D-Bus arrays are serialized as follows:

1. align to a 4-byte boundary (for the length)
2. uint32: the length of the serialized body in bytes
3. padding for the alignment of the body type (not included in the length)
4. the body.

Note that 3. is a no-op unless the body type is an 8-byte aligned type
(uint64, int64, double, struct, dict_entry), since you are always on a
4-byte boundary from aligning and writing the length.

So, an empty aax (that is, an array containing zero arrays of int64)
is serialized as follows:

1. align to a 4-byte boundary
2. length of the contents of this (empty) array, in bytes (0)
3. align to a 4-byte boundary (the child array's alignment requirement)
4. there is no body.

But previously, GDBus would recurse in step three to align not just for
the type of the child array, but for the nonexistent child array's
contents. This only affects the algorithm when the grandchild type has
8-byte alignment and the reader happened to not already be on an 8-byte
boundary, in which case 4 bytes were spuriously skipped.

https://bugzilla.gnome.org/show_bug.cgi?id=673612



Signed-off-by: David Zeuthen <davidz@redhat.com>