Fix midl.py fails when building a brand new IDL file

This change handles the cases of brand new IDL files added to a build,
or cases where files under third_party\win_build_output\midl are
(unexpectedly) missing. This change also has some minor refactoring of
the |writes_tlb| midl.gni option.

Bug: 1149764
Change-Id: If2e608f2e986bd3fbc13a3a7b699801660c2ffa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2543367
Commit-Queue: S. Ganesh <ganesh@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828319}

build/toolchain/win/midl.gni

@@ -87,7 +87,11 @@ template("midl") {
   dlldata_file = "{{source_name_part}}.dlldata.c"
   interface_identifier_file = "{{source_name_part}}_i.c"
   proxy_file = "{{source_name_part}}_p.c"
-  type_library_file = "{{source_name_part}}.tlb"
+  if (writes_tlb) {
+    type_library_file = "{{source_name_part}}.tlb"
+  } else {
+    type_library_file = "none"
+  }
 
   # TODO(crbug.com/1112471): Get this to run cleanly under Python 3.
   python2_action_foreach(action_name) {

build/toolchain/win/midl.py

@@ -234,7 +234,8 @@ def overwrite_guids(h_file, iid_file, proxy_file, tlb_file, dynamic_guids):
   overwrite_guids_h(h_file, dynamic_guids)
   overwrite_guids_iid(iid_file, dynamic_guids)
   overwrite_guids_proxy(proxy_file, dynamic_guids)
-  overwrite_guids_tlb(tlb_file, dynamic_guids)
+  if tlb_file:
+    overwrite_guids_tlb(tlb_file, dynamic_guids)
 
 
 # This function removes all occurrences of 'PLACEHOLDER-GUID-' from the
@@ -324,12 +325,40 @@ def main(arch, gendir, outdir, dynamic_guids, tlb, h, dlldata, iid, proxy,
   source = os.path.join(source, arch.split('.')[1])  # Append 'x86' or 'x64'.
   source = os.path.normpath(source)
 
+  source_exists = True
+  if not os.path.isdir(source):
+    source_exists = False
+    if sys.platform != 'win32':
+      print('Directory %s needs to be populated from Windows first' % source)
+      return 1
+
+    # This is a brand new IDL file that does not have outputs under
+    # third_party\win_build_output\midl. We create an empty directory for now.
+    os.makedirs(source)
+
   common_files = [h, dlldata, iid, proxy]
-  if os.path.isfile(os.path.join(source, tlb)):
+  if tlb != 'none':
     # Not all projects use tlb files.
     common_files += [tlb]
+  else:
+    tlb = None
+
   for source_file in common_files:
-    shutil.copy(os.path.join(source, source_file), outdir)
+    file_path = os.path.join(source, source_file)
+    if not os.path.isfile(file_path):
+      source_exists = False
+      if sys.platform != 'win32':
+        print('File %s needs to be generated from Windows first' % file_path)
+        return 1
+
+      # Either this is a brand new IDL file that does not have outputs under
+      # third_party\win_build_output\midl or the file is (unexpectedly) missing.
+      # We create an empty file for now. The rest of the machinery below will
+      # then generate the correctly populated file using the MIDL compiler and
+      # instruct the developer to copy that file under
+      # third_party\win_build_output\midl.
+      open(file_path, 'wb').close()
+    shutil.copy(file_path, outdir)
 
   if dynamic_guids != 'none':
     assert '=' in dynamic_guids
@@ -348,9 +377,10 @@ def main(arch, gendir, outdir, dynamic_guids, tlb, h, dlldata, iid, proxy,
     dynamic_guids = dynamic_guids.split(',')
     dynamic_guids = dict(s.split('=') for s in dynamic_guids)
     uuid5_substitutions(dynamic_guids)
-    overwrite_guids(os.path.join(outdir, h), os.path.join(outdir, iid),
-                    os.path.join(outdir, proxy), os.path.join(outdir, tlb),
-                    dynamic_guids)
+    if source_exists:
+      overwrite_guids(*(os.path.join(outdir, file) if file else None
+                        for file in [h, iid, proxy, tlb]),
+                      dynamic_guids=dynamic_guids)
   else:
     dynamic_guids = None
 
@@ -387,9 +417,9 @@ def main(arch, gendir, outdir, dynamic_guids, tlb, h, dlldata, iid, proxy,
   preprocessor_options = '-E -nologo -Wno-nonportable-include-path'
   preprocessor_options += ''.join(
       [' ' + flag for flag in flags if flag.startswith('/D')])
-  args = ['midl', '/nologo'] + list(flags) + [
-      '/tlb', tlb, '/h', h, '/dlldata', dlldata, '/iid', iid, '/proxy', proxy,
-      '/cpp_cmd', clang, '/cpp_opt', preprocessor_options, idl
+  args = ['midl', '/nologo'] + list(flags) + (['/tlb', tlb] if tlb else []) + [
+      '/h', h, '/dlldata', dlldata, '/iid', iid, '/proxy', proxy, '/cpp_cmd',
+      clang, '/cpp_opt', preprocessor_options, idl
   ]
 
   returncode, midl_output_dir = run_midl(args, env_dict)
@@ -399,6 +429,7 @@ def main(arch, gendir, outdir, dynamic_guids, tlb, h, dlldata, iid, proxy,
   # Now compare the output in midl_output_dir to the copied-over outputs.
   _, mismatch, errors = filecmp.cmpfiles(midl_output_dir, outdir, common_files)
   assert not errors
+
   if mismatch:
     print('midl.exe output different from files in %s, see %s' %
           (outdir, midl_output_dir))