lib/repo: Add ostree_repo_hash() and tests

Add a hash function for OstreeRepo instances, which relies on the repo
being open, and hence being able to hash the device and inode of its
root directory.

Add unit tests for this and ostree_repo_equal().

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://github.com/ostreedev/ostree/issues/1191

Closes: #1205
Approved by: cgwalters

src/libostree/ostree-repo.c

@@ -2609,6 +2609,36 @@ ostree_repo_get_dfd (OstreeRepo  *self)
   return self->repo_dir_fd;
 }
 
+/**
+ * ostree_repo_hash:
+ * @self: an #OstreeRepo
+ *
+ * Calculate a hash value for the given open repository, suitable for use when
+ * putting it into a hash table. It is an error to call this on an #OstreeRepo
+ * which is not yet open, as a persistent hash value cannot be calculated until
+ * the repository is open and the inode of its root directory has been loaded.
+ *
+ * This function does no I/O.
+ *
+ * Returns: hash value for the #OstreeRepo
+ * Since: 2017.12
+ */
+guint
+ostree_repo_hash (OstreeRepo *self)
+{
+  g_return_val_if_fail (OSTREE_IS_REPO (self), 0);
+
+  /* We cannot hash non-open repositories, since their hash value would change
+   * once they’re opened, resulting in false lookup misses and the inability to
+   * remove them from a hash table. */
+  g_assert (self->repo_dir_fd >= 0);
+
+  /* device and inode numbers are distributed fairly uniformly, so we can’t
+   * do much better than just combining them. No need to rehash to even out
+   * the distribution. */
+  return (self->device ^ self->inode);
+}
+
 /**
  * ostree_repo_equal:
  * @a: an #OstreeRepo