Make metacov run without errors.

1 files changed, 11 insertions, 3 deletions

diff --git a/coverage/collector.py b/coverage/collector.py
index 6e2112c..494ad2c 100644
--- a/coverage/collector.py
+++ b/coverage/collector.py
@@ -231,6 +231,17 @@ class Collector(object):
         """Start collecting trace information."""
         if self._collectors:
             self._collectors[-1].pause()
+
+        try:
+            # Install the tracer on this thread.
+            fn = self._start_tracer()
+        except:
+            if self._collectors:
+                self._collectors[-1].resume()
+            raise
+
+        # If _start_tracer succeeded, then we add ourselves to the global
+        # stack of collectors.
         self._collectors.append(self)
 
         # Check to see whether we had a fullcoverage tracer installed.
@@ -241,9 +252,6 @@ class Collector(object):
             if tracer0:
                 traces0 = getattr(tracer0, 'traces', [])
 
-        # Install the tracer on this thread.
-        fn = self._start_tracer()
-
         # Replay all the events from fullcoverage into the new trace function.
         for args in traces0:
             (frame, event, arg), lineno = args