Always start from WPT_BASE_MANIFEST.json when updating MANIFEST.json

This will avoid full manifest rebuilds for Chromium developers when
the manifest version changes.

Bug: 876717
Change-Id: I6d552ad03936fb07cf559d0f072508e5ea2184a4
Reviewed-on: https://chromium-review.googlesource.com/1185009
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585271}

third_party/blink/tools/blinkpy/common/host_mock.py

@@ -122,4 +122,4 @@ class MockHost(MockSystemHost):
         filesystem.maybe_make_directory(filesystem.join(external_dir, 'wpt'))
 
         manifest_base_path = filesystem.join(external_dir, 'WPT_BASE_MANIFEST.json')
-        filesystem.files[manifest_base_path] = '{}'
+        filesystem.files[manifest_base_path] = '{"manifest": "base"}'

third_party/blink/tools/blinkpy/w3c/wpt_manifest.py

@@ -172,7 +172,7 @@ class WPTManifest(object):
 
     @staticmethod
     def ensure_manifest(host):
-        """Generates the MANIFEST.json file if it does not exist."""
+        """Updates the MANIFEST.json file, or generates if it does not exist."""
         finder = PathFinder(host.filesystem)
         manifest_path = finder.path_from_layout_tests('external', 'wpt', 'MANIFEST.json')
         base_manifest_path = finder.path_from_layout_tests('external', 'WPT_BASE_MANIFEST.json')
@@ -181,11 +181,15 @@ class WPTManifest(object):
             _log.error('Manifest base not found at "%s".', base_manifest_path)
             host.filesystem.write_text_file(base_manifest_path, '{}')
 
-        if not host.filesystem.exists(manifest_path):
-            _log.debug('Manifest not found, copying from base "%s".', base_manifest_path)
-            host.filesystem.copyfile(base_manifest_path, manifest_path)
+        # Unconditionally replace MANIFEST.json with WPT_BASE_MANIFEST.json even
+        # if the former exists, to avoid regenerating the manifest from scratch
+        # when the manifest version changes. Remove the destination first as
+        # copyfile will fail if the two files are hardlinked or symlinked.
+        if host.filesystem.exists(manifest_path):
+            host.filesystem.remove(manifest_path)
+        host.filesystem.copyfile(base_manifest_path, manifest_path)
 
-        wpt_path = manifest_path = finder.path_from_layout_tests('external', 'wpt')
+        wpt_path = finder.path_from_layout_tests('external', 'wpt')
         WPTManifest.generate_manifest(host, wpt_path)
 
         _log.debug('Manifest generation completed.')

third_party/blink/tools/blinkpy/w3c/wpt_manifest_unittest.py

@@ -19,6 +19,7 @@ class WPTManifestUnitTest(unittest.TestCase):
         self.assertFalse(host.filesystem.exists(manifest_path))
         WPTManifest.ensure_manifest(host)
         self.assertTrue(host.filesystem.exists(manifest_path))
+        self.assertEqual(host.filesystem.written_files, {manifest_path: '{"manifest": "base"}'})
 
         webkit_base = '/mock-checkout/third_party/WebKit'
         self.assertEqual(
@@ -39,11 +40,12 @@ class WPTManifestUnitTest(unittest.TestCase):
         host = MockHost()
         manifest_path = '/mock-checkout/third_party/WebKit/LayoutTests/external/wpt/MANIFEST.json'
 
-        host.filesystem.write_text_file(manifest_path, '{}')
-        self.assertTrue(host.filesystem.exists(manifest_path))
+        host.filesystem.write_text_file(manifest_path, '{"manifest": "NOT base"}')
 
+        self.assertTrue(host.filesystem.exists(manifest_path))
         WPTManifest.ensure_manifest(host)
         self.assertTrue(host.filesystem.exists(manifest_path))
+        self.assertEqual(host.filesystem.written_files, {manifest_path: '{"manifest": "base"}'})
 
         webkit_base = '/mock-checkout/third_party/WebKit'
         self.assertEqual(