Add helpful error messages to check_gn_headers.py

build/check_gn_headers.py

Index: build/check_gn_headers.py
diff --git a/build/check_gn_headers.py b/build/check_gn_headers.py
index 0850196e31b1594e8923e263ae71468794bcba0b..e6f2b1e864ead291c34345ba74692b845c476c47 100755
--- a/build/check_gn_headers.py
+++ b/build/check_gn_headers.py
@@ -13,8 +13,10 @@ import argparse
 import json
 import os
 import re
+import shutil
 import subprocess
 import sys
+import tempfile
 from multiprocessing import Process, Queue
 
 
@@ -66,12 +68,22 @@ def ParseNinjaDepsOutput(ninja_out):
 
 def GetHeadersFromGN(out_dir, q):
   """Return all the header files from GN"""
-  subprocess.check_call(['gn', 'gen', out_dir, '--ide=json', '-q'])
-  gn_json = json.load(open(os.path.join(out_dir, 'project.json')))
-  q.put(ParseGNProjectJSON(gn_json))
 
-
-def ParseGNProjectJSON(gn):
+  tmp = None
+  try:
+    tmp = tempfile.mkdtemp()
+    shutil.copy2(os.path.join(out_dir, 'args.gn'),
+                 os.path.join(tmp, 'args.gn'))
+    # Do "gn gen" in a temp dir to prevent dirtying |out_dir|.
+    subprocess.check_call(['gn', 'gen', tmp, '--ide=json', '-q'])
+    gn_json = json.load(open(os.path.join(tmp, 'project.json')))
+  finally:
+    if tmp:
+      shutil.rmtree(tmp)
+  q.put(ParseGNProjectJSON(gn_json, out_dir, tmp))
+
+
+def ParseGNProjectJSON(gn, out_dir, tmp_out):
   """Parse GN output and get the header files"""
   all_headers = set()
 
@@ -85,6 +97,8 @@ def ParseGNProjectJSON(gn):
       if f.endswith('.h') or f.endswith('.hh'):
         if f.startswith('//'):
           f = f[2:]  # Strip the '//' prefix.
+          if f.startswith(tmp_out):
+            f = out_dir + f[len(tmp_out):]
           all_headers.add(f)
 
   return all_headers
@@ -125,14 +139,20 @@ def GetNonExistingFiles(lst):
 
 
 def main():
-  parser = argparse.ArgumentParser()
-  parser.add_argument('--out-dir', default='out/Release')
-  parser.add_argument('--json')
-  parser.add_argument('--whitelist')
-  parser.add_argument('args', nargs=argparse.REMAINDER)
+  parser = argparse.ArgumentParser(description='''
+      NOTE: Use ninja to build all targets in OUT_DIR before running
+      this script.''')
+  parser.add_argument('--out-dir', metavar='OUT_DIR', default='out/Release',
+                      help='output directory of the build')
+  parser.add_argument('--json',
+                      help='JSON output filename for missing headers')
+  parser.add_argument('--whitelist', help='file containing whitelist')
 
   args, _extras = parser.parse_known_args()
 
+  if not os.path.isdir(args.out_dir):
+    parser.error('OUT_DIR "%s" does not exist.' % args.out_dir)
+
   d_q = Queue()
   d_p = Process(target=GetHeadersFromNinja, args=(args.out_dir, d_q,))
   d_p.start()
@@ -146,8 +166,6 @@ def main():
   deps_p.start()
 
   d = d_q.get()
-  assert len(GetNonExistingFiles(d)) == 0, \
-      'Found non-existing files in ninja deps'
   gn = gn_q.get()
   missing = d - gn
   nonexisting = GetNonExistingFiles(gn)
@@ -160,6 +178,14 @@ def main():
   gn_p.join()
   deps_p.join()
 
+  if len(GetNonExistingFiles(d)) > 0:
+    parser.error('''Found non-existing files in ninja deps. You should
+        build all in OUT_DIR.''')
+  if len(d) == 0:
+    parser.error('OUT_DIR looks empty. You should build all there.')
+  if any((('/gen/' in i) for i in nonexisting)):
+    parser.error('OUT_DIR looks wrong. You should build all there.')
+
   if args.whitelist:
     whitelist = ParseWhiteList(open(args.whitelist).read())
     missing -= whitelist