Improve cache invalidation of (pseudo-)essential set

If a new pseudo-essential package was added to testing *and* it is
mutually-exclusive with an existing member, britney would incorrectly
flag it as uninstallable (unless another change triggered a cache
invalidation of the pseudo-essential set).

With this commit, the cache invalidation is now done based on whether
we add something that *might* be in the (effective) pseudo-essential
set.  It is an overapproximation as there are cases where the
invalidation is unnecessary, but at the moment it is not a performance
concern.

Closes: https://bugs.debian.org/944190
Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/installability/tester.py

@@ -17,7 +17,7 @@ from functools import partial
 import logging
 from itertools import chain, filterfalse
 
-from britney2.utils import iter_except
+from britney2.utils import iter_except, add_transitive_dependencies_flatten
 
 
 class InstallabilityTester(object):
@@ -52,12 +52,16 @@ class InstallabilityTester(object):
         # are essential and packages that will always follow.
         #
         # It may not be a complete essential set, since alternatives
-        # are not always resolved.  Noticably cases like "awk" may be
+        # are not always resolved.  Noticeably cases like "awk" may be
         # left out (since it could be either gawk, mawk or
         # original-awk) unless something in this sets depends strictly
         # on one of them
         self._cache_ess = {}
 
+        essential_w_transitive_deps = set(universe.essential_packages)
+        add_transitive_dependencies_flatten(universe, essential_w_transitive_deps)
+        self._cache_essential_transitive_dependencies = essential_w_transitive_deps
+
     def compute_installability(self):
         """Computes the installability of all the packages in the suite
 
@@ -137,8 +141,8 @@ class InstallabilityTester(object):
                 # Re-add broken packages as some of them may now be installable
                 self._suite_contents |= self._cache_broken
                 self._cache_broken = set()
-            if pkg_id in self._universe.essential_packages and pkg_id.architecture in self._cache_ess:
+            if pkg_id in self._cache_essential_transitive_dependencies and pkg_id.architecture in self._cache_ess:
-                # Adds new essential => "pseudo-essential" set needs to be
+                # Adds new possibly pseudo-essential => "pseudo-essential" set needs to be
                 # recomputed
                 del self._cache_ess[pkg_id.architecture]
 

britney2/utils.py

@@ -30,7 +30,7 @@ import time
 from collections import defaultdict
 from datetime import datetime
 from functools import partial
-from itertools import filterfalse
+from itertools import filterfalse, chain
 
 import yaml
 
@@ -127,6 +127,25 @@ def compute_reverse_tree(pkg_universe, affected):
     return None
 
 
+def add_transitive_dependencies_flatten(pkg_universe, initial_set):
+    """Find and include all transitive dependencies
+
+    This method updates the initial_set parameter to include all transitive
+    dependencies. The first argument is an instance of the BinaryPackageUniverse
+    and the second argument are a set of BinaryPackageId.
+
+    The set of initial packages will be updated in place and must
+    therefore be mutable.
+    """
+    remain = list(initial_set)
+    while remain:
+        pkg_id = remain.pop()
+        new_pkg_ids = [x for x in chain.from_iterable(pkg_universe.dependencies_of(pkg_id)) if x not in initial_set]
+        initial_set.update(new_pkg_ids)
+        remain.extend(new_pkg_ids)
+    return None
+
+
 def write_nuninst(filename, nuninst):
     """Write the non-installable report