Stop choosing $TMPDIR as a safe place to create hardlinks on OSX.

client/run_isolated.py

Index: client/run_isolated.py
diff --git a/client/run_isolated.py b/client/run_isolated.py
index dce643f110afd779391d9143572248a4dad9d60d..f964384181be76754d458645badd1640112d3292 100755
--- a/client/run_isolated.py
+++ b/client/run_isolated.py
@@ -88,11 +88,20 @@ def make_temp_dir(prefix, root_dir=None):
 
   If root_dir is given and /tmp is on same file system as root_dir, uses /tmp.
   Otherwise makes a new temp directory under root_dir.
+
+  Except on OSX, because it's dangerous to create hardlinks in $TMPDIR on OSX!
+  /System/Library/LaunchDaemons/com.apple.bsd.dirhelper.plist runs every day at
+  3:35am and deletes all files older than 3 days in $TMPDIR, but hardlinks do
+  not have the inode modification time updated, so they tend to be old, thus
+  they get deleted.
   """
   base_temp_dir = None
-  if (root_dir and
-      not file_path.is_same_filesystem(
-        root_dir, unicode(tempfile.gettempdir()))):
+  real_temp_dir = unicode(tempfile.gettempdir())
+  if sys.platform == 'darwin':
+    # Nope! Nope! Nope!
+    assert root_dir, 'It is unsafe to create hardlinks in $TMPDIR'
+    base_temp_dir = root_dir
+  elif root_dir and not file_path.is_same_filesystem(root_dir, real_temp_dir):
     base_temp_dir = root_dir
   return unicode(tempfile.mkdtemp(prefix=prefix, dir=base_temp_dir))