core: introduce cord_exit() function

cord_exit should be always called in the exiting thread. It's a single
place to call all the thread-specific module deinitalization routines.

In-scope-of #4264

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

src/lib/core/coio_task.c

@@ -122,6 +122,7 @@ static int
 coio_on_stop(void *data)
 {
 	(void) data;
+	cord_exit(cord());
 	cord_destroy(cord());
 	free(cord());
 	return 0;

src/lib/core/fiber.c

@@ -1560,6 +1560,13 @@ cord_add_garbage(struct cord *cord, struct fiber *f)
 		cord->garbage = f;
 }
 
+void
+cord_exit(struct cord *cord)
+{
+	assert(cord == cord());
+	(void)cord;
+}
+
 void
 cord_destroy(struct cord *cord)
 {
@@ -1625,6 +1632,9 @@ void *cord_thread_func(void *p)
 	                                            CORD_ON_EXIT_WONT_RUN);
 	if (!changed)
 		handler->callback(handler->argument);
+
+	cord_exit(cord());
+
 	return res;
 }
 
@@ -1874,6 +1884,7 @@ fiber_init(int (*invoke)(fiber_func f, va_list ap))
 void
 fiber_free(void)
 {
+	cord_exit(&main_cord);
 	cord_destroy(&main_cord);
 }
 

src/lib/core/fiber.h

@@ -771,6 +771,13 @@ extern __thread struct cord *cord_ptr;
 void
 cord_create(struct cord *cord, const char *name);
 
+/**
+ * Perform all the thread-specific deinitialization. Must be called in the
+ * exiting thread.
+ */
+void
+cord_exit(struct cord *cord);
+
 void
 cord_destroy(struct cord *cord);