Add more documentation to RepositoryCommon::checkMetaOffline

See discussion in #115

f#	../fail_state.tmpwb569uon/

src/libaktualizr/uptane/uptanerepository.h

@@ -27,7 +27,16 @@ class RepositoryCommon {
 
   /**
    * Load the initial state of the repository from storage.
-   * Note that this _required_ for correct initialization.
+   * Note that this _required_ for correct initialization. The API here is
+   * particularly error prone and should be improved:
+   *   - Until this is called the root will have a policy kRejectAll (i.e. no
+   *     updates are possible). It is therefore part of the initialization
+   *     process and so should be moved to the ctor.
+   *   - It throws an exception in a non-exceptional case: where the metadata
+   *     in the storage has expired.
+   *   - When it throws an exception, it changes the state and actually does
+   *     perform initialization, therefore violating the Strong Exception
+   *     Guarantee.
    * @throws UptaneException if the local metadata is stale (this is not a failure)
    */
   virtual void checkMetaOffline(INvStorage &storage) = 0;

src/virtual_secondary/managedsecondary.cc

@@ -63,7 +63,8 @@ ManagedSecondary::ManagedSecondary(Primary::ManagedSecondaryConfig sconfig_in) :
     director_repo_->checkMetaOffline(*storage_);
   } catch (const std::exception &e) {
     // This is actually safe. We've done enough initialization to get
-    // director_repo_ into a valid configuration
+    // director_repo_ into a valid configuration. See the documentation for
+    // RepositoryCommon::checkMetaOffline() for details.
     LOG_INFO << "No valid Director metadata found in storage: " << e.what();
   }
   try {