Enforce license requirement on new files

We have long had a presubmit that warns when a file lacks a suitable
license, but because this is a warning it can be ignored by the author
and is not visible to reviewers. Every failure that is not prevented
will occur, and indeed newly added files from this year (seen in
crrev.com/c/3706888 and crrev.com/c/3437860, and possibly others) are
missing valid licenses.

This change turns the warning into an error on newly added files.

Bug: 1361031
Change-Id: Ie17bd6e591918affd9d865a3bf2be6c2bf62d1ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3887721
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

presubmit_canned_checks.py

@@ -668,6 +668,7 @@ def CheckLicense(input_api, output_api, license_re=None, project_name=None,
 
   license_re = input_api.re.compile(license_re, input_api.re.MULTILINE)
   bad_files = []
+  bad_new_files = False
   for f in input_api.AffectedSourceFiles(source_file_filter):
     contents = input_api.ReadFile(f, 'r')
     if accept_empty_files and not contents:
@@ -675,8 +676,21 @@ def CheckLicense(input_api, output_api, license_re=None, project_name=None,
     # Search for key_line first to avoid fruitless and expensive regex searches.
     if (key_line and not key_line in contents):
       bad_files.append(f.LocalPath())
+      # f.OldContents() is expensive so don't call unless necessary.
+      if not f.OldContents():
+        bad_new_files = True
     elif not license_re.search(contents):
       bad_files.append(f.LocalPath())
+      # f.OldContents() is expensive so don't call unless necessary.
+      if not f.OldContents():
+        bad_new_files = True
+  if bad_new_files:
+    return [
+        output_api.PresubmitError(
+            'License must match:\n%s\n' % license_re.pattern +
+            'Found a bad license header in these files, some of which are new:',
+            items=bad_files)
+    ]
   if bad_files:
     return [output_api.PresubmitPromptWarning(
         'License must match:\n%s\n' % license_re.pattern +