Remove msgCtx in Resolvers after the XPath2.0 evaluation

We are keeping a reference to the message context in both GetPropertyFunctionResolver
and DOMSynapseXPathVariableResolver (hold by SynapseXpath ultimately).
But the resolveFunction method should always have the new message context to be evaluated.
Hence removing the msgCtx in Resolvers after the XPath2.0 evaluation.

Fixes: wso2/api-manager#1977

modules/core/src/main/java/org/apache/synapse/util/xpath/DOMSynapseXPathVariableResolver.java

@@ -31,4 +31,8 @@ public Object resolveVariable(QName variable) {
             throw new SynapseException("DOM Synapse XPATH variable resolution failed",e);
         }
     }
+
+    public void setSynCtx(MessageContext synCtx) {
+        this.synCtx = synCtx;
+    }
 }

modules/core/src/main/java/org/apache/synapse/util/xpath/GetPropertyFunctionResolver.java

@@ -15,9 +15,14 @@ public GetPropertyFunctionResolver(MessageContext synCtx) {
     }
 
     public XPathFunction resolveFunction(QName functionName, int arity) {
+
         if (SynapseXPathConstants.GET_PROPERTY_FUNCTION.equals(functionName.getLocalPart())) {
             return new GetPropertyFunction(synCtx);
         }
         return null;
     }
+
+    public void setSynCtx(MessageContext synCtx) {
+        this.synCtx = synCtx;
+    }
 }

modules/core/src/main/java/org/apache/synapse/util/xpath/SynapseXPath.java

@@ -670,12 +670,19 @@ public synchronized String evaluateDOMXPath(MessageContext synCtx) throws XPathE
             doomElement = convertToDOOM(element);
         }
         domXpath.setNamespaceContext(domNamespaceMap);
-        domXpath.setXPathFunctionResolver(new GetPropertyFunctionResolver(synCtx));
-        domXpath.setXPathVariableResolver(new DOMSynapseXPathVariableResolver(this.getVariableContext(), synCtx));
+        GetPropertyFunctionResolver getPropertyFunctionResolver = new GetPropertyFunctionResolver(synCtx);
+        DOMSynapseXPathVariableResolver domSynapseXPathVariableResolver =
+                new DOMSynapseXPathVariableResolver(this.getVariableContext(), synCtx);
+        domXpath.setXPathFunctionResolver(getPropertyFunctionResolver);
+        domXpath.setXPathVariableResolver(domSynapseXPathVariableResolver);
         /* Compile the original expression again with Saxon to be evaluated with XPath 2.0 */
         XPathExpression expr = domXpath.compile(getExpression());
         Object result = expr.evaluate(doomElement);
 
+        // Set message contexts in resolvers to null to avoid memory leaks. Resolvers are initialized for each evaluation
+        domSynapseXPathVariableResolver.setSynCtx(null);
+        getPropertyFunctionResolver.setSynCtx(null);
+
         if (result != null) {
             return result.toString();
         }