Register C globals holding Ruby objects with the GC

[plribeiro3000]

Summary

• tiny_tds_ext.c, client.c and result.c keep TinyTds, TinyTds::Error, TinyTds::Client, TinyTds::Result, Kernel and Date in C globals that were never registered with the GC.
• After compaction cTinyTdsError can point to a moved object, so the next FreeTDS message or error handled by rb_tinytds_raise_error crashes the process with a segmentation fault instead of raising TinyTds::Error.
• Each of them is now registered with rb_global_variable, the same way opt_escape_regex, opt_escape_dblquote and the opt_* values in result.c already are.

Test

test/gc_compaction_test.rb runs the minimal repro from #608 in a subprocess: it forces compaction with GC.verify_compaction_references and connects to a closed port. It needs no SQL Server and is skipped where compaction is unsupported.

Verified locally on macOS, Ruby 4.0.6, FreeTDS 1.5.4:

• without the patch: the subprocess dies with SIGSEGV (rb_exc_new -> unexpected_type)
• with the patch: TinyTds::Error is raised and the test passes

bundle exec rake format leaves the C files unchanged.

Fixes #608

[aharpervc]

ci failed,

GcCompactionTest::after GC compaction
  test_0001_raises TinyTds::Error instead of crashing when a connection fails FAIL (0.20s)
        expected TinyTds::Error to be raised, got #<Process::Status: pid 2511 exit 1>:
        <internal:gc>:230:in `verify_compaction_references': unknown keyword: :expand_heap (ArgumentError)
        	from -e:3:in `<main>'
        /home/runner/work/tiny_tds/tiny_tds/test/gc_compaction_test.rb:26:in `block (2 levels) in <class:GcCompactionTest>'

Does this need to be conditional for ruby 3.2+?

[plribeiro3000]

Yes, good catch @aharpervc. expand_heap: only exists from Ruby 3.2 on; 3.0 and 3.1 call it double_heap:. The test now picks the keyword based on RUBY_VERSION (7ab12b9).

[andyundso]

small thing here: can you call rb_global_variable directly after the assignment of mTinyTds?

[plribeiro3000]

Done, each rb_global_variable now comes right after the assignment it registers, so mTinyTds is registered before cTinyTdsError is looked up. This also matches how client.c and result.c already do it. Amended into df658bc.