Bug 857557 - Avoid autoconf.mk being modified for new other-platform-specific variables. r=gps

python/mozbuild/mozbuild/backend/configenvironment.py

@@ -89,7 +89,10 @@ class ConfigEnvironment(object):
     and another additional subst variable from all the other substs:
       - ALLSUBSTS contains the substs in the form NAME = VALUE, in sorted
         order, for use in autoconf.mk. It includes ACDEFINES, but doesn't
-        include ALLDEFINES.
+        include ALLDEFINES. Only substs with a VALUE are included, such that
+        the resulting file doesn't change when new empty substs are added.
+        This results in less invalidation of build dependencies in the case
+        of autoconf.mk..
 
     ConfigEnvironment expects a "top_srcdir" subst to be set with the top
     source directory, in msys format on windows. It is used to derive a
@@ -109,7 +112,7 @@ class ConfigEnvironment(object):
         self.substs['ACDEFINES'] = ' '.join(['-D%s=%s' % (name,
             shell_escape(self.defines[name])) for name in global_defines])
         self.substs['ALLSUBSTS'] = '\n'.join(sorted(['%s = %s' % (name,
-            self.substs[name]) for name in self.substs]))
+            self.substs[name]) for name in self.substs if self.substs[name]]))
         self.substs['ALLDEFINES'] = '\n'.join(sorted(['#define %s %s' % (name,
             self.defines[name]) for name in global_defines]))
 

python/mozbuild/mozbuild/test/backend/test_configenvironment.py

@@ -30,7 +30,8 @@ class TestEnvironment(unittest.TestCase):
                               ('abc', 'def'), ('extra', 'foobar') ],
                   non_global_defines = ['extra', 'ignore'],
                   substs = [ ('FOO', 'bar'), ('ABC', 'def'),
-                             ('bar', 'baz qux'), ('zzz', '"abc def"') ])
+                             ('bar', 'baz qux'), ('zzz', '"abc def"'),
+                             ('qux', '') ])
         # non_global_defines should be filtered out in ACDEFINES and
         # ALLDEFINES.
         # Original order of the defines need to be respected in ACDEFINES