Allow gather phase reuse on product_id change.

I've analysed multiple nigthly composes and often the only difference
in the configuration between two nightly composes is different
`product_id` commit hash.

The `product_id` is used in later `createrepo` phase and does not
influence the gather phase at all. I therefore think it can be
whitelisted in gather phase reuse code.

Signed-off-by: Jan Kaluza <jkaluza@redhat.com>

pungi/phases/gather/__init__.py

@@ -251,6 +251,13 @@ def reuse_old_gather_packages(compose, arch, variant, package_sets):
                 )
                 return
             continue
+
+        # Skip checking for frequently changing configuration options which do *not*
+        # influence Gather phase.
+        config_whitelist = ["product_id"]
+        if opt in config_whitelist:
+            continue
+
         if opt not in config or config[opt] != value:
             compose.log_info(
                 log_msg % ("compose configuration option %s changed." % opt)

tests/test_gather_phase.py

@@ -1173,6 +1173,28 @@ class TestReuseOldGatherPackages(helpers.PungiTestCase):
         )
         self.assertEqual(result, None)
 
+    @mock.patch("pungi.phases.gather.load_old_gather_result")
+    @mock.patch("pungi.phases.gather.load_old_compose_config")
+    def test_reuse_compose_config_different_whitelist(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        for whitelist_opt in ["product_id"]:
+            load_old_gather_result.return_value = {
+                "rpm": [{"path": "/build/bash-1.0.0-1.x86_64.rpm"}],
+                "srpm": [],
+                "debuginfo": [],
+            }
+
+            compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+            compose_conf_copy = dict(compose.conf)
+            compose_conf_copy[whitelist_opt] = "different"
+            load_old_compose_config.return_value = compose_conf_copy
+
+            result = gather.reuse_old_gather_packages(
+                compose, "x86_64", compose.variants["Server"], []
+            )
+            self.assertEqual(result, {"rpm": [], "srpm": [], "debuginfo": []})
+
     def _prepare_package_sets(
         self, load_old_gather_result, extra_global_pkgs=None, **kwargs
     ):