Bug 1117860 - part 2 - factor out unified file writing from _add_unified_build_rules; r=mshal

Writing the unified files is another thing that will have to be moved
out of recursivemake.py eventually.  And it doesn't belong inline amidst
makefile rules and variables.  Move its logic to a separate function as
well.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -632,6 +632,32 @@ class RecursiveMakeBackend(CommonBackend):
             just_the_filenames = list(filter_out_dummy(unified_group))
             yield '%s%d.%s' % (unified_prefix, i, unified_suffix), just_the_filenames
 
+    def _write_unified_file(self, unified_file, source_filenames,
+                            output_directory, poison_windows_h=False):
+        with self._write_file(mozpath.join(output_directory, unified_file)) as f:
+            f.write('#define MOZ_UNIFIED_BUILD\n')
+            includeTemplate = '#include "%(cppfile)s"'
+            if poison_windows_h:
+                includeTemplate += (
+                    '\n'
+                    '#ifdef _WINDOWS_\n'
+                    '#error "%(cppfile)s included windows.h"\n'
+                    "#endif")
+            includeTemplate += (
+                '\n'
+                '#ifdef PL_ARENA_CONST_ALIGN_MASK\n'
+                '#error "%(cppfile)s uses PL_ARENA_CONST_ALIGN_MASK, '
+                'so it cannot be built in unified mode."\n'
+                '#undef PL_ARENA_CONST_ALIGN_MASK\n'
+                '#endif\n'
+                '#ifdef INITGUID\n'
+                '#error "%(cppfile)s defines INITGUID, '
+                'so it cannot be built in unified mode."\n'
+                '#undef INITGUID\n'
+                '#endif')
+            f.write('\n'.join(includeTemplate % { "cppfile": s } for
+                              s in source_filenames))
+
     def _add_unified_build_rules(self, makefile, files, output_directory,
                                  unified_prefix='Unified',
                                  unified_suffix='cpp',
@@ -669,29 +695,9 @@ class RecursiveMakeBackend(CommonBackend):
             # blown away and we need to regenerate them.  The rule doesn't correctly
             # handle source files being added/removed/renamed.  Therefore, we
             # generate them here also to make sure everything's up-to-date.
-            with self._write_file(mozpath.join(output_directory, unified_file)) as f:
-                f.write('#define MOZ_UNIFIED_BUILD\n')
-                includeTemplate = '#include "%(cppfile)s"'
-                if poison_windows_h:
-                    includeTemplate += (
-                        '\n'
-                        '#ifdef _WINDOWS_\n'
-                        '#error "%(cppfile)s included windows.h"\n'
-                        "#endif")
-                includeTemplate += (
-                    '\n'
-                    '#ifdef PL_ARENA_CONST_ALIGN_MASK\n'
-                    '#error "%(cppfile)s uses PL_ARENA_CONST_ALIGN_MASK, '
-                    'so it cannot be built in unified mode."\n'
-                    '#undef PL_ARENA_CONST_ALIGN_MASK\n'
-                    '#endif\n'
-                    '#ifdef INITGUID\n'
-                    '#error "%(cppfile)s defines INITGUID, '
-                    'so it cannot be built in unified mode."\n'
-                    '#undef INITGUID\n'
-                    '#endif')
-                f.write('\n'.join(includeTemplate % { "cppfile": s } for
-                                  s in source_filenames))
+            self._write_unified_file(unified_file, source_filenames,
+                                     output_directory,
+                                     poison_windows_h=poison_windows_h)
 
         if include_curdir_build_rules:
             makefile.add_statement('\n'