dirs._addpath: don't mutate Python strings after exposing them (issue4589)
One of the rules of Python strings is that they're immutable. dirs._addpath
breaks this assumption for performance, which is fine as long as it is done
safely -- once a string is no longer internal-only it shouldn't be mutated.
Unfortunately, we weren't being safe here -- we were mutating 'key' even after
adding it to a dictionary.
This only really affects other C code that reads strings, so it's somewhat hard
to write a test for this without poking into the internal representation of the
string via ctypes or similar. There is currently no C code that reads the
output of the string, but there will likely be some soon as the bug indicates.
There's no significant difference in performance.
--- a/mercurial/dirs.c Mon Apr 06 08:23:27 2015 -0700
+++ b/mercurial/dirs.c Mon Apr 06 10:46:44 2015 -0700
@@ -93,11 +93,11 @@
if (ret == -1)
goto bail;
- if (pos != 0)
- PyString_AS_STRING(key)[pos] = '/';
- else
- key = NULL;
- Py_CLEAR(key);
+ /* Clear the key out since we've already exposed it to Python
+ and can't mutate it further. key's refcount is currently 2 so
+ we can't just use Py_CLEAR. */
+ Py_DECREF(key);
+ key = NULL;
}
ret = 0;