Bug 1233282 - Add generated FinalTargetFiles as optional_exists in install manifests. r=gps

Otherwise there's a possibility of race condition in some cases.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -1304,10 +1304,12 @@ INSTALL_TARGETS += %(prefix)s
                           if path else target).replace('/', '_')
             have_objdir_files = False
             for f in files:
+                dest = mozpath.join(reltarget, path,
+                                    mozpath.basename(f.full_path))
                 if not isinstance(f, ObjDirPath):
-                    dest = mozpath.join(reltarget, path, mozpath.basename(f))
                     install_manifest.add_symlink(f.full_path, dest)
                 else:
+                    install_manifest.add_optional_exists(dest)
                     backend_file.write('%s_FILES += %s\n' % (
                         target_var, self._pretty_path(f, backend_file)))
                     have_objdir_files = True

python/mozbuild/mozbuild/test/backend/test_recursivemake.py

@@ -401,11 +401,15 @@ class TestRecursiveMakeBackend(BackendTester):
         # EXPORTS files should appear in the dist_include install manifest.
         m = InstallManifest(path=mozpath.join(env.topobjdir,
             '_build_manifests', 'install', 'dist_include'))
-        self.assertEqual(len(m), 4)
+        self.assertEqual(len(m), 8)
         self.assertIn('foo.h', m)
         self.assertIn('mozilla/mozilla1.h', m)
         self.assertIn('mozilla/dom/dom1.h', m)
         self.assertIn('gfx/gfx.h', m)
+        self.assertIn('bar.h', m)
+        self.assertIn('mozilla/mozilla2.h', m)
+        self.assertIn('mozilla/dom/dom2.h', m)
+        self.assertIn('mozilla/dom/dom3.h', m)
         # EXPORTS files that are also GENERATED_FILES should be handled as
         # INSTALL_TARGETS.
         backend_path = mozpath.join(env.topobjdir, 'backend.mk')