[evolution] I#53 - Address Helgrind warning about lock acquire order

[Milan Crha <mcrha src gnome org>]

commit 8e1ebeffe08e1b199ad275e7bf8ac78c253865f5
Author: Milan Crha <mcrha redhat com>
Date:   Wed Jul 11 16:23:03 2018 +0200

    I#53 - Address Helgrind warning about lock acquire order
    
    Closes https://gitlab.gnome.org/GNOME/evolution/issues/53

 src/libemail-engine/e-mail-session.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)
---
diff --git a/src/libemail-engine/e-mail-session.c b/src/libemail-engine/e-mail-session.c
index 2270d7c0ab..65c3420b2e 100644
--- a/src/libemail-engine/e-mail-session.c
+++ b/src/libemail-engine/e-mail-session.c
@@ -1693,6 +1693,17 @@ mail_session_create_vfolder_context (EMailSession *session)
        return em_vfolder_context_new ();
 }
 
+static gpointer
+mail_session_init_null_provider_once (gpointer unused)
+{
+       camel_null_store_register_provider ();
+
+       /* Make sure ESourceCamel picks up the "none" provider. */
+       e_source_camel_generate_subtype ("none", CAMEL_TYPE_SETTINGS);
+
+       return NULL;
+}
+
 static void
 e_mail_session_class_init (EMailSessionClass *class)
 {
@@ -1879,19 +1890,19 @@ e_mail_session_class_init (EMailSessionClass *class)
                G_TYPE_STRING, 2,
                G_TYPE_UINT,
                G_TYPE_STRING);
-
-       camel_null_store_register_provider ();
-
-       /* Make sure ESourceCamel picks up the "none" provider. */
-       e_source_camel_generate_subtype ("none", CAMEL_TYPE_SETTINGS);
 }
 
 static void
 e_mail_session_init (EMailSession *session)
 {
+       static GOnce init_null_provider_once = G_ONCE_INIT;
        GHashTable *auto_refresh_table;
        GHashTable *junk_filters;
 
+       /* Do not call this from the class_init(), to avoid a claim from Helgrind about a lock
+          order violation between CamelProvider's global lock and glib's GType lock. */
+       g_once (&init_null_provider_once, mail_session_init_null_provider_once, NULL);
+
        auto_refresh_table = g_hash_table_new_full (
                (GHashFunc) g_str_hash,
                (GEqualFunc) g_str_equal,