config: Speed up ACO & sorcery initialization

When starting Asterisk in the foreground, there is a perceptible delay
when loading modules that use the ACO and sorcery config frameworks.
For example, a lightly configured res_pjsip took 853ms to load on my
VM.

I tracked down the slowness to the XPath queries used to associate the
relevant documentation with the config options. One improvement was
adding a call to xmlXPathOrderDocElems after loading an XML document.
From the libxml2 docs:

  Call this routine to speed up XPath computation on static documents.

The second change was to remove recursive descent and wildcard
operators from the XPath queries. After these changes, res_pjsip takes
85ms to load on my VM and there is no longer a perceptible delay when
starting Asterisk in the foreground.

Change-Id: I45d457f1580e26bf5a2b0dab16e8e9ae46dcbd82

main/config_options.c

@@ -998,11 +998,11 @@ static int xmldoc_update_config_type(const char *module, const char *name, const
 
 	/* If we already have a syntax element, bail. This isn't an error, since we may unload a module which
 	 * has updated the docs and then load it again. */
-	if ((results = ast_xmldoc_query("//configInfo[@name='%s']/*/configObject[@name='%s']/syntax", module, name))) {
+	if ((results = ast_xmldoc_query("/docs/configInfo[@name='%s']/configFile/configObject[@name='%s']/syntax", module, name))) {
 		return 0;
 	}
 
-	if (!(results = ast_xmldoc_query("//configInfo[@name='%s']/*/configObject[@name='%s']", module, name))) {
+	if (!(results = ast_xmldoc_query("/docs/configInfo[@name='%s']/configFile/configObject[@name='%s']", module, name))) {
 		ast_log(LOG_WARNING, "Cannot update type '%s' in module '%s' because it has no existing documentation!\n", name, module);
 		return XMLDOC_STRICT ? -1 : 0;
 	}
@@ -1068,7 +1068,7 @@ static int xmldoc_update_config_option(struct aco_type **types, const char *modu
 		return XMLDOC_STRICT ? -1 : 0;
 	}
 
-	if (!(results = ast_xmldoc_query("//configInfo[@name='%s']/*/configObject[@name='%s']/configOption[@name='%s']", module, object_name, name))) {
+	if (!(results = ast_xmldoc_query("/docs/configInfo[@name='%s']/configFile/configObject[@name='%s']/configOption[@name='%s']", module, object_name, name))) {
 		ast_log(LOG_WARNING, "Could not find option '%s' with type '%s' in module '%s'\n", name, object_name, module);
 		return XMLDOC_STRICT ? -1 : 0;
 	}

main/xml.c

@@ -96,6 +96,9 @@ struct ast_xml_doc *ast_xml_open(char *filename)
 	ast_log(LOG_NOTICE, "XSLT support not found. XML documentation may be incomplete.\n");
 #endif /* HAVE_LIBXSLT */
 
+	/* Optimize for XPath */
+	xmlXPathOrderDocElems(doc);
+
 	return (struct ast_xml_doc *) doc;
 }