Add-ons: new Addon methods to access the storage area beneath $FG_HOME/Export

New methods Addon::createStorageDir() and Addon::getStoragePath() with
corresponding Nasal bindings in the addons.Addon ghost:

  createStorageDir() method (returns the dir, doesn't fail if it already
                     exists)
  storagePath        read-only attribute to get the dir

The directory reserved for each add-on is
$FG_HOME/Export/Addons/ADDON_ID, but please use the above methods (or
the corresponding C++ ones) to avoid hardcoding such paths in your code.

Also create directory $FG_HOME/Export/Addons in fgInitConfig() as a way
of reserving the namespace, in order to prevent future failures in case
someone would have the strange idea to create it as a file...

src/Add-ons/Addon.cxx

@@ -24,6 +24,7 @@
 #include <utility>
 #include <vector>
 
+#include <simgear/misc/sg_dir.hxx>
 #include <simgear/misc/sg_path.hxx>
 #include <simgear/misc/strutils.hxx>
 #include <simgear/nasal/cppbind/Ghost.hxx>
@@ -34,6 +35,7 @@
 
 #include <Main/fg_props.hxx>
 #include <Main/globals.hxx>
+#include <Main/util.hxx>
 #include <Scripting/NasalSys.hxx>
 
 #include "addon_fwd.hxx"
@@ -93,6 +95,7 @@ Addon::Addon(std::string id, AddonVersion version, SGPath basePath,
     _version(
       shared_ptr_traits<AddonVersionRef>::makeStrongRef(std::move(version))),
     _basePath(std::move(basePath)),
+    _storagePath(globals->get_fg_home() / ("Export/Addons/" + _id)),
     _minFGVersionRequired(std::move(minFGVersionRequired)),
     _maxFGVersionRequired(std::move(maxFGVersionRequired)),
     _addonNode(addonNode)
@@ -187,6 +190,48 @@ SGPath Addon::getBasePath() const
 void Addon::setBasePath(const SGPath& addonBasePath)
 { _basePath = addonBasePath; }
 
+SGPath Addon::getStoragePath() const
+{ return _storagePath; }
+
+SGPath Addon::createStorageDir() const
+{
+  if (_storagePath.exists()) {
+    if (!_storagePath.isDir()) {
+      string msg =
+        "Unable to create add-on storage directory because the entry already "
+        "exists, but is not a directory: '" + _storagePath.utf8Str() + "'";
+      // Log + throw, because if called from Nasal, only throwing would cause
+      // the exception message to 1) be truncated and 2) only appear in the
+      // log, not stopping the sim. Then users would have to figure out why
+      // their add-on doesn't work...
+      SG_LOG(SG_GENERAL, SG_POPUP, msg);
+      throw errors::unable_to_create_addon_storage_dir(msg);
+    }
+  } else {
+    SGPath authorizedPath = fgValidatePath(_storagePath, true /* write */);
+
+    if (authorizedPath.isNull()) {
+      string msg =
+        "Unable to create add-on storage directory because of the FlightGear "
+        "security policy (refused by fgValidatePath()): '" +
+        _storagePath.utf8Str() + "'";
+      SG_LOG(SG_GENERAL, SG_POPUP, msg);
+      throw errors::unable_to_create_addon_storage_dir(msg);
+    } else {
+      simgear::Dir(authorizedPath).create(0777);
+    }
+  }
+
+  // The sensitive operation (creating the directory) is behind us; return
+  // _storagePath instead of authorizedPath for consistency with the
+  // getStoragePath() method (_storagePath and authorizedPath could be
+  // different in case the former contains symlink components). Further
+  // sensitive operations beneath _storagePath must use fgValidatePath() again
+  // every time, of course (otherwise attackers could use symlinks in
+  // _storagePath to bypass the security policy).
+  return _storagePath;
+}
+
 std::string Addon::resourcePath(const std::string& relativePath) const
 {
   if (strutils::starts_with(relativePath, "/")) {
@@ -448,6 +493,8 @@ void Addon::setupGhost(nasal::Hash& addonsModule)
     .member("licenseUrl", &Addon::getLicenseUrl)
     .member("tags", &Addon::getTags)
     .member("basePath", &Addon::getBasePath)
+    .member("storagePath", &Addon::getStoragePath)
+    .method("createStorageDir", &Addon::createStorageDir)
     .method("resourcePath", &Addon::resourcePath)
     .member("minFGVersionRequired", &Addon::getMinFGVersionRequired)
     .member("maxFGVersionRequired", &Addon::getMaxFGVersionRequired)

src/Add-ons/Addon.hxx

@@ -127,6 +127,13 @@ public:
   SGPath getBasePath() const;
   void setBasePath(const SGPath& addonBasePath);
 
+  // Return $FG_HOME/Export/Addons/ADDON_ID as an SGPath instance.
+  SGPath getStoragePath() const;
+  // Create directory $FG_HOME/Export/Addons/ADDON_ID, including any parent,
+  // if it doesn't already exist. Throw an exception in case of problems.
+  // Return an SGPath instance for the directory (same as getStoragePath()).
+  SGPath createStorageDir() const;
+
   // Return a resource path suitable for use with the simgear::ResourceManager.
   // 'relativePath' is relative to the add-on base path, and should not start
   // with a '/'.
@@ -218,6 +225,8 @@ private:
 
   std::vector<std::string> _tags;
   SGPath _basePath;
+  // $FG_HOME/Export/Addons/ADDON_ID
+  const SGPath _storagePath;
 
   // To be used with simgear::strutils::compare_versions()
   std::string _minFGVersionRequired;

src/Add-ons/addon_fwd.hxx

@@ -61,6 +61,7 @@ class duplicate_registration_attempt;
 class fg_version_too_old;
 class fg_version_too_recent;
 class invalid_resource_path;
+class unable_to_create_addon_storage_dir;
 
 } // of namespace errors
 

src/Add-ons/exceptions.hxx

@@ -65,6 +65,9 @@ class fg_version_too_recent : public error
 class invalid_resource_path : public error
 { using error::error; };
 
+class unable_to_create_addon_storage_dir : public error
+{ using error::error; };
+
 } // of namespace errors
 
 } // of namespace addons

src/Main/fg_init.cxx

@@ -488,6 +488,21 @@ bool fgInitHome()
     return result;
 }
 
+static void createBaseStorageDirForAddons(const SGPath& exportDir)
+{
+    SGPath addonStorageBasePath = exportDir / "Addons";
+    if (addonStorageBasePath.exists()) {
+      if (!addonStorageBasePath.isDir()) {
+        throw sg_error(
+          "Unable to create add-on storage base directory, because the entry "
+          "already exists but is not a directory: '" +
+          addonStorageBasePath.utf8Str() + "'");
+      }
+    } else {
+      simgear::Dir(addonStorageBasePath).create(0777); // respect user's umask
+    }
+}
+
 // Read in configuration (file and command line)
 int fgInitConfig ( int argc, char **argv, bool reinit )
 {
@@ -498,6 +513,10 @@ int fgInitConfig ( int argc, char **argv, bool reinit )
       exportDir.create(0755);
     }
 
+    // Reserve a directory where add-ons can write. There will be a subdir for
+    // each add-on, see Addon::createStorageDir() and Addon::getStoragePath().
+    createBaseStorageDirForAddons(exportDir.path());
+
     // Set /sim/fg-home.  Use FG_HOME if necessary.
     // deliberately not a tied property, for fgValidatePath security
     // write-protect to avoid accidents