gather: Always get latest packages

If lookaside contains an older version of a package, but with a
different arch, the depsolver doesn't notice that and prefers the
lookaside version.

This is not correct. The latest package should be used no matter if
there are different arches available.

The filtering in DNF doesn't ensure this, so we have to build it
ourselves. To limit the performance impact, only run this filtering when
there actually are some lookaside repos configured.

JIRA: RHELCMP-11728

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/gather_dnf.py

@@ -15,12 +15,14 @@
 
 
 from enum import Enum
-from itertools import count
+from functools import cmp_to_key
+from itertools import count, groupby
 import logging
 import os
 import re
 
 from kobo.rpmlib import parse_nvra
+import rpm
 
 import pungi.common
 import pungi.dnf_wrapper
@@ -246,6 +248,33 @@ class Gather(GatherBase):
         # from lookaside. This can be achieved by removing any package that is
         # also in lookaside from the list.
         lookaside_pkgs = set()
+
+        if self.opts.lookaside_repos:
+            # We called `latest()` to get the highest version packages only.
+            # However, that is per name and architecture. If a package switches
+            # from arched to noarch or the other way, it is possible that the
+            # package_list contains different versions in main repos and in
+            # lookaside repos.
+            # We need to manually filter the latest version.
+            def vercmp(x, y):
+                return rpm.labelCompare(x[1], y[1])
+
+            # Annotate the packages with their version.
+            versioned_packages = [
+                (pkg, (str(pkg.epoch) or "0", pkg.version, pkg.release))
+                for pkg in package_list
+            ]
+            # Sort the packages newest first.
+            sorted_packages = sorted(
+                versioned_packages, key=cmp_to_key(vercmp), reverse=True
+            )
+            # Group packages by version, take the first group and discard the
+            # version info from the tuple.
+            package_list = list(
+                x[0] for x in next(groupby(sorted_packages, key=lambda x: x[1]))[1]
+            )
+
+            # Now we can decide what is used from lookaside.
             for pkg in package_list:
                 if pkg.repoid in self.opts.lookaside_repos:
                     lookaside_pkgs.add("{0.name}-{0.evr}".format(pkg))