Allow reusing gather phase results.

- Get also requires/provides of RPMs in package set.
- Store the results of gather phase as pickle file.
- Reuse old gather phase results in case Pungi configuration
  did not change, the "names" of RPMs in global package set
  did not change and their requires/provides did not change.
- Add `gather_allow_reuse` option to enable this feature.

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

Add gather_allow_reuse, add more tests and better handling of gather_lookaside_repos.

doc/configuration.rst

@@ -642,6 +642,10 @@ Options
     (*bool*) -- When set to ``True``, *Pungi* will build a self-hosting tree by
     following build dependencies. Only use when ``gather_method = "deps"``.
 
+**gather_allow_reuse** = False
+    (*bool*) -- When set to ``True``, *Pungi* will try to reuse gather results
+    from old compose specified by ``--old-composes``.
+
 **greedy_method**
     (*str*) -- This option controls how package requirements are satisfied in
     case a particular ``Requires`` has multiple candidates.

pungi/checks.py

@@ -641,6 +641,7 @@ def make_schema():
                 "default": _get_default_gather_backend(),
             },
             "gather_profiler": {"type": "boolean", "default": False},
+            "gather_allow_reuse": {"type": "boolean", "default": False},
             "pkgset_source": {"type": "string", "enum": ["koji", "repos"]},
             "createrepo_c": {"type": "boolean", "default": True},
             "createrepo_checksum": {

pungi/paths.py

@@ -154,6 +154,24 @@ class WorkPaths(object):
         path = os.path.join(path, file_name)
         return path
 
+    def gather_result(self, arch=None, variant=None, create_dir=True):
+        """
+        Examples:
+            work/x86_64/gather_result/x86_64.result
+            work/x86_64/gather_result/Server.x86_64.result
+        """
+        arch = arch or "global"
+        file_name = ""
+        if variant:
+            file_name += variant.uid + "."
+        file_name += arch + "."
+        file_name += "result"
+        path = os.path.join(self.topdir(arch, create_dir=create_dir), "gather_result")
+        if create_dir:
+            makedirs(path)
+        path = os.path.join(path, file_name)
+        return path
+
     def pungi_conf(self, arch=None, variant=None, create_dir=True, source_name=None):
         """
         Examples:

pungi/phases/gather/__init__.py

@@ -18,6 +18,8 @@ import json
 import os
 import shutil
 import threading
+import six
+from six.moves import cPickle as pickle
 
 from kobo.rpmlib import parse_nvra
 from kobo.shortcuts import run
@@ -166,6 +168,206 @@ def get_gather_methods(compose, variant):
     return global_method_name, methods
 
