Attributes are not stored in dense storage

[radistmorse]

Describe the bug
The attributes are not stored in the dense storage if the header version is less than H5O_VERSION_2, which it is by default. Creating the large attribute will cause an error, even if the dense storage was chosen. This is the minimal example to reproduce the bug.

#include "hdf5.h"

int main(void)
{
        hid_t props, file, space, attr;
        hsize_t dims[1] = {1000000ul};
 
        props = H5Pcreate(H5P_FILE_CREATE);
        
        H5Pset_attr_phase_change(props, 0, 0);
 
        file = H5Fcreate("test.h5", H5F_ACC_TRUNC, props, H5P_DEFAULT);
        
        space = H5Screate_simple(1, dims, NULL);
        
        attr = H5Acreate2(file, "test", H5T_NATIVE_INT, space, H5P_DEFAULT, H5P_DEFAULT);

        return 0;
}

To workaround this bug and force the header version to H5O_VERSION_2, one could enable creation order tracking by calling

H5Pset_attr_creation_order(props, H5P_CRT_ORDER_TRACKED);

but this behaviour is undocumented and unintuitive.

Expected behavior
Attribute should be written to the dense storage even without creation order tracking.

Platform (please complete the following information)

• HDF5 version 1.12.1, but also tested with upstream 562c53c

[fortnern]

I see two issues here:

1. Setting an attribute phase change value should cause the object header version to upgrade
2. Creating an attribute larger than 64K (including the header) should cause the object header version to upgrade

I will try to address this soon