Michal/mnesia/add_tests_for_compare_storage_type

[Mikaka27]

Those tests are related to #9311.
When investigating problem during schema merge it was found that when some nodes in the cluster are down, it's still possible to call mnesia:change_table_copy_type/3 for 2 special cases:

• ram_copies -> disc_copies
• disc_copies -> disc_only_copies

The cause of this is that mnesia_schema:compare_storage_types/3 does not consider the two (mentioned above) conversions as incompatible, this can be seen here:

otp/lib/mnesia/src/mnesia_schema.erl

Lines 3661 to 3664 in 5dab31e

	compare_storage_type(_Retry, ram_copies, disc_copies) ->
	{diff, disc_copies};
	compare_storage_type(_Retry, disc_copies, disc_only_copies) ->
	{diff, disc_only_copies};

The cause is in mnesia_schema:make_change_table_copy_type/3, one can see that only when storage type is incompatible, it calls ensure_active(Cs)

otp/lib/mnesia/src/mnesia_schema.erl

Lines 1891 to 1898 in 5dab31e

	case compare_storage_type(false, FromS, ToSExp) of
	{same, _} ->
	mnesia:abort({already_exists, Tab, Node, ToSExp});
	{diff, _} ->
	ignore;
	incompatible ->
	ensure_active(Cs)
	end,

In addition to that:

compare_storage_type(_Retry, ram_copies, disc_copies) ->
    {diff, disc_copies};

is used for merging a schema after it's removal (when node rejoins the cluster), this can be seen in mnesia_config_test:dynamic_basic/1 when doing following operations:

otp/lib/mnesia/test/mnesia_config_test.erl

Lines 1114 to 1120 in 5dab31e

	?match([], mnesia_test_lib:kill_mnesia([N2])),
	?match(ok, mnesia:delete_schema([N2])),
	?match(ok, mnesia:dirty_write({tab1, 1, 1})),
	?match(ok, mnesia:dirty_write({tab2, 1, 1})),
	?match(ok, rpc:call(N2, mnesia, start, [[{extra_db_nodes, [N1]}, {schema, ?BACKEND}]])),

The cause of that is when we remove node's schema it restarts with ram_copies schema, and later has to join a cluster and convert it's schema to disc_copies.
This is mentioned in the documentation: https://www.erlang.org/doc/apps/mnesia/mnesia.html#module-configuration-parameters

opt_disc - Optional disc. The schema can reside on disc or in RAM. If the schema is found on disc, Mnesia starts as a disc-based node and the storage type of the schema table is disc_copies. If no schema is found on disc, Mnesia starts as a disc-less node and the storage type of the schema table is ram_copies. Default value for the application parameter is opt_disc.

but

compare_storage_type(_Retry, disc_copies, disc_only_copies) ->
    {diff, disc_only_copies};

is not covered by any tests, so add tests for both of those cases.

[github-actions]

CT Test Results

  2 files   59 suites   19m 47s ⏱️
683 tests 535 ✅ 148 💤 0 ❌
737 runs  574 ✅ 163 💤 0 ❌

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