Correct fallback and mtime in distributed ThinLTO scripts

This change makes two improvements to the distributed ThinLTO scripts:
 1. When no code generation is necessary, invokes the local linker
    correctly even when --gomacc or --jobs options are specified.
 2. Change create_file to ensure_file to only create files if they
    do not already exist, reducing unnecessary rebuilding.

R=gbiv,hans,rnk,thakis
BUG=877722

Change-Id: I29ffa028202c31f77df95d306432f726e3f7bda9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2039996
Auto-Submit: Bob Haarman <inglorion@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738684}

tools/clang/scripts/goma_link.py

@@ -55,14 +55,6 @@ def autoninja():
     return name
 
 
-def create_file(path):
-  """
-  Creates an empty file at path, creating parent directories if needed.
-  """
-  ensure_dir(os.path.dirname(path))
-  open(path, 'wb').close()
-
-
 def ensure_dir(path):
   """
   Creates path as a directory if it does not already exist.
@@ -76,6 +68,20 @@ def ensure_dir(path):
       raise
 
 
+def ensure_file(path):
+  """
+  Creates an empty file at path if it does not already exist.
+  Also creates directories as needed.
+  """
+  ensure_dir(os.path.dirname(path))
+  try:
+    fd = os.open(path, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0o644)
+    os.close(fd)
+  except OSError as e:
+    if e.errno != errno.EEXIST:
+      raise
+
+
 def exe_suffix():
   if os.name == 'nt':
     return '.exe'
@@ -395,7 +401,7 @@ class GomaLinkBase(object):
             index = common_index
           else:
             index = obj_dir + '/' + param + '.thinlto.bc'
-            create_file(index)
+            ensure_file(index)
           codegen.append((os.path.normpath(native), param, index))
         else:
           final_params.append(param)
@@ -549,10 +555,10 @@ class GomaLinkBase(object):
     # If we determined that no distributed code generation need be done, just
     # invoke the original command.
     if params is None:
-      return subprocess.call(argv[1:])
+      return subprocess.call([args.linker] + args.linker_args)
     if use_common_objects:
       objs = [x[0] for x in params.codegen]
-      create_file(common_dir + '/empty.thinlto.bc')
+      ensure_file(common_dir + '/empty.thinlto.bc')
     else:
       objs = self.thin_link(params, gen_dir)
     self.codegen_and_link(params, gen_dir, objs)

tools/clang/scripts/goma_link_tests.py

@@ -107,26 +107,30 @@ class GomaLinkUnitTest(unittest.TestCase):
   Unit tests for goma_link.
   """
 
-  def test_create_file_no_dir(self):
+  def test_ensure_file_no_dir(self):
     with NamedDirectory() as d, WorkingDirectory(d):
       self.assertFalse(os.path.exists('test'))
-      goma_link.create_file('test')
+      goma_link.ensure_file('test')
       self.assertTrue(os.path.exists('test'))
 
-  def test_create_file_existing(self):
+  def test_ensure_file_existing(self):
     with NamedDirectory() as d, WorkingDirectory(d):
       self.assertFalse(os.path.exists('foo/test'))
-      goma_link.create_file('foo/test')
+      goma_link.ensure_file('foo/test')
       self.assertTrue(os.path.exists('foo/test'))
-      goma_link.create_file('foo/test')
+      os.utime('foo/test', (0, 0))
+      statresult = os.stat('foo/test')
+      goma_link.ensure_file('foo/test')
       self.assertTrue(os.path.exists('foo/test'))
+      newstatresult = os.stat('foo/test')
+      self.assertEqual(newstatresult.st_mtime, statresult.st_mtime)
 
-  def test_create_file_error(self):
+  def test_ensure_file_error(self):
     with NamedDirectory() as d, WorkingDirectory(d):
       self.assertFalse(os.path.exists('test'))
-      goma_link.create_file('test')
+      goma_link.ensure_file('test')
       self.assertTrue(os.path.exists('test'))
-      self.assertRaises(IOError, goma_link.create_file, 'test/impossible')
+      self.assertRaises(OSError, goma_link.ensure_file, 'test/impossible')
 
 
 class GomaLinkIntegrationTest(unittest.TestCase):