 
+def load_old_gather_result(compose, arch, variant):
+    """
+    Helper method to load `gather_packages` result from old compose.
+    """
+    gather_result = compose.paths.work.gather_result(variant=variant, arch=arch)
+    old_gather_result = compose.paths.old_compose_path(gather_result)
+    if not old_gather_result:
+        return None
+
+    compose.log_info("Loading old GATHER phase results: %s", old_gather_result)
+    with open(old_gather_result, "rb") as f:
+        old_result = pickle.load(f)
+        return old_result
+
+
+def load_old_compose_config(compose):
+    """
+    Helper method to load Pungi config dump from old compose.
+    """
+    config_dump_full = compose.paths.log.log_file("global", "config-dump")
+    config_dump_full = compose.paths.old_compose_path(config_dump_full)
+    if not config_dump_full:
+        return None
+
+    compose.log_info("Loading old config file: %s", config_dump_full)
+    with open(config_dump_full, "r") as f:
+        old_config = json.load(f)
+        return old_config
+
+
+def reuse_old_gather_packages(compose, arch, variant, package_sets):
+    """
+    Tries to reuse `gather_packages` result from older compose.
+
+    :param Compose compose: Compose instance.
+    :param str arch: Architecture to reuse old gather data for.
+    :param str variant: Variant to reuse old gather data for.
+    :param list package_sets: List of package sets to gather packages from.
+    :return: Old `gather_packages` result or None if old result cannot be used.
+    """
+    log_msg = "Cannot reuse old GATHER phase results - %s"
+    if not compose.conf["gather_allow_reuse"]:
+        compose.log_info(log_msg % "reuse of old gather results is disabled.")
+        return
+
+    old_result = load_old_gather_result(compose, arch, variant)
+    if old_result is None:
+        compose.log_info(log_msg % "no old gather results.")
+        return
+
+    old_config = load_old_compose_config(compose)
+    if old_config is None:
+        compose.log_info(log_msg % "no old compose config dump.")
+        return
+
+    # The dumps/loads is needed to convert all unicode strings to non-unicode ones.
+    config = json.loads(json.dumps(compose.conf))
+    for opt, value in old_config.items():
+        # Gather lookaside repos are updated during the gather phase. Check that
+        # the gather_lookaside_repos except the ones added are the same.
+        if opt == "gather_lookaside_repos" and opt in config:
+            value_to_compare = []
+            # Filter out repourls which starts with `compose.topdir` and also remove
+            # their parent list in case it would be empty.
+            for variant, per_arch_repos in config[opt]:
+                per_arch_repos_to_compare = {}
+                for arch, repourl in per_arch_repos.items():
+                    # The gather_lookaside_repos config allows setting multiple repourls
+                    # using list, but `_update_config` always uses strings. Therefore we
+                    # only try to filter out string_types.
+                    if isinstance(repourl, six.string_types):
+                        continue
+                    if not repourl.startswith(compose.topdir):
+                        per_arch_repos_to_compare[arch] = repourl
+                if per_arch_repos_to_compare:
+                    value_to_compare.append([variant, per_arch_repos_to_compare])
+            if value != value_to_compare:
+                compose.log_info(
+                    log_msg
+                    % ("compose configuration option gather_lookaside_repos changed.")
+                )
+                return
+            continue
+        if opt not in config or config[opt] != value:
+            compose.log_info(
+                log_msg % ("compose configuration option %s changed." % opt)
+            )
+            return
+
+    result = {
+        "rpm": [],
+        "srpm": [],
+        "debuginfo": [],
+    }
+
+    for pkgset in package_sets:
+        global_pkgset = pkgset["global"]
+
+        # Return in case the old file cache does not exist.
+        if global_pkgset.old_file_cache is None:
+            compose.log_info(log_msg % "old file cache does not exist.")
+            return
+
+        # Do quick check to find out the number of input RPMs is the same in both
+        # old and new cache.
+        if len(global_pkgset.old_file_cache) != len(global_pkgset.file_cache):
+            compose.log_info(log_msg % "some RPMs have been added/removed.")
+            return
+
+        # Create temporary dict mapping RPM path to record in `old_result`. This
