Safer repository pointer extraction

Casting a pointer to a non-pointer type is something which you should
never do. Instead, do something a bit more convoluted, but which is
guaranteed to give us the right pointer, as well as making sure that the
memory we exchange between python/cffi and the C extension is of the
right pointer size.

While we're touching this code, fix which object we pass to the Remote
constructor to keep alive. We need to pass the Repository object to
stop it from becoming unreferenced (and thus freeing memory the remote
needs) instead of the git_repository pointer.

pygit2/repository.py

@@ -41,6 +41,15 @@ from .remote import Remote
 
 class Repository(_Repository):
 
+    def __init__(self, *args, **kwargs):
+        super(Repository, self).__init__(*args, **kwargs)
+
+        # Get the pointer as the contents of a buffer and store it for
+        # later access
+        repo_cptr = ffi.new('git_repository **')
+        ffi.buffer(repo_cptr)[:] = self._pointer[:]
+        self._repo = repo_cptr[0]
+
     #
     # Mapping interface
     #
@@ -72,36 +81,30 @@ class Repository(_Repository):
         Creates a new remote.
         """
 
-        repo_cptr = ffi.new('git_repository **')
-        repo_cptr[0] = ffi.cast('git_repository *', self._pointer)
         cremote = ffi.new('git_remote **')
 
-        repo = repo_cptr[0]
+        err = C.git_remote_create(cremote, self._repo, to_str(name), to_str(url))
-        err = C.git_remote_create(cremote, repo, to_str(name), to_str(url))
         check_error(err)
 
-        return Remote(repo, cremote[0])
+        return Remote(self, cremote[0])
 
     @property
     def remotes(self):
         """Returns all configured remotes"""
 
-        repo_cptr = ffi.new('git_repository **')
-        repo_cptr[0] = ffi.cast('git_repository *', self._pointer)
         names = ffi.new('git_strarray *')
 
-        repo = repo_cptr[0]
         try:
-            err = C.git_remote_list(names, repo)
+            err = C.git_remote_list(names, self._repo)
             check_error(err)
 
             l = [None] * names.count
             cremote = ffi.new('git_remote **')
             for i in range(names.count):
-                err = C.git_remote_load(cremote, repo, names.strings[i])
+                err = C.git_remote_load(cremote, self._repo, names.strings[i])
                 check_error(err)
 
-                l[i] = Remote(repo, cremote[0])
+                l[i] = Remote(self, cremote[0])
             return l
         finally:
             C.git_strarray_free(names)

src/repository.c

@@ -1293,13 +1293,8 @@ PyDoc_STRVAR(Repository__pointer__doc__, "Get the repo's pointer. For internal u
 PyObject *
 Repository__pointer__get__(Repository *self)
 {
-    /*
+    /* Bytes means a raw buffer */
-     * This is pretty bad. We shouldn't be casting a pointer into an
+    return PyBytes_FromStringAndSize((char *) &self->repo, sizeof(git_repository *));
-     * integer, but we can't access the contents of a PyCapsule from
-     * python code, which we need to do in order to get a type that
-     * cffi likes.
-     */
-    return PyLong_FromLongLong((long long) self->repo);
 }
 
 PyDoc_STRVAR(Repository_checkout_head__doc__,