[glib: 1/2] gutils: Add a comment specifying reason of omitting g_mkdir

[Philip Withnall <pwithnall src gnome org>]

commit 5484bf1cef0a22fd68d5e4af85a6cbe6191eb66e
Author: Egor Bychin <e bychin drweb com>
Date:   Thu Oct 21 15:25:25 2021 +0300

    gutils: Add a comment specifying reason of omitting g_mkdir

 glib/gutils.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)
---
diff --git a/glib/gutils.c b/glib/gutils.c
index 4bccd7229..e4fffa4d2 100644
--- a/glib/gutils.c
+++ b/glib/gutils.c
@@ -1900,12 +1900,21 @@ g_build_user_runtime_dir (void)
   const gchar *runtime_dir_env = g_getenv ("XDG_RUNTIME_DIR");
 
   if (runtime_dir_env && runtime_dir_env[0])
-    runtime_dir = g_strdup (runtime_dir_env);
+    {
+      runtime_dir = g_strdup (runtime_dir_env);
+
+      /* If the XDG_RUNTIME_DIR environment variable is set, we are being told by
+       * the OS that this directory exists and is appropriately configured
+       * already.
+       */
+    }
   else
     {
       runtime_dir = g_build_user_cache_dir ();
 
-      /* The user should be able to rely on the directory existing
+      /* Fallback case: the directory may not yet exist.
+       *
+       * The user should be able to rely on the directory existing
        * when the function returns.  Probably it already does, but
        * let's make sure.  Just do mkdir() directly since it will be
        * no more expensive than a stat() in the case that the