🔧 Allow providing README for license.py in 3p subdirectories

Additionally to scaning the first level of third_party for
README.chromium files, these directories can now contain an
additional_readme_paths.json file that lists subdirectories
that will contain the actual third_party dependencies to
take into account for credits generation.

Bug: 794210
Change-Id: If6eef9fcd615bd96b9a260f9ae4d0c0c4fd6405a
Reviewed-on: https://chromium-review.googlesource.com/822612
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533270}

tools/licenses.py

@@ -17,6 +17,7 @@ Commands:
 
 import argparse
 import cgi
+import json
 import os
 import shutil
 import re
@@ -104,6 +105,11 @@ PRUNE_DIRS = (VCS_METADATA_DIRS +
               ('out', 'Debug', 'Release',  # build files
                'layout_tests'))            # lots of subdirs
 
+# A third_party directory can define this file, containing a list of
+# subdirectories to process instead of itself. Intended for directories that
+# contain multiple others as transitive dependencies.
+ADDITIONAL_PATHS_FILENAME = 'additional_readme_paths.json'
+
 ADDITIONAL_PATHS = (
     os.path.join('chrome', 'common', 'extensions', 'docs', 'examples'),
     os.path.join('chrome', 'test', 'chromeos', 'autotest'),
@@ -358,7 +364,7 @@ def ParseDir(path, root, require_license_file=True, optional_keys=None):
         readme_path = os.path.join(root, path, 'README.chromium')
         if not os.path.exists(readme_path):
             raise LicenseError("missing README.chromium or licenses.py "
-                               "SPECIAL_CASES entry in %s" % path)
+                               "SPECIAL_CASES entry in %s\n" % path)
 
         for line in open(readme_path):
             line = line.strip()
@@ -395,7 +401,7 @@ def ParseDir(path, root, require_license_file=True, optional_keys=None):
         metadata["License File"] = license_path
 
     if errors:
-        raise LicenseError(";\n".join(errors))
+        raise LicenseError("Errors in %s:\n %s\n" % (path, ";\n ".join(errors)))
     return metadata
 
 
@@ -438,7 +444,14 @@ def FindThirdPartyDirs(prune_paths, root):
             # Add all subdirectories that are not marked for skipping.
             for dir in dirs:
                 dirpath = os.path.join(path, dir)
-                if dirpath not in prune_paths:
+                additional_paths_file = os.path.join(
+                        dirpath, ADDITIONAL_PATHS_FILENAME)
+                if os.path.exists(additional_paths_file):
+                    with open(additional_paths_file) as paths_file:
+                        extra_paths = json.load(paths_file)
+                        third_party_dirs.update([
+                                os.path.join(dirpath, p) for p in extra_paths])
+                elif dirpath not in prune_paths:
                     third_party_dirs.add(dirpath)
 
             # Don't recurse into any subdirs from here.