+        # is needed later to make things faster.
+        old_result_cache = {}
+        for old_result_key, old_result_records in old_result.items():
+            for old_result_record in old_result_records:
+                old_result_cache[old_result_record["path"]] = [
+                    old_result_key,
+                    old_result_record,
+                ]
+
+        # The `old_file_cache` contains all the input RPMs from old pkgset. Some
+        # of these RPMs will be in older versions/releases than the ones in the
+        # new `file_cache`. This is OK, but we need to be able to pair them so we
+        # know that particular RPM package from `old_file_cache` has been updated
+        # by another package in the new `file_cache`.
+        # Following code uses "`rpm_obj.arch`-`rpm_obj.sourcerpm`-`rpm_obj.name`"
+        # as a key to map RPMs from `old_file_cache` to RPMs in `file_cache`.
+        #
+        # At first, we need to create helper dict with the mentioned key. The value
+        # is tuple in (rpm_obj, old_result_key, old_result_record) format.
+        key_to_old_rpm_obj = {}
+        for rpm_path, rpm_obj in global_pkgset.old_file_cache.items():
+            key = "%s-%s-%s" % (
+                rpm_obj.arch,
+                rpm_obj.sourcerpm or rpm_obj.name,
+                rpm_obj.name,
+            )
+
+            # With the current aproach, we cannot reuse old gather result in case
+            # there are multiple RPMs with the same arch, sourcerpm and name.
+            if key in key_to_old_rpm_obj:
+                compose.log_info(
+                    log_msg % ("two RPMs with the same key exist: %s." % key)
+                )
+                return
+
+            old_result_key, old_result_record = old_result_cache.get(
+                rpm_path, [None, None]
+            )
+            key_to_old_rpm_obj[key] = [rpm_obj, old_result_key, old_result_record]
+
+        # The `key_to_old_rpm_obj` now contains all the RPMs in the old global
+        # package set. We will now compare these old RPMs with the RPMs in the
+        # current global package set.
+        for rpm_path, rpm_obj in global_pkgset.file_cache.items():
+            key = "%s-%s-%s" % (
+                rpm_obj.arch,
+                rpm_obj.sourcerpm or rpm_obj.name,
+                rpm_obj.name,
+            )
+
+            # Check that this RPM existed even in the old package set.
+            if key not in key_to_old_rpm_obj:
+                compose.log_info(log_msg % "some RPMs have been added.")
+                return
+
+            # Check that requires or provides of this RPM is stil the same.
+            old_rpm_obj, old_result_key, old_result_record = key_to_old_rpm_obj[key]
+            if (
+                old_rpm_obj.requires != rpm_obj.requires
+                or old_rpm_obj.provides != rpm_obj.provides
+            ):
+                compose.log_info(
+                    log_msg % "requires or provides of some RPMs have changed."
+                )
+                return
+
+            # Add this RPM into the current result in case it has been in the
+            # old result.
+            if old_result_key and old_result_record:
+                # Update the path to RPM, because in the `old_result_record`,
+                # we might have path to old build of this RPM, but the rpm_path
+                # contains the updated one with the same requires/provides.
+                old_result_record["path"] = rpm_path
+                result[old_result_key].append(old_result_record)
+
+            # Delete the key from key_to_old_rpm_obj so we can find out later if all
+            # RPMs from the old package set have their counterpart in the current
+            # package set.
+            del key_to_old_rpm_obj[key]
+
+        # Check that all the RPMs from old_file_cache has been mapped to some RPM
+        # in the new file cache.
+        for per_arch_dict in key_to_old_rpm_obj.values():
+            if len(per_arch_dict) != 0:
+                compose.log_info(log_msg % "some RPMs have been removed.")
+                return
+
+    return result
+
+
 def gather_packages(compose, arch, variant, package_sets, fulltree_excludes=None):
     # multilib white/black-list is per-arch, common for all variants
     multilib_whitelist = get_multilib_whitelist(compose, arch)
@@ -189,7 +391,10 @@ def gather_packages(compose, arch, variant, package_sets, fulltree_excludes=None
     prepopulate = get_prepopulate_packages(compose, arch, variant)
     fulltree_excludes = fulltree_excludes or set()
 
