Generate an error message if an external auth username is already in use.

If changing authentication method, or authentication username for a user
and that username is already in use within that auth method, Mahara was
hitting an unrecoverable error because of unique constraints in the
database.

Adding this to the validation stage give sa much more appropriate error
message.
Signed-off-by: Andrew Robert Nicols <andrew.nicols@luns.net.uk>

htdocs/admin/users/edit.php

@@ -164,12 +164,25 @@ $siteform = pieform(array(
 
 function edituser_site_validate(Pieform $form, $values) {
     global $USER, $SESSION;
+    if (!$user = get_record('usr', 'id', $values['id'])) {
+        return false;
+    }
     $maxquotaenabled = get_config_plugin('artefact', 'file', 'maxquotaenabled');
     $maxquota = get_config_plugin('artefact', 'file', 'maxquota');
     if ($maxquotaenabled && $values['quota'] > $maxquota) {
         $form->set_error('quota', get_string('maxquotaexceededform', 'artefact.file', display_size($maxquota)));
         $SESSION->add_error_msg(get_string('maxquotaexceeded', 'artefact.file', display_size($maxquota)));
     }
+
+    // Check that the external username isn't already in use
+    if ($usedby = get_record_select('auth_remote_user',
+        'authinstance = ? AND remoteusername = ? AND localusr != ?',
+        array($values['authinstance'], $values['remoteusername'], $values['id']))
+    ) {
+        $usedbyuser = get_field('usr', 'username', 'id', $usedby->localusr);
+        $SESSION->add_error_msg(get_string('duplicateremoteusername', 'auth', $usedbyuser));
+        $form->set_error('remoteusername', get_string('duplicateremoteusernameformerror', 'auth'));
+    }
 }
 
 function edituser_site_submit(Pieform $form, $values) {

htdocs/lang/en.utf8/auth.php

@@ -81,4 +81,8 @@ $string['requiredfieldsset'] = 'Required profile fields set';
 $string['noauthpluginconfigoptions'] = 'There are no configuration options associated with this plugin';
 
 $string['hostwwwrootinuse'] = 'WWW root already in use by another institution (%s)';
+
+// Error messages for external authentication usernames
+$string['duplicateremoteusername'] = 'This external authentication username is already in use by the user %s. External authentication usernames must be unique within an authentication method.';
+$string['duplicateremoteusernameformerror'] = 'External authentication usernames must be unique within an authentication method.';
 ?>