static public String protocolAndHost {
        get {
            if (protocolAndHost == null) {
                //memoize
                String orgId = UserInfo.getOrganizationId();
                String userId = UserInfo.getUserId();
                String sessionId = UserInfo.getSessionId();

                //use getSalesforceBaseUrl within batches and schedules (not Visualforce), and fix inconsistent protocol
                if (sessionId == null) return Url.getSalesforceBaseUrl().toExternalForm().replace('http:', 'https:');

                PageReference pr = new PageReference('/id/' + orgId + '/' + userId);
                pr.getParameters().put('oauth_token', sessionId);
                pr.getParameters().put('format', 'json');

                //within test context use url class, else derive from identity api
                String data = Test.isRunningTest() ? '{"urls": {"rest": "' + Url.getSalesforceBaseUrl().toExternalForm() + '"}}' : pr.getContent().toString();
                Map<String,Object> result = (Map<String,Object>)Json.deserializeUntyped(data);
                Map<String,Object> urls = (Map<String,Object>)result.get('urls');

                //compose pod from the REST endpoint
                Url rest = new Url((String)urls.get('rest'));
                protocolAndHost = rest.getProtocol() + '://' + rest.getHost();
            }

            return protocolAndHost;
        }
    }

403 status code return from request to https://na16.salesforce.com/id/00Dj0000001tO6KEAU/005j000000BYfmLAAT?format=json&oauth_token=00Dj0000001tO6K%21AQwAQHsiQOYRd7vpLI2.iY0vRYMzUbbiXUkZQiLdx593O2hxAbtIrbjU3TfYa6KTNPBAeVA.Pg3pUQQOrWlJgbsbG7ppWcU8&inline=1
An unexpected error has occurred. Your solution provider has been notified.