Clang update.py: check for LLVM_FORCE_HEAD_REVISION in the environment

tools/clang/scripts/update.py

Index: tools/clang/scripts/update.py
diff --git a/tools/clang/scripts/update.py b/tools/clang/scripts/update.py
index 89c5b1f1787884af8c2c7f5f8c6af3002e1ad57d..519c62c21ff4f604cd51b58e7a91e28d0a7bfdb4 100755
--- a/tools/clang/scripts/update.py
+++ b/tools/clang/scripts/update.py
@@ -24,6 +24,10 @@ if (sys.platform in ['win32', 'cygwin'] and
     re.search(r'\b(asan)=1', os.environ.get('GYP_DEFINES', ''))):

[Nico, 2014/06/19 21:22:06]

Can this say `and not os.environ.get('LLVM_WIN_REVISION') == 'HEAD' here? It
looks like that's all you need; I'd rather not have general support for pinning
to arbitrary revisions in bot configs.

[hans, 2014/06/19 21:27:05]

It seems confusing to a user if they see LLVM_WIN_REVISION=HEAD, think they can
use that to get a specific revision they want, and we then ignore it silently
because it only supports one magic value.
I'd rather check for HEAD in the if below and print an error that non-HEAD is
not supported if anyone tries to misuse it.

   LLVM_WIN_REVISION = '210586'
 
+# It's possible to override LLVM_WIN_REVISION from the environment.
+if 'LLVM_WIN_REVISION' in os.environ:
+  LLVM_WIN_REVISION = os.environ['LLVM_WIN_REVISION']
+
 # Path constants. (All of these should be absolute paths.)
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
 CHROMIUM_DIR = os.path.abspath(os.path.join(THIS_DIR, '..', '..', '..'))