-    if methods == "hybrid":
+    reused_result = reuse_old_gather_packages(compose, arch, variant, package_sets)
+    if reused_result:
+        result = reused_result
+    elif methods == "hybrid":
         # This variant is using a hybrid solver. Gather all inputs and run the
         # method once.
 
@@ -257,6 +462,10 @@ def gather_packages(compose, arch, variant, package_sets, fulltree_excludes=None
             for t in ("rpm", "srpm", "debuginfo"):
                 result[t].extend(pkg_map.get(t, []))
 
+    gather_result = compose.paths.work.gather_result(variant=variant, arch=arch)
+    with open(gather_result, "wb") as f:
+        pickle.dump(result, f, protocol=pickle.HIGHEST_PROTOCOL)
+
     compose.log_info("[DONE ] %s" % msg)
     return result
 

pungi/phases/gather/methods/method_deps.py

@@ -25,6 +25,7 @@ from pungi.wrappers.pungi import PungiWrapper
 
 from pungi.arch import tree_arch_to_yum_arch
 import pungi.phases.gather
+from pungi.phases.pkgset.pkgsets import ExtendedRpmWrapper
 
 import pungi.phases.gather.method
 
@@ -97,7 +98,7 @@ def _format_packages(pkgs):
     """Sort packages and merge name with arch."""
     result = set()
     for pkg, pkg_arch in pkgs:
-        if type(pkg) in [SimpleRpmWrapper, RpmWrapper]:
+        if type(pkg) in [SimpleRpmWrapper, RpmWrapper, ExtendedRpmWrapper]:
             pkg_name = pkg.name
         else:
             pkg_name = pkg

pungi/phases/gather/methods/method_nodeps.py

@@ -21,6 +21,7 @@ import six
 import pungi.arch
 from pungi.util import pkg_is_rpm, pkg_is_srpm, pkg_is_debug
 from pungi.wrappers.comps import CompsWrapper
+from pungi.phases.pkgset.pkgsets import ExtendedRpmWrapper
 
 import pungi.phases.gather.method
 from kobo.pkgset import SimpleRpmWrapper, RpmWrapper
@@ -83,7 +84,8 @@ class GatherMethodNodeps(pungi.phases.gather.method.GatherMethodBase):
                 ):
                     continue
                 elif (
-                    type(gathered_pkg) in [SimpleRpmWrapper, RpmWrapper]
+                    type(gathered_pkg)
+                    in [SimpleRpmWrapper, RpmWrapper, ExtendedRpmWrapper]
                     and pkg.nevra != gathered_pkg.nevra
                 ):
                     continue

pungi/phases/pkgset/pkgsets.py

@@ -34,6 +34,19 @@ from pungi.util import pkg_is_srpm
 from pungi.arch import get_valid_arches, is_excluded
 
 
+class ExtendedRpmWrapper(kobo.pkgset.SimpleRpmWrapper):
+    """
+    ExtendedRpmWrapper extracts only certain RPM fields instead of
+    keeping the whole RPM header in memory.
+    """
+
+    def __init__(self, file_path, ts=None, **kwargs):
+        kobo.pkgset.SimpleRpmWrapper.__init__(self, file_path, ts=ts)
+        header = kobo.rpmlib.get_rpm_header(file_path, ts=ts)
+        self.requires = set(kobo.rpmlib.get_header_field(header, "requires"))
+        self.provides = set(kobo.rpmlib.get_header_field(header, "provides"))
+
+
 class ReaderPool(ThreadPool):
     def __init__(self, package_set, logger=None):
         ThreadPool.__init__(self, logger)
@@ -55,14 +68,19 @@ class ReaderThread(WorkerThread):
 
         # In case we have old file cache data, try to reuse it.
         if self.pool.package_set.old_file_cache:
-            # The kobo.pkgset.FileCache does not have any method to check if
-            # the RPM is in cache. Instead it just re-uses the cached RPM data,
-            # if available, in .add() method.
-            # Therefore we call `old_file_cache.add()` which either returns the
-            # cached RPM object fast or just loads it from filesystem. We then
-            # add the returned RPM object to real `file_cache` directly.
-            rpm_obj = self.pool.package_set.old_file_cache.add(rpm_path)
-            self.pool.package_set.file_cache[rpm_path] = rpm_obj
+            # Try to find the RPM in old_file_cache and reuse it instead of
+            # reading its headers again.
+            try:
+                rpm_obj = self.pool.package_set.old_file_cache[rpm_path]
+            except KeyError:
+                rpm_obj = None
+
+            # Also reload rpm_obj if it's not ExtendedRpmWrapper object
+            # to get the requires/provides data into the cache.
+            if rpm_obj and isinstance(rpm_obj, ExtendedRpmWrapper):
+                self.pool.package_set.file_cache[rpm_path] = rpm_obj
+            else:
+                rpm_obj = self.pool.package_set.file_cache.add(rpm_path)
         else:
             rpm_obj = self.pool.package_set.file_cache.add(rpm_path)
         self.pool.package_set.rpms_by_arch.setdefault(rpm_obj.arch, []).append(rpm_obj)
@@ -90,7 +108,7 @@ class PackageSetBase(kobo.log.LoggingBase):
     ):
         super(PackageSetBase, self).__init__(logger=logger)
         self.name = name
-        self.file_cache = kobo.pkgset.FileCache(kobo.pkgset.SimpleRpmWrapper)
+        self.file_cache = kobo.pkgset.FileCache(ExtendedRpmWrapper)
         self.old_file_cache = None
         self.sigkey_ordering = tuple(sigkey_ordering or [None])
         self.arches = arches

