How do I determine if a url or path alias is being used?

My apologies, a careful re-reading of the documentation, and it seems that my first impressions were wrong.

I assumed that most drupal pages were aliases of one type or another and that any drupal paths like /admin, admin/config, etc., were system urls. They are common to all drupal sites, after all.

I've developed a module with a hook_menu that uses a path that appears in a form - i.e. the site admin can specify a custom path. When specifying a new path, the validate functions checks to see if the path is available - i.e. /admin should NOT be available, therefore is not valid.

I assumed drupal_lookup_path( ... ) would be a fast lookup to see if that path is taken. That approach did not work, of course. Now I am using drupal_http_request() for the path validation, with a pass if a 404 code is encountered.

I'm not proud of this, it's an ugly hack. If you could advise of a better method, I would be grateful. ;-)

And how are path conflicts resolved in hook_menus of different contributed modules?

Thank you for that reply Mark - sorry I'm so late in replying.

OK - so I did some real testing with menu_get_item() in the context I was using.

I have a module that creates a page at some arbitrary url, defaults to 'members'.

To test this function I created a basic page at /node/122 and assigned it the url 'members' before enabling the module.

After enabling the module, the '/node/122' page is rendered up instead of the page I was expecting.

I tried every variation of $path in menu_get_item($path). NULL was returned in every case.

So I am reverting to drupal_http_request() to determine if a url is already in use.

Change to Issue title

Well that was a curve ball. Thank you Mark, for the response and the code sample.

It has been an interesting day. I tried a whole bunch of stuff.

First, the use case:

I have a module that creates a custom page. A feature of this module is that the admin can specify the url or page alias for the page the module creates. This location is stored as a site variable. This is easy to do:

/**
 * Implements hook_menu().
 */
function members_menu() {
  $user_path = variable_get('members_register_url', 'register/user');
  $items[$user_path] = array(
    'title' => 'Create new account',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('members_user_register_form'),
    'access callback' => 'user_is_anonymous',
    'type' => MENU_NORMAL_ITEM,
  );

  $members_path = variable_get('members_page_url', 'members');
  $items[$members_path] = array(
    'title' => 'Members',
    'description' => 'Login or register a username.',
    'page callback' => 'members_page',
    'access callback' => TRUE,
    'menu_name' => 'main-menu',
    'type' => MENU_SUGGESTED_ITEM,
  );

Now this module, to properly install itself, must determine if the default url for the variable is an available location, otherwise it tries another similar string until it find a url that is free.

The code for drupal_get_path() is not small or simple. And the code for menu_get_item() is hardly intuitive either. I simply tried what I thought might work without really trying to figure out how these two functions worked.

Here is the result of my trial and error testing:

In Drupal 7, I fell back to using drupal_http_request() before my test case installed correctly. I never could get menu_get_item() or drupal_http_request() to work the way I expected.

Then I switched to Drupal 8. The drupal_http_request() that worked in D7 method gave me a WSOD in D8. I also tried wrapping the code you provided above in a function. It also failed but did return a page. It tried about 1600 different urls before it gave up.

So I wrote another function that uses the php curl library. It works quite well and seems to be the fastest of any method I have tried.

This module (Members Page) is a sandbox project waiting for approval. The D8 version has all the latest test code in a file called members.inc, including a couple of test functions that did not work.

The default url's for the Members Page module are 'members' and 'register/user'.

To test, I did a fresh D8 install, then created some bogus pages and gave then url's of 'members' and 'register/user' respectively. Since these pages now existed, a successful install would create create the default pages at members2 and register/user2 instead.

The code below works for me with D8. I haven't tested yet with D7 or D6 yet but I think this may work across all versions since its basically just php and drupal independent.

To summarize: A function - say drupal_check_url_free($url) - is needed that provides a simple TRUE/FALSE return for pages that implement a relocatable menu router item. Something like this:

/**
 * Checks if the $url is available.
 *
 * @param string $url
 *  The url or path alias to be set.  This should be a simple string without any
 *  protocol (http:) leader, and without leading or trailing /'s.
 * @returns boolean TRUE | FALSE
 */
function drupal_check_url_free($url) {
  $url = 'http://' . $_SERVER['SERVER_NAME'] . '/' . $url;
  $cptr = curl_init();                    
  curl_setopt($cptr, CURLOPT_URL, $url);     // set url
  curl_setopt($cptr, CURLOPT_NOBODY, true);  // no body, just header
  curl_setopt($cptr, CURLOPT_CONNECTTIMEOUT, 2);
  $content = curl_exec($cptr);
  $info = curl_getinfo($cptr);
  $errs = curl_error($cptr);
  if (!empty($errs)){ 
    echo "<pre>"; print_r($errs); echo "</pre>"; 
  }
  curl_close($cptr);
  return $info['http_code'] == '404' ? TRUE : FALSE;
}

Is there any reason you're not doing this the most straightforward way?

The reason is simple - it's a rather effective security by obscurity technique.

As an example - every drupal website has a url '/user/register'. That is where every web bot first goes to try to register so they can leave spam.

Set up another separate page, say at /register/user (or wherever) and leave the original in place as a kind of honey trap for the web bots. Create a link to the real page that a real user would use. Web bots have to be far more clever to find the real page. If it's a moving target and maybe different at other websites, all the better.

The D8 version of Members Page has all the test code I tried, including a function that had the same code you had suggested. It would take all of five minutes (if you already have a D8 test bed set up) to try an install/uninstall cycle with the two different functions. The differences are quite stark.