MDL-72283 caching: internationalize redis session handler error

The 'unable to obtain session lock'-exception raised by the Redis
session handler is hardcoded in English and not all that useful
to the end user.

This change adds the error message to the lang/error.php and gives
the user further hints why the error might have occured and how it
could be fixed.

Signed-off-by: Daniel Ziegenberg <daniel@ziegenberg.at>

lang/en/error.php

@@ -532,6 +532,9 @@
 $string['sessionipnomatch'] = 'Sorry, but your IP number seems to have changed from when you first logged in.  This security feature prevents crackers stealing your identity while logged in to this site.  Normal users should not be seeing this message - please ask the site administrator for help.';
 $string['sessionipnomatch2'] = '<p>Sorry, but your IP number seems to have changed from when you first logged in. This security feature prevents crackers stealing your identity while logged in to this site. You may see this error if you use wireless networks or if you are roaming between different networks. Please ask the site administrator for more help.</p>
 <p>If you want to continue please press F5 key to refresh this page.</p>';
+$string['sessioncannotobtainlock'] = '<p>Unable to obtain lock for session id {$a->id} within {$a->acquiretimeout}.</p>
+<p>It is likely that another page ({$a->whohaslock}) is still running in another browser tab, or it did not release the lock due to an error.</p>
+<p>You can wait until the session lock timeout ({$a->lockexpire}) or you can restart your browser session. If this error persists, please notify the server administrator.</p>';
 $string['shortnametaken'] = 'Short name is already used for another course ({$a})';
 $string['sitepolicynotagreed'] = 'Site policy not agreed: <a href="{$a}">Click here to open the site policy.</a>';
 $string['scheduledbackupsdisabled'] = 'Scheduled backups have been disabled by the server admin';

lib/classes/session/redis.php

@@ -479,7 +479,14 @@ protected function lock_session($id) {
                 // phpcs:ignore
                 error_log("Cannot obtain session lock for sid: $id within $this->acquiretimeout seconds. " .
                     "It is likely another page ($whohaslock) has a long session lock, or the session lock was never released.");
-                throw new exception("Unable to obtain session lock");
+                $acquiretimeout = format_time($this->acquiretimeout);
+                $lockexpire = format_time($this->lockexpire);
+                $a = (object)[
+                    'id' => substr($id, 0, 10),
+                    'acquiretimeout' => $acquiretimeout,
+                    'whohaslock' => $whohaslock,
+                    'lockexpire' => $lockexpire];
+                throw new exception("sessioncannotobtainlock", 'error', '', $a);
             }
 
             if ($this->time() < $startlocktime + 5) {

lib/tests/session_redis_test.php

@@ -46,6 +46,11 @@ class core_session_redis_testcase extends advanced_testcase {
     protected $keyprefix = null;
     /** @var $redis The current testing redis connection */
     protected $redis = null;
+    /** @var int $acquiretimeout how long we wait for session lock in seconds when testing Redis */
+    protected $acquiretimeout = 1;
+    /** @var int $lockexpire how long to wait in seconds before expiring the lock when testing Redis */
+    protected $lockexpire = 70;
+
 
     public function setUp(): void {
         global $CFG;
@@ -72,8 +77,8 @@ public function setUp(): void {
 
         // Set a very short lock timeout to ensure tests run quickly.  We are running single threaded,
         // so unless we lock and expect it to be there, we will always see a lock.
-        $CFG->session_redis_acquire_lock_timeout = 1;
-        $CFG->session_redis_lock_expire = 70;
+        $CFG->session_redis_acquire_lock_timeout = $this->acquiretimeout;
+        $CFG->session_redis_lock_expire = $this->lockexpire;
 
         $this->redis = new Redis();
         $this->redis->connect(TEST_SESSION_REDIS_HOST);
@@ -165,7 +170,9 @@ public function test_session_blocks_with_existing_session() {
             $sessblocked->handler_read('sess1');
             $this->fail('Session lock must fail to be obtained.');
         } catch (\core\session\exception $e) {
-            $this->assertStringContainsString("Unable to obtain session lock", $e->getMessage());
+            $this->assertStringContainsString("Unable to obtain lock for session id sess1", $e->getMessage());
+            $this->assertStringContainsString('within 1 sec.', $e->getMessage());
+            $this->assertStringContainsString('session lock timeout (1 min 10 secs) ', $e->getMessage());
             $this->assertStringContainsString('Cannot obtain session lock for sid: sess1', file_get_contents($errorlog));
         }