ninja windows: emulate msvs generator behaviour for msvs_error_on_missing_sources=1

pylib/gyp/msvs_emulation.py

Index: pylib/gyp/msvs_emulation.py
===================================================================
--- pylib/gyp/msvs_emulation.py	(revision 1525)
+++ pylib/gyp/msvs_emulation.py	(working copy)
@@ -690,3 +690,19 @@
     f = open_out(os.path.join(toplevel_build_dir, 'environment.' + arch), 'wb')
     f.write(env_block)
     f.close()
+
+def VerifyMissingSources(sources, build_dir, generator_flags, gyp_to_ninja):
+  """Emulate behavior of msvs_error_on_missing_sources present in the msvs
+  generator: Check that all regular source files, i.e. not created at run time,
+  exist on disk. Missing files cause needless recompilation when building via
+  VS, and we want this check to match for people/bots that build using ninja,
+  so they're not surprised when the VS build fails."""
+  if int(generator_flags.get('msvs_error_on_missing_sources', 0)):
+    no_specials = filter(lambda x: '$' not in x, sources)
+    relative = [os.path.join(build_dir, gyp_to_ninja(s)) for s in no_specials]
+    missing = filter(lambda x: not os.path.exists(x), relative)
+    if missing:
+      # They'll look like out\Release\..\..\stuff\things.cc, so normalize the
+      # path for a slightly less crazy looking output.
+      cleaned_up = [os.path.normpath(x) for x in missing]
+      raise Exception('Missing input files:\n%s' % '\n'.join(cleaned_up))