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Node on RHEL7

In August 2017 we implemented Swarthmore College’s PDF accessibility tool for Moodle. This required us to stand up a Node.js application, which was a new experience for us. Our environment was RHEL7, and our preferred web server Apache.

Application deployment

We followed our usual Capistrano principles for deploying the application. We created a simple project with all the Capistrano configuration and then mounted the Swarthmore project as a git submodule in a top-level directory named public. We configured the Capistrano npm module to use public as its working directory to ensure that the various node modules are installed on deployment.

PM2

PM2 is a Node process manager; its role is to ensure that the application runs at boot. To use it, we first need to install it globally:

sudo npm install -g pm2

Next, we create an ecosystem.json file. This needs to be in the root of the project repository; since we’re using Capistrano we define it in shared/public and symlink it on deploy. This is what ours looked like:

{
"apps": [{
    "name": "{NAME}",
    "script": "./index.js",
    "cwd": "/var/www/{NAME}/current/public",
    "error_file": "/var/www/{NAME}/current/logs/{NAME}.err.log",
    "out_file": "/var/www/{NAME}/current/logs/app.out.log",
    "exec_mode": "fork_mode"
}] }

All straightforward. We create a new user on the unix platform to own this job, have it start the process:

sudo -u {USER} pm2 start ecosystem.json

We can run a second command which generates the necessary syntax for setting up the systemd commands:

sudo -u {USER} pm2 startup

Apache

Having done all that, the node application is happily running on port 8080. We’re not interested in exposing that port in our environment, so we add a proxy pass to our standard Apache configuration for that virtual host:

        ProxyRequests on
        ProxyPass / http://localhost:8080/

We’ll have to revisit this if we ever want to have a second node application on the system, but for now it works.

Featured image by Hermann Luyken [CC BY-SA 3.0 or GFDL], from Wikimedia Commons.

Critical pagination

This is a story about Moodle, PHP, Active Directory, OpenLDAP, and how I stared a problem in the face for two days without realizing what I was looking at.

Pagination

I assumed maintenance of the LDAP syncing scripts plugin (local_ldap) in 2016. One thing I did was to add PHPUnit coverge which I wrote about in Writing LDAP unit tests for a Moodle plugin.

I’ve received reports about a possible bug with the plugin, Active Directory, and large numbers of users. After standing up a test Active Directory server (which is a story for another day), I’ve been extending the unit tests for the local_ldap module to take advantage of pagination.

PHP added support for LDAP pagination in PHP 5.4. Moodle added support soon after in Moodle 2.4 (MDL-36119). Beyond some range queries on the Active Directory side, there wasn’t anything in the plugin using pagination. The queries quickly revealed problems with the Active Directory code:

1) local_ldap_sync_testcase::test_cohort_group_sync
ldap_list(): Partial search results returned: Sizelimit exceeded

/var/www/moodle/htdocs/local/ldap/locallib.php:144
/var/www/moodle/htdocs/local/ldap/locallib.php:626
/var/www/moodle/htdocs/local/ldap/tests/sync_test.php:176
/var/www/moodle/htdocs/lib/phpunit/classes/advanced_testcase.php:80

Adding pagination is fairly straightforward, but you have to take care because not all LDAP implementations support it. Here’s an example cribbed from Moodle’s implementation:

$connection = $this->ldap_connect(); // Connection and bind.
$ldappagedresults = ldap_paged_results_supported($this->config->ldap_version, $connection);
$ldapcookie = '';
do {
	if ($ldappagedresults) {
    	ldap_control_paged_result($connection, $this->config->pagesize, true, $ldapcookie);
    }
    ... // Whatever LDAP task you're doing
    if ($ldappagedresults) {
    	ldap_control_paged_result_response($connection, $ldapresult, $ldapcookie);
    }
} while ($ldappagedresults && $ldapcookie !== null && $ldapcookie != '');

If the LDAP server doesn’t support pagination all the extra code is a no-op and you pass through the do…while loop once.

I wound up adding pagination in three places: the code that gets all the groups from the LDAP server, the code that searches for all the distinct attribute values (e.g. all possible values of eduPersonAffiliation), and the code in the unit test itself which tears down the test container in LDAP. Everything passed in Active Directory, so I made what I figured would be a pro forma push to Travis to test the code against OpenLDAP. I came back from lunch to read an error message I’d never even heard of:

ldap_delete(): Delete: Critical extension is unavailable

Critically unavailable

I’ve spent a fair amount of time debugging bizarre LDAP problems but “Critical extension is unavailable” was a new one on me:

1) local_ldap_sync_testcase::test_cohort_group_sync
ldap_delete(): Delete: Critical extension is unavailable
/home/travis/build/moodle/local/ldap/tests/sync_test.php:405
/home/travis/build/moodle/local/ldap/tests/sync_test.php:67
/home/travis/build/moodle/lib/phpunit/classes/advanced_testcase.php:80

Researching this phrase led me to a discovery: if you’ve run a paginated query against LDAP in PHP, you need to tear down that connection and reconnect to the server afterwards. The code in question was in the PHPUnit code which tore down the environment between tests. It runs two queries; one deletes all the users and groups while the second deletes the organizational units. This was code I’d taken from the Moodle LDAP authentication module (auth_ldap) and extended with pagination when the Active Directory tests failed.

Mocked up, this is what the code did before I modified it:

Establish LDAP connection
Get the top-level information about the test container
Get the users and groups from the test container
Delete those users and groups from the test container
Get the organizational units from the test container
Delete the organizational units from the test container
Delete the test container

After adding pagination and connection closures, the code did this:

Establish LDAP connection
Get the top-level information about the test container
Do
	Get the users and groups from the test container
	Delete those users and groups from the test container
Loop
Close LDAP connection
Establish LDAP connection
Do
	Get the organizational units from the test container
	Delete the organizational units from the test container
Loop
Close LDAP connection
Establish LDAP connection
Delete the test container

And it still didn’t work. I played around with various permutations for hours but didn’t make any progress. It was Friday, I was tired, I went home for a three-day weekend and didn’t think about it at all (maybe a little). When I got back in the office the problem was staring me right in the face.

In the old, unpaginated code, it wasn’t a problem to invoke ldap_delete after retrieving results, using the same LDAP connection. With pagination that logic doesn’t work anymore; we need to ensure we have all the results first, recreate the connection, then separately run the deletes. Thus modified, the logic is like this:

Establish LDAP connection
Get the top-level information about the test container
Do
	Get the users and groups from the test container
Loop
Close LDAP connection
Establish LDAP connection
Delete those users and groups from the test container
Do
	Get the organizational units from the test container
Loop
Close LDAP connection
Establish LDAP connection
Delete the organizational units from the test container
Delete the test container

The tests pass now. It’s interesting to me that this was never an issue with Active Directory, only with OpenLDAP. It’s a valuable lesson about when to refactor your code and check and your assumptions.

Featured image by Nevit Dilmen [GFDL or CC-BY-SA-3.0], from Wikimedia Commons

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