Configure angular $http with a value obtained from server

Question

I'm building my (first) angular app that will have tokens inserted into headers (the content shown is for the most part taken from here)

angular.module('myApp')

.factory('sessionInjector', ['SessionService', function(SessionService) {  
   var sessionInjector = {
        request: function(config) {
            config.headers['x-session-token'] = SessionService.getToken();
            return config;
        }
    };
    return sessionInjector;
}])

.config(['$httpProvider', function($httpProvider) {  
    $httpProvider.interceptors.push('sessionInjector');
}])

The trouble I'm having is with SessionService - how can I initialize this with call to the server?

For example, this didn't work:

.factory('SessionService', ['$injector', function($injector){
    var token = "";

    return {
        getToken: function () {
            var http = $injector.get('$http');
            if (token === "") {
                http.get('http://localhost/api/auth/getToken').success(function (ret) {
                    token = ret;
                });
            }
            return token; 
            //I can see a $q/deferred/promise should be used somehow here... 
            //but I'm not sure it solves the problem I'm having...
        }
    }
}]);

because it just overloads my cpu to 100%...

Since it's my first angular app, I'm sure I'm missing something, but... what?

EDIT:
Another take on the matter... still doesn't work though... (again, uses up cpu, probably infinite loop)

  .factory('sessionData', function () {
    var currentToken = '[uninitialized-token]';

    return {
        getToken: function () {
            return currentToken;
        },
        setToken: function (token) {
            currentToken = token;
        }
    }
})

.factory('sessionInjector', ['sessionData', '$injector', '$q', function (sessionData, $injector, $q) {
    var sessionInjector = {
        request: function (config) {
            var deferred = $q.defer();
            var http = $injector.get('$http');
            http.get('http://localhost/api/auth/getToken').success(function (ret) {
                sessionData.setToken(ret);
                console.log("successfully authenticated with token " + sessionData.getToken());
                config.headers['x-header-sessionID'] = sessionData.getToken();
                deferred.resolve(config);
            })
            .error(function(){
                console.log("failed to authenticate");
                deferred.resolve(config);
            });

            return deferred.promise;
        }
    };
    return sessionInjector;
}])

.config(['$httpProvider', function ($httpProvider) {
    $httpProvider.interceptors.push('sessionInjector');
}])

.run(['$http', 'sessionData', function ($http, configs, sessionData) {
    $http.get('http://localhost/api/auth/testMethod').then(function (ret) {
        //do something...
    });
}])

Answer 1

Answer was actually pretty straight forward - if the targeted URL is for login, then don't inject anything (look for the comment the fix):

.factory('sessionData', function () {
    var currentToken = '[uninitialized-token]';

    return {
        getToken: function () {
            return currentToken;
        },
        setToken: function (token) {
            currentToken = token;
        }
    }
})

.factory('sessionInjector', ['sessionData', '$injector', '$q', function (sessionData, $injector, $q) {
    var sessionInjector = {
        request: function (config) {

            //The fix:
            if(config.url === 'http://localhost/api/auth/getToken')
                return config;

            var deferred = $q.defer();
            var http = $injector.get('$http');
            http.get('http://localhost/api/auth/getToken').success(function (ret) {
                sessionData.setToken(ret);
                console.log("successfully authenticated with token " + sessionData.getToken());
                config.headers['x-header-sessionID'] = sessionData.getToken();
                deferred.resolve(config);
            })
            .error(function(){
                console.log("failed to authenticate");
                deferred.resolve(config);
            });

            return deferred.promise;
        }
    };
    return sessionInjector;
}])

.config(['$httpProvider', function ($httpProvider) {
    $httpProvider.interceptors.push('sessionInjector');
}])

.run(['$http', 'sessionData', function ($http, configs, sessionData) {
    $http.get('http://localhost/api/auth/testMethod').then(function (ret) {
        //do something...
    });
}])