{
+    if (userDataPath.isNull()) {
+        userDataPath = get_fg_home();
+    }
+
+    // Remember that we have (tried) to load any existing autosave file
     haveUserSettings = true;
 
-    SGPath autosaveFile = simgear::Dir(dataPath).file(autosaveName());
+    SGPath autosaveFile = autosaveFilePath(userDataPath);
     SGPropertyNode autosave;
     if (autosaveFile.exists()) {
-      SG_LOG(SG_INPUT, SG_INFO, "Reading user settings from " << autosaveFile);
+      SG_LOG(SG_INPUT, SG_INFO,
+             "Reading user settings from " << autosaveFile);
       try {
           readProperties(autosaveFile, &autosave, SGPropertyNode::USERARCHIVE);
       } catch (sg_exception& e) {
@@ -702,10 +716,19 @@ FGGlobals::loadUserSettings(const SGPath& dataPath)
     copyProperties(&autosave, globals->get_props());
 }
 
-// Save user settings in autosave.xml
+// Save user settings to the autosave file.
+//
+// When calling this method, make sure the value of 'userDataPath' is
+// trustworthy. In particular, make sure it can't be influenced by Nasal code,
+// not even indirectly via a Nasal-writable place such as the property tree.
+//
+// Note: the default value, which causes the autosave file to be written to
+//       $FG_HOME, is safe---if not, it would be a bug.
 void
-FGGlobals::saveUserSettings()
+FGGlobals::saveUserSettings(SGPath userDataPath)
 {
+    if (userDataPath.isNull()) userDataPath = get_fg_home();
+
     // only save settings when we have (tried) to load the previous
     // settings (otherwise user data was lost)
     if (!haveUserSettings)
@@ -715,8 +738,7 @@ FGGlobals::saveUserSettings()
       // don't save settings more than once on shutdown
       haveUserSettings = false;
 
-      SGPath autosaveFile(globals->get_fg_home());
-      autosaveFile.append(autosaveName());
+      SGPath autosaveFile = autosaveFilePath(userDataPath);
       autosaveFile.create_dir( 0700 );
       SG_LOG(SG_IO, SG_INFO, "Saving user settings to " << autosaveFile);
       try {

src/Main/globals.hxx

@@ -373,15 +373,29 @@ public:
     inline void set_channellist( FGTACANList *c ) { channellist = c; }
 
     /**
-     * Load user settings from autosave.xml
+     * Return an SGPath instance for the autosave file under 'userDataPath',
+     * which defaults to $FG_HOME.
      */
-    void loadUserSettings(const SGPath& datapath);
+    SGPath autosaveFilePath(SGPath userDataPath = SGPath()) const;
+    /**
+     * Load user settings from the autosave file under 'userDataPath',
+     * which defaults to $FG_HOME.
+     */
+    void loadUserSettings(SGPath userDatapath = SGPath());
 
     /**
-     * Save user settings in autosave.xml
+     * Save user settings to the autosave file under 'userDataPath', which
+     * defaults to $FG_HOME.
+     *
+     * When calling this method, make sure the value of 'userDataPath' is
+     * trustworthy. In particular, make sure it can't be influenced by Nasal
+     * code, not even indirectly via a Nasal-writable place such as the
+     * property tree.
+     *
+     * Note: the default value is safe---if not, it would be a bug.
      */
-    void saveUserSettings();
-    
+    void saveUserSettings(SGPath userDatapath = SGPath());
+
     void addListenerToCleanup(SGPropertyChangeListener* l);
   
     simgear::pkg::Root* packageRoot();

src/Main/main.cxx

@@ -191,10 +191,8 @@ static void fgSetVideoOptions()
             }
 
             flightgear::Options* options = flightgear::Options::sharedInstance();
-            if (!options->isOptionSet("ignore-autosave"))
-            {
-                SGPath dataPath = globals->get_fg_home();
-                globals->loadUserSettings(dataPath);
+            if (!options->isOptionSet("ignore-autosave")) {
+                globals->loadUserSettings();
             }
         }
     }