inst-tester: Correctly handle unresolved essential choices

Britney has a special case for essential packages to ensure that any
package that with essential packages are not installable.  This check
did not account for a case, where a package is not co-installable with
two or more pseudo-essential package part of the same OR dependency.
A contrived example based on real world data:

  Package: foo
  # Conflict with all providers of "awk"
  Conflicts: mawk | gawk | original-awk

This alone is actually not sufficient to trigger the bug, as
_get_min_pseudo_ess_set is in theory some times smart enough to pick
an "obvious" solution between the pseudo-essential option.
  When it does, one of the above ends up in the (de-facto) essential set
and then the installability tester correctly rejects "foo".

Though, even with the fix above, the handling for this is probably not
correct if the essential set is not (fully co-)installable.  However,
that basically only happens if we are bootstrapping an architecture
(or testing is royally broken, in which case this is the least of our
worries).

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/installability/tester.py

@@ -300,9 +300,9 @@ class InstallabilityTester(object):
             if t.architecture not in self._cache_ess:
                 # The minimal essential set cache is not present -
                 # compute it now.
-                (start, ess_never) = self._get_min_pseudo_ess_set(t.architecture)
+                (start, ess_never, ess_choices) = self._get_min_pseudo_ess_set(t.architecture)
             else:
-                (start, ess_never) = self._cache_ess[t.architecture]
+                (start, ess_never, ess_choices) = self._cache_ess[t.architecture]
 
             if t in ess_never:
                 # t conflicts with something in the essential set or the essential
@@ -313,6 +313,7 @@ class InstallabilityTester(object):
                 return False
             musts.update(start)
             never.update(ess_never)
+            choices.update(ess_choices)
 
         # curry check_loop
         check_loop = partial(self._check_loop, universe, testing,
@@ -630,7 +631,7 @@ class InstallabilityTester(object):
 
             for x in start:
                 ess_never.update(universe[x][1])
-            self._cache_ess[arch] = (frozenset(start), frozenset(ess_never))
+            self._cache_ess[arch] = (frozenset(start), frozenset(ess_never), frozenset(ess_choices))
 
         return self._cache_ess[arch]
 

tests/test_inst_tester.py

@@ -49,6 +49,31 @@ class TestInstTester(unittest.TestCase):
         universe.add_testing_binary(pkg_awk)
         assert universe.is_installable(pkg_lintian)
 
+    def test_basic_essential_conflict(self):
+        builder = new_pkg_universe_builder()
+        pseudo_ess1 = builder.new_package('pseudo-essential1')
+        pseudo_ess2 = builder.new_package('pseudo-essential2')
+        essential_simple = builder.new_package('essential-simple').is_essential()
+        essential_with_deps = builder.new_package('essential-with-deps').is_essential().\
+            depends_on_any_of(pseudo_ess1, pseudo_ess2)
+        conflict1 = builder.new_package('conflict1').conflicts_with(essential_simple)
+        conflict2 = builder.new_package('conflict2').conflicts_with(pseudo_ess1, pseudo_ess2)
+        conflict_installable1 = builder.new_package('conflict-inst1').conflicts_with(pseudo_ess1)
+        conflict_installable2 = builder.new_package('conflict-inst2').conflicts_with(pseudo_ess2)
+
+        universe = builder.build()
+
+        assert universe.is_installable(essential_simple.pkg_id)
+        assert universe.is_installable(essential_with_deps.pkg_id)
+        assert universe.is_installable(conflict_installable1.pkg_id)
+        assert universe.is_installable(conflict_installable2.pkg_id)
+        assert not universe.is_installable(conflict1.pkg_id)
+        assert not universe.is_installable(conflict2.pkg_id)
+
+        for line in universe.stats.stats():
+            print(line)
+        assert universe.stats.conflicts_essential == 1
+
     def test_basic_simple_choice(self):
         builder = new_pkg_universe_builder()
         root_pkg = builder.new_package('root')