No more speed penalty for settrace(gettrace()). #397.

2 files changed, 8 insertions, 0 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 610ea4bf..dbfd1908 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -15,9 +15,13 @@ Version 4.1
   are seeing corrupt data files, but this lets them continue combining anyway.
   Prompted by `issue 418`_.
 
+- Code that uses ``sys.settrace(sys.gettrace())`` used to incur more than a 2x
+  speed penalty.  Now there's no penalty at all. Fixes `issue 397`_.
+
 - Pyexpat C code will no longer be recorded as a source file, fixing
   `issue 419`_.
 
+.. _issue 397: https://bitbucket.org/ned/coveragepy/issues/397/stopping-and-resuming-coverage-with
 .. _issue 418: https://bitbucket.org/ned/coveragepy/issues/418/json-parse-error
 .. _issue 419: https://bitbucket.org/ned/coveragepy/issues/419/nosource-no-source-for-code-path-to-c
 
diff --git a/coverage/ctracer/tracer.c b/coverage/ctracer/tracer.c
index 23472575..e5f39d09 100644
--- a/coverage/ctracer/tracer.c
+++ b/coverage/ctracer/tracer.c
@@ -903,6 +903,10 @@ CTracer_call(CTracer *self, PyObject *args, PyObject *kwds)
     /* Clean up. */
     frame->f_lineno = orig_lineno;
 
+    /* For better speed, install ourselves the C way so that future calls go
+       directly to CTracer_trace, without this intermediate function. */
+    PyEval_SetTrace((Py_tracefunc)CTracer_trace, (PyObject*)self);
+
 done:
     return ret;
 }