GVariant: fix normal-form checking for tuples
GVariant has the concept of fixed-sized types (ie: types for which all values of the type will have the same size). Examples are booleans, integers, doubles, etc. Tuples containing only these types are also fixed size. When GVariant is trying to deal with a fixed-sized value for which it doesn't have a sufficient backing store (eg: the case where a fixed-sized value was created with g_variant_new_data() with an incorrect number of bytes) it denotes this by setting the size of the value to the correct fixed size but using a NULL data pointer. This is well-documented in several code comments and also in the public API documentation for g_variant_get_data() which describes the situation number which NULL could be returned. The decision to deal with this case in this way was changed at the last minute around the time that GVariant was merged -- originally we had an elaborate setup involving allocating an internal buffer of sufficient size to be shared between all invalid values. Unfortunately, when making this change a small detail was missed. gvs_tuple_get_child() (the function responsible for deserialising tuples) was updated to properly check for this case (and it contains a comment about why it must). gvs_tuple_is_normal() (the function responsible for verifying if a tuple is in normal form) was not. We add the check now. Note that this problem does not exist with any other container type because tuples are the only container capable of being fixed-sized. All other container types (arrays, maybes, variants) can contain a variable number of items or items of variable types (note: we consider dictionary entries to be two-tuples). The code for validating non-container values also contains a check for the case of NULL data. The problem also does not occur in the only other function dealing with serialised tuples: gvs_tuple_n_children(). Whereas other container types would have to inspect the serialised data to determine the number of children, for tuples it can be determined directly from the type.
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