excuse, excusefinder: Allow excuses to invalidate each other

Sometimes (for the LinuxPolicy), excuses don't have enough information
when they are processed. They need to defer some of their work until
another excuse has been evaluated and then perhaps get invalidated.

Introduce the concept of "external invalidation" to deal with this. A
policy can keep references to other excuses around, and then invalidate
them later (when processing another one). The excuse finder needs to
know this happened to remove the excuse from the list of "actionable
excuses".

For example. The Linux policy invalidates linux-foo if linux-meta-foo is
invalid. If linux-foo is encountered first, we will not yet know if
linux-meta-foo is going to be invalid. We effectively defer the
evaluation until after linux-meta-foo has been dealt with, and then we
know whether we need to invalidate linux-foo or not.

britney2/excuse.py

@@ -156,6 +156,7 @@ class Excuse(object):
         self.hints = []
         self.forced = False
         self._policy_verdict = PolicyVerdict.REJECTED_PERMANENTLY
+        self.invalidated_externally = False
 
         self.all_deps = []
         self.break_deps = []
@@ -219,6 +220,29 @@ class Excuse(object):
             value = PolicyVerdict.PASS_HINTED
         self._policy_verdict = value
 
+    @property
+    def tentative_policy_verdict(self):
+        """If we've not finished running all of the policies, we can find out
+        what all of the policies that have run so far said."""
+        all_verdicts = {
+            info["verdict"]
+            if isinstance(info["verdict"], PolicyVerdict)
+            else PolicyVerdict[info["verdict"]]
+            for info in self.policy_info.values()
+        }
+
+        return max(all_verdicts)
+
+    def invalidate_externally(self, verdict):
+        """A policy might want to invalidate an excuse other than the one it
+        is currently looking at, e.g. if it later learns of a fact that it
+        didn't know when it was processing the first excuse.
+
+        We need to know, so that we can remove this excuse from the list of
+        actionable excuses."""
+        self.policy_verdict = verdict
+        self.invalidated_externally = True
+
     def set_vers(self, tver, uver):
         """Set the versions of the item from target and source suite"""
         if tver and uver:

britney2/excusefinder.py

@@ -520,6 +520,9 @@ class ExcuseFinder(object):
                         if should_upgrade_srcarch(item):
                             actionable_items_add(item)
 
+        # If anything was discarded by a different excuse, it's not actionable any more
+        actionable_items = {item for item in actionable_items if not self.excuses[item.name].invalidated_externally}
+
         # process the `remove' hints, if the given package is not yet in actionable_items
         for hint in self.hints['remove']:
             src = hint.package