dirs: resolve fuzzer OOM situation by disallowing deep directory hierarchies

It seems like 2048 directories ought to be enough for any reasonable
use of Mercurial?

A previous version of this patch scanned for slashes before any allocations
occurred. That approach is slower than this in the happy path, but much faster
than this in the case that too many slashes are encountered. We may want to
revisit it in the future using memchr() so it'll be well-optimized by the libc
we're using.

.. bc:

    Mercurial will now defend against OOMs by refusing to operate on
    paths with 2048 or more components. This means that _extremely_
    deep path hierarchies will be rejected, but we anticipate nobody
    is using hierarchies this deep.

Differential Revision: https://phab.mercurial-scm.org/D7411

mercurial/cext/dirs.c

@@ -9,6 +9,7 b''
 
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
+#include <string.h>
 
 #include "util.h"
 
@@ -48,12 +49,19 b' static inline Py_ssize_t _finddir(const '
 	return pos;
 }
 
+/* Mercurial will fail to run on directory hierarchies deeper than
+ * this constant, so we should try and keep this constant as big as
+ * possible.
+ */
+#define MAX_DIRS_DEPTH 2048
+
 static int _addpath(PyObject *dirs, PyObject *path)
 {
 	const char *cpath = PyBytes_AS_STRING(path);
 	Py_ssize_t pos = PyBytes_GET_SIZE(path);
 	PyObject *key = NULL;
 	int ret = -1;
+	size_t num_slashes = 0;
 
 	/* This loop is super critical for performance. That's why we inline
 	 * access to Python structs instead of going through a supported API.
@@ -65,6 +73,12 b' static int _addpath(PyObject *dirs, PyOb'
 	 * unnoticed. */
 	while ((pos = _finddir(cpath, pos - 1)) != -1) {
 		PyObject *val;
+		++num_slashes;
+		if (num_slashes > MAX_DIRS_DEPTH) {
+			PyErr_SetString(PyExc_ValueError,
+			                "Directory hierarchy too deep.");
+			goto bail;
+		}
 
 		/* Sniff for trailing slashes, a marker of an invalid input. */
 		if (pos > 0 && cpath[pos - 1] == '/') {