Only write gyp-win-tool when needed

build/toolchain/win/setup_toolchain.py

Index: build/toolchain/win/setup_toolchain.py
diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index a766cc1541573c8527e4c6c2efd6b72e0012177e..df674a8248af189c518fdce2e9ff8d945be77d1b 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -147,11 +147,22 @@ def _CopyTool(source_path):
     tool_source = source_file.readlines()
 
   # Add header and write it out to the current directory (which should be the
-  # root build dir).
-  with open("gyp-win-tool", 'w') as tool_file:
-    tool_file.write(''.join([tool_source[0],
-                             '# Generated by setup_toolchain.py do not edit.\n']
-                            + tool_source[1:]))
+  # root build dir). Don't write the file if a matching file already exists
+  # because that causes a cascade of unnecessary rebuilds.
+  match = False
+  contents = ''.join([tool_source[0],
+                      '# Generated by setup_toolchain.py do not edit.\n']
+                     + tool_source[1:])
+  try:

[scottmg, 2016/04/29 02:11:40]

filename = 'gyp-win-tool'

and use in both places.

[brucedawson, 2016/04/29 17:25:28]

Done.

+    with open("gyp-win-tool", 'r') as tool_file:

[scottmg, 2016/04/29 02:11:40]

" -> ' while you're here (and below).

[scottmg, 2016/04/29 02:11:40]

'rb' here, and 'wb' below since we're sort of doing a binary diff.

[scottmg, 2016/04/29 02:11:40]

Maybe call this read_tool_file and the one below write_tool_file since they're
in the same scope.

[brucedawson, 2016/04/29 17:25:28]

Yeah, I noticed that and wondered if there was some obscure Python style rule
that had caused double quotes to be used in that one place. Fixed.

[brucedawson, 2016/04/29 17:25:28]

Done.

[brucedawson, 2016/04/29 17:25:28]

Done.

+      existing_contents = tool_file.read()
+    if existing_contents == contents:
+      match = True
+  except:
+    pass
+  if not match:
+    with open("gyp-win-tool", 'w') as tool_file:
+      tool_file.write(contents)
 
 
 def main():