Avoid assert failure when NULL EC is expected

After 5680c38, postponed job APIs now
expect to be called on native threads not managed by Ruby and handles
getting a NULL execution context. However, in debug builds the change
runs into an assertion failure with GET_EC() which asserts that EC is
non-NULL. Avoid the assertion failure by passing `false` for `expect_ec`
instead as the intention is to handle when there is no EC.

Add a test from John Crepezzi and John Hawthorn to exercise this
situation.

[Bug #17573]

Co-authored-by: John Hawthorn <john@hawthorn.email>
Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>

ext/-test-/postponed_job/postponed_job.c

@@ -58,12 +58,43 @@ pjob_call_direct(VALUE self, VALUE obj)
     return self;
 }
 
+#ifdef HAVE_PTHREAD_H
+#include <pthread.h>
+
+static void *
+pjob_register_in_c_thread_i(void *obj)
+{
+    rb_postponed_job_register_one(0, pjob_one_callback, (void *)obj);
+    rb_postponed_job_register_one(0, pjob_one_callback, (void *)obj);
+    rb_postponed_job_register_one(0, pjob_one_callback, (void *)obj);
+    return NULL;
+}
+
+static VALUE
+pjob_register_in_c_thread(VALUE self, VALUE obj)
+{
+    pthread_t thread;
+    if (pthread_create(&thread, NULL, pjob_register_in_c_thread_i, (void *)obj)) {
+        return Qfalse;
+    }
+
+    if (pthread_join(thread, NULL)) {
+        return Qfalse;
+    }
+
+    return Qtrue;
+}
+#endif
+
 void
 Init_postponed_job(VALUE self)
 {
     VALUE mBug = rb_define_module("Bug");
     rb_define_module_function(mBug, "postponed_job_register", pjob_register, 1);
     rb_define_module_function(mBug, "postponed_job_register_one", pjob_register_one, 1);
     rb_define_module_function(mBug, "postponed_job_call_direct", pjob_call_direct, 1);
+#ifdef HAVE_PTHREAD_H
+    rb_define_module_function(mBug, "postponed_job_register_in_c_thread", pjob_register_in_c_thread, 1);
+#endif
 }
 

test/-ext-/postponed_job/test_postponed_job.rb

@@ -25,4 +25,11 @@ def test_register
     Bug.postponed_job_register_one(ary = [])
     assert_equal [1], ary
   end
+
+  if Bug.respond_to?(:postponed_job_register_in_c_thread)
+    def test_register_in_c_thread
+      assert Bug.postponed_job_register_in_c_thread(ary = [])
+      assert_equal [1], ary
+    end
+  end
 end

vm_trace.c

@@ -1585,7 +1585,7 @@ postponed_job_register(rb_execution_context_t *ec, rb_vm_t *vm,
 static rb_execution_context_t *
 get_valid_ec(rb_vm_t *vm)
 {
-    rb_execution_context_t *ec = GET_EC();
+    rb_execution_context_t *ec = rb_current_execution_context(false);
     if (ec == NULL) ec = rb_vm_main_ractor_ec(vm);
     return ec;
 }