tests/test_gather_phase.py

@@ -24,12 +24,14 @@ class MockPackageSet(dict):
 
 
 class MockPkg(object):
-    def __init__(self, path, is_system_release=False):
+    def __init__(self, path, is_system_release=False, **kwargs):
         self.path = path
         self.is_system_release = is_system_release
         filename = os.path.basename(path)
         self.nvr, self.arch, _ = filename.rsplit(".", 2)
         self.name, self.version, self.release = self.nvr.rsplit("-", 2)
+        for k, v in kwargs.items():
+            setattr(self, k, v)
 
     def __repr__(self):
         return self.nvr
@@ -1080,6 +1082,11 @@ class TestGatherPackages(helpers.PungiTestCase):
             if s == "comps"
             else (None, None, None)
         )
+        get_gather_method.return_value.return_value.return_value = {
+            "rpm": [],
+            "srpm": [],
+            "debuginfo": [],
+        }
         compose = helpers.DummyCompose(self.topdir, {"gather_method": "hybrid"})
         variant = compose.variants["Server"]
         pkg_set = mock.Mock()
@@ -1094,6 +1101,297 @@ class TestGatherPackages(helpers.PungiTestCase):
         self.assertEqual(method_kwargs["groups"], groups)
 
 
+class TestReuseOldGatherPackages(helpers.PungiTestCase):
+    @mock.patch("pungi.phases.gather.load_old_gather_result")
+    def test_reuse_no_old_gather_result(self, load_old_gather_result):
+        load_old_gather_result.return_value = None
+
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], []
+        )
+        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_no_old_compose_config(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        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})
+        load_old_compose_config.return_value = None
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], []
+        )
+        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(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        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["gather_method"] = "nodeps"
+        load_old_compose_config.return_value = compose_conf_copy
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], []
+        )
+        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(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        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["gather_method"] = "nodeps"
+        load_old_compose_config.return_value = compose_conf_copy
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], []
+        )
+        self.assertEqual(result, None)
+
+    def _prepare_package_sets(
+        self, load_old_gather_result, extra_global_pkgs=None, **kwargs
+    ):
+        old_rpm = "/build/bash-1.0.0-1.x86_64.rpm"
+        new_rpm = "/build/bash-1.0.0-2.x86_64.rpm"
+
+        pkg_old = MockPkg(old_rpm, sourcerpm="bash", requires=[], provides=[])
+        pkg_new = MockPkg(new_rpm, sourcerpm="bash", **kwargs)
+        pkgset = MockPackageSet(pkg_new)
+        pkgset.old_file_cache = {pkg_old.path: pkg_old}
+        pkgset.file_cache = {pkg_new.path: pkg_new}
+        package_sets = [{"global": pkgset, "x86_64": pkgset}]
+
+        load_old_gather_result.return_value = {
+            "rpm": [{"path": old_rpm}],
+            "srpm": [],
+            "debuginfo": [],
+        }
+        return package_sets
+
+    @mock.patch("pungi.phases.gather.load_old_gather_result")
+    @mock.patch("pungi.phases.gather.load_old_compose_config")
+    def test_reuse(self, load_old_compose_config, load_old_gather_result):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        self.assertEqual(
+            result,
+            {
+                "debuginfo": [],
+                "rpm": [{"path": "/build/bash-1.0.0-2.x86_64.rpm"}],
+                "srpm": [],
+            },
+        )
+
+    @mock.patch("pungi.phases.gather.load_old_gather_result")
