Adjust snt < max_cpu calculation

[Bug #20146]

Previously we dealt with the main Ractor not being enabled for M:N by
incrementing snt_cnt++. This worked for comparing against ractor count,
but meant that we always had one less SNT than was specified by
RUBY_MAX_CPU.

This was notably a problem for RUBY_MAX_CPU=1, which would cause Ractors
to hang.

This commit instead of adjusting snt, adjusts a
"schedulable_ractor_cnt". This way snt_cnt will actually reach
RUBY_MAX_CPU.

Author: jhawthorn

test/ruby/test_ractor.rb

@@ -202,6 +202,13 @@ def ractor_job(job_count, array_size)
     RUBY
   end
 
+  # [Bug #20146]
+  def test_max_cpu_1
+    assert_ractor(<<~'RUBY', args: [{ "RUBY_MAX_CPU" => "1" }])
+      assert_equal :ok, Ractor.new { :ok }.value
+    RUBY
+  end
+
   def assert_make_shareable(obj)
     refute Ractor.shareable?(obj), "object was already shareable"
     Ractor.make_shareable(obj)

thread_pthread_mn.c

@@ -397,11 +397,15 @@ native_thread_check_and_create_shared(rb_vm_t *vm)
 
     rb_native_mutex_lock(&vm->ractor.sched.lock);
     {
-        unsigned int snt_cnt = vm->ractor.sched.snt_cnt;
-        if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads) snt_cnt++; // do not need snt for main ractor
+        unsigned int schedulable_ractor_cnt = vm->ractor.cnt;
+        RUBY_ASSERT(schedulable_ractor_cnt >= 1);
+
+        if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads)
+            schedulable_ractor_cnt--; // do not need snt for main ractor
 
+        unsigned int snt_cnt = vm->ractor.sched.snt_cnt;
         if (((int)snt_cnt < MINIMUM_SNT) ||
-            (snt_cnt < vm->ractor.cnt  &&
+            (snt_cnt < schedulable_ractor_cnt  &&
              snt_cnt < vm->ractor.sched.max_cpu)) {
 
             RUBY_DEBUG_LOG("added snt:%u dnt:%u ractor_cnt:%u grq_cnt:%u",