MJIT: Handle JIT failure properly

mjit.c

@@ -244,7 +244,15 @@ check_unit_queue(void)
     current_cc_ms = real_ms_time();
     current_cc_unit = unit;
     current_cc_pid = start_mjit_compile(unit);
-    // TODO: handle -1
+
+    // JIT failure
+    if (current_cc_pid == -1) {
+        current_cc_pid = 0;
+        current_cc_unit->iseq->body->jit_func = (mjit_func_t)NOT_COMPILED_JIT_ISEQ_FUNC;
+        current_cc_unit = NULL;
+        return;
+    }
+
     if (mjit_opts.wait) {
         mjit_wait(unit->iseq->body);
     }

test/ruby/test_mjit.rb

@@ -1198,6 +1198,18 @@ class TestMJIT < Test::Unit::TestCase
     end
   end if defined?(fork)
 
+  def test_jit_failure
+    _, err = eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", min_calls: 1, verbose: 1)
+    begin;
+      1.times do
+        class A
+        end
+      end
+    end;
+    assert_match(/^MJIT warning: .+ unsupported instruction: defineclass/, err)
+    assert_match(/^JIT failure: block in <main>/, err)
+  end
+
   private
 
   # The shortest way to test one proc