[bugfix] guard against NPE in securitymanager

fixes #4670

Author: Nico Verwer

exist-core/src/main/java/org/exist/xquery/functions/securitymanager/IdFunction.java

@@ -77,13 +77,18 @@ private org.exist.dom.memtree.DocumentImpl functionId() {
 
             builder.startElement(new QName("id", SecurityManagerModule.NAMESPACE_URI, SecurityManagerModule.PREFIX), null);
 
-            builder.startElement(new QName("real", SecurityManagerModule.NAMESPACE_URI, SecurityManagerModule.PREFIX), null);
-            subjectToXml(builder, context.getRealUser());
-            builder.endElement();
+            final Subject realUser = context.getRealUser();
+            if (realUser != null) {
+                builder.startElement(new QName("real", SecurityManagerModule.NAMESPACE_URI, SecurityManagerModule.PREFIX), null);
+                subjectToXml(builder, realUser);
+                builder.endElement();
+            }
 
-            if (!sameUserWithSameGroups(context.getRealUser(), context.getEffectiveUser())) {
+            final Subject effectiveUser = context.getEffectiveUser();
+            if (effectiveUser != null && (
+                    realUser == null || !sameUserWithSameGroups(realUser, effectiveUser))) {
                 builder.startElement(new QName("effective", SecurityManagerModule.NAMESPACE_URI, SecurityManagerModule.PREFIX), null);
-                subjectToXml(builder, context.getEffectiveUser());
+                subjectToXml(builder, effectiveUser);
                 builder.endElement();
             }
 

exist-core/src/test/java/org/exist/xquery/functions/securitymanager/IdFunctionTest.java

@@ -183,15 +183,15 @@ public void differingByGroupRealAndEffectiveUsers() throws XPathException, Xpath
         expect(mckContext.getDocumentBuilder()).andReturn(new MemTreeBuilder());
         mckContext.popDocumentContext();
         expectLastCall().once();
-        expect(mckContext.getRealUser()).andReturn(mckRealUser).times(2);
+        expect(mckContext.getRealUser()).andReturn(mckRealUser);
         expect(mckRealUser.getName()).andReturn(realUsername);
         expect(mckRealUser.getGroups()).andReturn(new String[]{"realGroup1"});
         expect(mckRealUser.getId()).andReturn(101);
         expect(mckRealUser.getGroupIds()).andReturn(new int[] {101});
 
         final Subject mckEffectiveUser = EasyMock.createMock(Subject.class);
         final String effectiveUsername = "user1";
-        expect(mckContext.getEffectiveUser()).andReturn(mckEffectiveUser).times(2);
+        expect(mckContext.getEffectiveUser()).andReturn(mckEffectiveUser);
         expect(mckEffectiveUser.getId()).andReturn(101);
         expect(mckEffectiveUser.getName()).andReturn(effectiveUsername);
         expect(mckEffectiveUser.getGroups()).andReturn(new String[]{"realGroup1", "effectiveGroup1"});