Bug 1360764 - Record warnings seen during the current operation; r=froydnj

Currently, the build monitor has a single compiler warnings database
that unions warnings from previous runs with warnings from the current
invocation. I want to introduce functionality that treats warnings
seen during the current invocation differently from "all warnings."
To facilitate this, this commit introduces a 2nd, non-persisted
warnings database to record warnings just for the current invocation.

MozReview-Commit-ID: FIY0GiarDmr

--HG--
extra : rebase_source : b2002e1c248ea65b2c0ee45a78b1e74d61a26f3c

python/mozbuild/mozbuild/compilation/warnings.py

@@ -58,6 +58,12 @@ class CompilerWarning(dict):
         self['message'] = None
         self['flag'] = None
 
+    def copy(self):
+        """Returns a copy of this compiler warning."""
+        w = CompilerWarning()
+        w.update(self)
+        return w
+
     # Since we inherit from dict, functools.total_ordering gets confused.
     # Thus, we define a key function, a generic comparison, and then
     # implement all the rich operators with those; approach is from:

python/mozbuild/mozbuild/controller/building.py

@@ -172,6 +172,10 @@ class BuildMonitor(MozbuildObject):
             except ValueError:
                 os.remove(warnings_path)
 
+        # Contains warnings unique to this invocation. Not populated with old
+        # warnings.
+        self.instance_warnings = WarningsDatabase()
+
         def on_warning(warning):
             filename = warning['filename']
 
@@ -180,6 +184,8 @@ class BuildMonitor(MozbuildObject):
                                 filename)
 
             self.warnings_database.insert(warning)
+            # Make a copy so mutations don't impact other database.
+            self.instance_warnings.insert(warning.copy())
 
         self._warnings_collector = WarningsCollector(on_warning,
                                                      objdir=self.topobjdir)