Patchwork dirs._addpath: don't mutate Python strings after exposing them (issue4589)

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Submitter Siddharth Agarwal
Date April 6, 2015, 7:44 p.m.
Message ID <07327bdebb35586d5b3e.1428349451@devbig136.prn2.facebook.com>
Download mbox | patch
Permalink /patch/8513/
State Accepted
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Comments

Siddharth Agarwal - April 6, 2015, 7:44 p.m.
# HG changeset patch
# User Siddharth Agarwal <sid0@fb.com>
# Date 1428342404 25200
#      Mon Apr 06 10:46:44 2015 -0700
# Node ID 07327bdebb35586d5b3e950a32070db16b65d63b
# Parent  4a4018831d2ebc3c9cae9c6613e6a2497b4f0993
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.
Matt Mackall - April 6, 2015, 9:07 p.m.
On Mon, 2015-04-06 at 12:44 -0700, Siddharth Agarwal wrote:
> # HG changeset patch
> # User Siddharth Agarwal <sid0@fb.com>
> # Date 1428342404 25200
> #      Mon Apr 06 10:46:44 2015 -0700
> # Node ID 07327bdebb35586d5b3e950a32070db16b65d63b
> # Parent  4a4018831d2ebc3c9cae9c6613e6a2497b4f0993
> dirs._addpath: don't mutate Python strings after exposing them (issue4589)

Queued for default, thanks.

Patch

diff --git a/mercurial/dirs.c b/mercurial/dirs.c
--- a/mercurial/dirs.c
+++ b/mercurial/dirs.c
@@ -93,11 +93,11 @@  static int _addpath(PyObject *dirs, PyOb
 		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;