Dtrace on OSX 10.7 uses a negative number for the 3rd parameter.

tools/isolate/trace_inputs.py

Index: tools/isolate/trace_inputs.py
diff --git a/tools/isolate/trace_inputs.py b/tools/isolate/trace_inputs.py
index c8c0c537418d15a0336cc9fb6c50c10712ca50dc..e74b2a12c38a8d999737024e1fd62a2052cfde5a 100755
--- a/tools/isolate/trace_inputs.py
+++ b/tools/isolate/trace_inputs.py
@@ -714,7 +714,7 @@ class Dtrace(object):
 
     # Arguments parsing.
     RE_CHDIR = re.compile(r'^\"(.+?)\"$')
-    RE_OPEN = re.compile(r'^\"(.+?)\", (\d+), (\d+)$')
+    RE_OPEN = re.compile(r'^\"(.+?)\", (\d+), (-?\d+)$')
     RE_RENAME = re.compile(r'^\"(.+?)\", \"(.+?)\"$')
 
     O_DIRECTORY = 0x100000
@@ -776,8 +776,10 @@ class Dtrace(object):
     def handle_open_nocancel(self, ppid, pid, function, args, result):
       return self.handle_open(ppid, pid, function, args, result)
 
-    def handle_open(self, _ppid, pid, _function, args, result):
-      args = self.RE_OPEN.match(args).groups()
+    def handle_open(self, _ppid, pid, function, args, result):
+      match = self.RE_OPEN.match(args)
+      assert match, (pid, function, args, result)
+      args = match.groups()
       flag = int(args[1])
       if self.O_DIRECTORY & flag == self.O_DIRECTORY:
         # Ignore directories.