MDL-74586 rss: Make rss_get_userid_from_token() use table index

Due to missing `AND k.script = 'rss'` condition, the query did not make
use of the existing script-value compound index defined for the table.
So it had to perform the full sequential scan for all rows when
searching for the token. This had serious performance issues on sites
with many users, especially in case on non-existing token / key.

lib/rsslib.php

@@ -413,10 +413,15 @@ function rss_geterrorxmlfile($errortype = 'rsserror') {
 function rss_get_userid_from_token($token) {
     global $DB;
 
-    $sql = 'SELECT u.id FROM {user} u
-            JOIN {user_private_key} k ON u.id = k.userid
-            WHERE u.deleted = 0 AND u.confirmed = 1
-            AND u.suspended = 0 AND k.value = ?';
+    $sql = "SELECT u.id
+              FROM {user} u
+              JOIN {user_private_key} k ON u.id = k.userid
+             WHERE u.deleted = 0
+               AND u.confirmed = 1
+               AND u.suspended = 0
+               AND k.script = 'rss'
+               AND k.value = ?";
+
     return $DB->get_field_sql($sql, array($token), IGNORE_MISSING);
 }
 

lib/tests/rsslib_test.php

@@ -20,7 +20,7 @@
 
 global $CFG;
 require_once($CFG->libdir.'/simplepie/moodle_simplepie.php');
-
+require_once($CFG->libdir . '/rsslib.php');
 
 /**
  * These tests rely on the rsstest.xml file on download.moodle.org,
@@ -140,4 +140,19 @@ public function test_redirect() {
         $this->assertSame('Moodle News', $feed->get_title());
         $this->assertSame('http://moodle.org/mod/forum/view.php?f=1', $feed->get_link());
     }
+
+    /**
+     * Test that we can get the right user ID based on the provided private key (token).
+     *
+     * @covers ::rss_get_userid_from_token
+     */
+    public function test_rss_get_userid_from_token() {
+        global $USER;
+
+        $this->resetAfterTest();
+        $this->setGuestUser();
+
+        $key = rss_get_token($USER->id);
+        $this->assertSame(rss_get_userid_from_token($key), $USER->id);
+    }
 }