+    @mock.patch("pungi.phases.gather.load_old_compose_config")
+    def test_reuse_update_gather_lookaside_repos(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = copy.deepcopy(compose.conf)
+
+        gather._update_config(compose, "Server", "x86_64", compose.topdir)
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        self.assertEqual(
+            result,
+            {
+                "debuginfo": [],
+                "rpm": [{"path": "/build/bash-1.0.0-2.x86_64.rpm"}],
+                "srpm": [],
+            },
+        )
+
+    @mock.patch("pungi.phases.gather.load_old_gather_result")
+    @mock.patch("pungi.phases.gather.load_old_compose_config")
+    def test_reuse_update_gather_lookaside_repos_different_initial_repos(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        lookasides = compose.conf["gather_lookaside_repos"]
+        lookasides.append(("^Server$", {"x86_64": "http://localhost/real.repo"}))
+        load_old_compose_config.return_value = copy.deepcopy(compose.conf)
+
+        gather._update_config(compose, "Server", "x86_64", compose.topdir)
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_no_old_file_cache(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        package_sets[0]["global"].old_file_cache = {
+            "/build/foo-1-1.x86_64.rpm": MockPkg("foo-1-1.x86_64.rpm", sourcerpm="foo")
+        }
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_no_old_file_cache(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        package_sets[0]["global"].old_file_cache = {
+            "/build/foo-1-1.x86_64.rpm": MockPkg("foo-1-1.x86_64.rpm", sourcerpm="foo")
+        }
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_two_rpms_from_same_source(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        bash_pkg = MockPkg(
+            "bash-1-2.x86_64.rpm", sourcerpm="bash", requires=[], provides=[]
+        )
+        pkg_set = package_sets[0]["global"]
+        pkg_set.old_file_cache["/build/bash-1-2.x86_64.rpm"] = bash_pkg
+        pkg_set.file_cache["/build/bash-1-2.x86_64.rpm"] = bash_pkg
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_rpm_added_removed(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=[]
+        )
+        foo_pkg = MockPkg(
+            "foo-1-1.x86_64.rpm", sourcerpm="foo", requires=[], provides=[]
+        )
+        file_pkg = MockPkg(
+            "file-1-1.x86_64.rpm", sourcerpm="file", requires=[], provides=[]
+        )
+        pkg_set = package_sets[0]["global"]
+        pkg_set.old_file_cache["/build/file-1-1.x86_64.rpm"] = file_pkg
+        pkg_set.file_cache["/build/foo-1-1.x86_64.rpm"] = foo_pkg
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_different_packages(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=["foo"]
+        )
+        package_sets[0]["global"].old_file_cache = None
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_requires_changed(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=["foo"], provides=[]
+        )
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        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_provides_changed(
+        self, load_old_compose_config, load_old_gather_result
+    ):
+        package_sets = self._prepare_package_sets(
+            load_old_gather_result, requires=[], provides=["foo"]
+        )
+        compose = helpers.DummyCompose(self.topdir, {"gather_allow_reuse": True})
+        load_old_compose_config.return_value = compose.conf
+
+        result = gather.reuse_old_gather_packages(
+            compose, "x86_64", compose.variants["Server"], package_sets
+        )
+        self.assertEqual(result, None)
+
+
 class TestWritePrepopulate(helpers.PungiTestCase):
     def test_without_config(self):
         compose = helpers.DummyCompose(self.topdir, {})