[gdome] Fix thread issues

[Gustavo Sverzut Barbieri <Gustavo Barbieri indt org br>]

While developing a Gdome application using threads, I found an issue with the 
way it handles singletons, like dome-xml-domimpl.c and gdome-xpath-xpeval.c.

You first test if global instance is NULL, if not you allocate the new, 
_EMPTY_ instance directly to the global instance, then goes to fill needed 
fields, like "vtab".

Problem happens if you allocate and is preempted by system. Global pointer is 
not NULL anymore, however you don't have any required field, like "vtab", and 
can segfault your application.

This could be solved easily without any locks, just use a local variable while 
the instance is not setup, just then make it global.

Attached patch fix this problem.

Thanks,

-- 
Gustavo Sverzut Barbieri
------------------------
INdT, Recife, Brazil

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diff -ur gdome2-fix/libgdome/gdomecore/gdome-xml-domimpl.c gdome2-orig/libgdome/gdomecore/gdome-xml-domimpl.c
--- gdome2-fix/libgdome/gdomecore/gdome-xml-domimpl.c	2006-05-18 15:02:22.000000000 -0300
+++ gdome2-orig/libgdome/gdomecore/gdome-xml-domimpl.c	2005-05-13 04:05:56.000000000 -0300
@@ -70,11 +70,10 @@
 GdomeDOMImplementation *
 gdome_xml_di_mkref (void) {
 	if (gdome_xml_DOMImplementation == NULL) {
-		Gdome_xml_DOMImplementation *n;
-		n = g_new0 (Gdome_xml_DOMImplementation, 1);
-		n->refcnt = 1;
-		n->vtab = &gdome_xml_di_vtab;
-		gdome_xml_DOMImplementation = n;
+		gdome_xml_DOMImplementation = g_new (Gdome_xml_DOMImplementation, 1);
+		memset(gdome_xml_DOMImplementation, 0, sizeof(Gdome_xml_DOMImplementation));
+		gdome_xml_DOMImplementation->refcnt = 1;
+		gdome_xml_DOMImplementation->vtab = &gdome_xml_di_vtab;
 	} else
 		gdome_xml_DOMImplementation->refcnt++;
 
diff -ur gdome2-fix/libgdome/xpath/gdome-xpath-xpeval.c gdome2-orig/libgdome/xpath/gdome-xpath-xpeval.c
--- gdome2-fix/libgdome/xpath/gdome-xpath-xpeval.c	2006-05-18 15:01:55.000000000 -0300
+++ gdome2-orig/libgdome/xpath/gdome-xpath-xpeval.c	2006-05-18 15:11:48.000000000 -0300
@@ -53,11 +53,9 @@
 gdome_xpath_xpeval_mkref (void)
 {
   if (gdome_xpath_XPathEvaluator == NULL) {
-		Gdome_xpath_XPathEvaluator *n;
-		n = g_new0 (Gdome_xpath_XPathEvaluator, 1);
-    n->refcnt = 1;
-    n->vtab = &gdome_xpath_xpeval_vtab;
-		gdome_xpath_XPathEvaluator = n;
+    gdome_xpath_XPathEvaluator = g_new (Gdome_xpath_XPathEvaluator, 1);
+    gdome_xpath_XPathEvaluator->refcnt = 1;
+    gdome_xpath_XPathEvaluator->vtab = &gdome_xpath_xpeval_vtab;
   } else
     gdome_xpath_XPathEvaluator->refcnt++;