keep-harder.patch

diff --git a/DistUpgrade/DistUpgradeController.py b/DistUpgrade/DistUpgradeController.py
index 43d3624eb..79d4ae6b4 100644
--- a/DistUpgrade/DistUpgradeController.py
+++ b/DistUpgrade/DistUpgradeController.py
@@ -2025,7 +2025,24 @@ class DistUpgradeController(object):
             # resolve them by keeping back the obsolete packages.
             self.cache._startAptResolverLog()
             pr = apt.ProblemResolver(self.cache)
-            pr.resolve_by_keep()
+
+            # Manually keep dependencies of now-broken packages
+            while self.cache.broken_count > 0:
+                logging.info("Resolving %d broken dependencies", self.cache.broken_count)
+                pr.resolve_by_keep()
+                # There is a bug in resolve_by_keep() where it doesn't undo purge
+                # requests, so let's do it ourselves.
+                with self.cache.actiongroup():
+                    for package in self.cache:
+                        if package.is_inst_broken:
+                            for dep in package.installed.get_dependencies("PreDepends", "Depends"):
+                                for target_version in dep.installed_target_versions:
+                                    if target_version.package.marked_delete:
+                                        logging.debug("Keeping %s due to %s", target_version.package, dep)
+                                        target_version.package.mark_keep()
+                                        if not target_version.package.marked_keep:
+                                            logging.error("Could not keep %s", target_version.package)
+
             self.cache._stopAptResolverLog()
 
             # resolve_by_keep() will revert any unsafe removals, so we need to list them here again.