system-upgrade: Handle _execute errors

dnf5.spec

@@ -663,6 +663,7 @@ config-manager, copr, and repoclosure commands.
 
 %files -n dnf5-plugins -f dnf5-plugin-builddep.lang -f dnf5-plugin-changelog.lang -f dnf5-plugin-config-manager.lang -f dnf5-plugin-copr.lang -f dnf5-plugin-needs-restarting.lang -f dnf5-plugin-repoclosure.lang
 %config %{_unitdir}/dnf5-offline-transaction.service
+%config %{_unitdir}/dnf5-offline-transaction-cleanup.service
 %{_libdir}/dnf5/plugins/*.so
 %{_mandir}/man8/dnf5-builddep.8.*
 %{_mandir}/man8/dnf5-copr.8.*

dnf5/commands/offline/offline.cpp

@@ -412,6 +412,9 @@ void OfflineExecuteCommand::run() {
 
     auto transaction = goal->resolve();
     if (transaction.get_problems() != libdnf5::GoalProblem::NO_PROBLEM) {
+        std::cerr << "Failed to resolve transaction. This indicates some bigger problem, since the offline transaction "
+                     "was already successfully resolved before. Was the cache at "
+                  << datadir << " modified?" << std::endl;
         throw libdnf5::cli::GoalResolveError(transaction);
     }
 

dnf5/config/systemd/system/dnf5-offline-transaction-cleanup.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Offline upgrade/transaction using DNF 5 failed
+DefaultDependencies=no
+
+[Service]
+Type=oneshot
+# Remove the symlink if it's still there, to protect against reboot loops.
+ExecStart=/usr/bin/rm -fv /system-update
+# If anything goes wrong, reboot back to the normal system.
+ExecStart=/usr/bin/systemctl --no-block reboot

dnf5/config/systemd/system/dnf5-offline-transaction.service

@@ -7,6 +7,7 @@ DefaultDependencies=no
 Requires=sysinit.target
 After=sysinit.target systemd-journald.socket system-update-pre.target
 Before=poweroff.target reboot.target shutdown.target system-update.target
+OnFailure=dnf5-offline-transaction-cleanup.service
 
 [Service]
 # We are done when the script exits, not before