Fix error code propagation in 'mb gen'.

If the underlying call to 'gn gen' (or gyp_chromium) failed,
mb was swallowing the error.

R=brettw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1078913003

Cr-Commit-Position: refs/heads/master@{#326892}

tools/mb/mb.py

@@ -122,12 +122,11 @@ class MetaBuildWrapper(object):
   def CmdGen(self):
     vals = self.GetConfig()
     if vals['type'] == 'gn':
-      self.RunGNGen(self.args.path[0], vals)
-    elif vals['type'] == 'gyp':
-      self.RunGYPGen(self.args.path[0], vals)
-    else:
-      raise MBErr('Unknown meta-build type "%s"' % vals['type'])
-    return 0
+      return self.RunGNGen(self.args.path[0], vals)
+    if vals['type'] == 'gyp':
+      return self.RunGYPGen(self.args.path[0], vals)
+
+    raise MBErr('Unknown meta-build type "%s"' % vals['type'])
 
   def CmdLookup(self):
     vals = self.GetConfig()

tools/mb/mb_unittest.py

@@ -100,12 +100,12 @@ TEST_CONFIG = """\
 
 
 class UnitTest(unittest.TestCase):
-  def fake_mbw(self, files):
+  def fake_mbw(self, files=None):
     mbw = FakeMBW()
+    mbw.files.setdefault(mbw.default_config, TEST_CONFIG)
     if files:
       for path, contents in files.items():
         mbw.files[path] = contents
-    mbw.files.setdefault(mbw.default_config, TEST_CONFIG)
     return mbw
 
   def check(self, args, mbw=None, files=None, out=None, err=None, ret=None):
@@ -180,6 +180,12 @@ class UnitTest(unittest.TestCase):
     self.check(['gen', '-c', 'gn_debug', '//out/Default'], ret=0)
     self.check(['gen', '-c', 'gyp_rel_bot', '//out/Release'], ret=0)
 
+  def test_gen_fails(self):
+    mbw = self.fake_mbw()
+    mbw.Call = lambda cmd: (1, '', '')
+    self.check(['gen', '-c', 'gn_debug', '//out/Default'], mbw=mbw, ret=1)
+    self.check(['gen', '-c', 'gyp_rel_bot', '//out/Release'], mbw=mbw, ret=1)
+
   def test_goma_dir_expansion(self):
     self.check(['lookup', '-c', 'gyp_rel_bot', '-g', '/foo'], ret=0,
                out=("python build/gyp_chromium -G 'output_dir=<path>' "