Bug 1117860 - part 1 - factor sources->unified file computation out of _add_unified_build_rules; r=mshal

_add_unified_build_rules does quite a lot of work besides adding
makefile rules and variables.  The divying up of source files into
unified files is one part of that, so move it out into its own function.
When we eventually move that computation out of recursivemake.py, this
refactoring will make it easier to verify that's what we've done.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -610,6 +610,28 @@ class RecursiveMakeBackend(CommonBackend):
                 mozpath.join(self.environment.topobjdir, 'root-deps.mk')) as root_deps:
             root_deps_mk.dump(root_deps, removal_guard=False)
 
+    def _group_unified_files(self, files, unified_prefix, unified_suffix,
+                             files_per_unified_file):
+        "Return an iterator of (unified_filename, source_filenames) tuples."
+        # Our last returned list of source filenames may be short, and we
+        # don't want the fill value inserted by izip_longest to be an
+        # issue.  So we do a little dance to filter it out ourselves.
+        dummy_fill_value = ("dummy",)
+        def filter_out_dummy(iterable):
+            return itertools.ifilter(lambda x: x != dummy_fill_value,
+                                     iterable)
+
+        # From the itertools documentation, slightly modified:
+        def grouper(n, iterable):
+            "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
+            args = [iter(iterable)] * n
+            return itertools.izip_longest(fillvalue=dummy_fill_value, *args)
+
+        for i, unified_group in enumerate(grouper(files_per_unified_file,
+                                                  files)):
+            just_the_filenames = list(filter_out_dummy(unified_group))
+            yield '%s%d.%s' % (unified_prefix, i, unified_suffix), just_the_filenames
+
     def _add_unified_build_rules(self, makefile, files, output_directory,
                                  unified_prefix='Unified',
                                  unified_suffix='cpp',
@@ -630,32 +652,15 @@ class RecursiveMakeBackend(CommonBackend):
             "# rebuild time, and compiler memory usage." % files_per_unified_file
         makefile.add_statement(explanation)
 
-        def unified_files():
-            "Return an iterator of (unified_filename, source_filenames) tuples."
-            # Our last returned list of source filenames may be short, and we
-            # don't want the fill value inserted by izip_longest to be an
-            # issue.  So we do a little dance to filter it out ourselves.
-            dummy_fill_value = ("dummy",)
-            def filter_out_dummy(iterable):
-                return itertools.ifilter(lambda x: x != dummy_fill_value,
-                                         iterable)
-
-            # From the itertools documentation, slightly modified:
-            def grouper(n, iterable):
-                "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
-                args = [iter(iterable)] * n
-                return itertools.izip_longest(fillvalue=dummy_fill_value, *args)
-
-            for i, unified_group in enumerate(grouper(files_per_unified_file,
-                                                      files)):
-                just_the_filenames = list(filter_out_dummy(unified_group))
-                yield '%s%d.%s' % (unified_prefix, i, unified_suffix), just_the_filenames
-
-        all_sources = ' '.join(source for source, _ in unified_files())
+        unified_source_mapping = list(self._group_unified_files(files,
+                                                                unified_prefix=unified_prefix,
+                                                                unified_suffix=unified_suffix,
+                                                                files_per_unified_file=files_per_unified_file))
+        all_sources = ' '.join(source for source, _ in unified_source_mapping)
         makefile.add_statement('%s := %s' % (unified_files_makefile_variable,
                                                all_sources))
 
-        for unified_file, source_filenames in unified_files():
+        for unified_file, source_filenames in unified_source_mapping:
             if extra_dependencies:
                 rule = makefile.create_rule([unified_file])
                 rule.add_dependencies(extra_dependencies)