tests/constructor: Support systems where dlclose is a no-op

POSIX allows dlclose() implementations that are no-ops,
and in such case library destructors run at application
exit rather than dlclose().

That's the case, for example, of UNIX systems with the
Musl LibC.

glib/tests/constructor-helper.c

@@ -61,6 +61,9 @@ void string_add_exclusive (const char *string);
 MODULE_EXPORT
 int string_remove (const char *string);
 
+MODULE_EXPORT
+int string_find (const char *string);
+
 MODULE_EXPORT
 void string_check (const char *string);
 
@@ -204,6 +207,19 @@ string_remove (const char *string)
   return 1;
 }
 
+/**< private >
+ * string_find:
+ *
+ * @string: NULL-terminated string. Must not be empty
+ *
+ * Returns 1 if the string is present, 0 otherwise
+ */
+MODULE_EXPORT
+int string_find (const char *string)
+{
+  return string_find_index_ (string) < sizeof (buffer);
+}
+
 /**< private >
  * string_check:
  *

glib/tests/constructor.c

@@ -40,6 +40,8 @@ void string_add_exclusive (const char *string);
 MODULE_IMPORT
 void string_check (const char *string);
 
+MODULE_IMPORT
+int string_find (const char *string);
 
 #if G_HAS_CONSTRUCTORS
 
@@ -66,9 +68,13 @@ dtor (void)
 {
   string_add_exclusive (G_STRINGIFY (PREFIX) "_" "dtor");
 
-#ifdef BUILD_TEST_EXECUTABLE
+  if (string_find ("app_dtor") && string_find ("lib_dtor"))
+    {
+      /* All destructors were invoked, this is the last.
+       * Call _Exit (EXIT_SUCCESS) to exit immediately
+       * with a success code */
       _Exit (EXIT_SUCCESS);
-#endif
+    }
 }
 
 #endif /* G_HAS_CONSTRUCTORS */
@@ -210,7 +216,12 @@ test_lib (gconstpointer data)
   unload_library ();
 
 #if G_HAS_DESTRUCTORS
-  string_check ("lib_" "dtor");
+  /* Destructors in dynamically-loaded libraries do not
+   * necessarily run on dlclose. On some systems dlclose
+   * is effectively a no-op (e.g with the Musl LibC) and
+   * destructors run at program exit */
+  g_test_message ("Destructors run on module unload: %s\n",
+                  string_find ("lib_" "dtor") ? "yes" : "no");
 #endif
 #if defined (_WIN32) && defined (G_HAS_TLS_CALLBACKS)
   string_check ("lib_" "tlscb_process_detach");