MDL-30026 improve session lock acquire timeouts and other minor cleanup

This is partially based on original patch by Tony Levi.

lang/en/error.php

@@ -433,6 +433,7 @@
 $string['sectionnotexist'] = 'This section does not exist';
 $string['sendmessage'] = 'Send message';
 $string['servicedonotexist'] = 'The service does not exist';
+$string['sessionwaiterr'] = 'Timed out while waiting for session lock.<br />Wait for your current requests to finish and try again later.';
 $string['sessioncookiesdisable'] = 'Incorrect use of require_key_login() - session cookies must be disabled!';
 $string['sessiondiskfull'] = 'The session partition is full. It is not possible to login at this time.<br /><br />Please notify server administrator.';
 $string['sessionerroruser'] = 'Your session has timed out.  Please login again.';

lib/dml/moodle_database.php

@@ -2204,9 +2204,10 @@ public function session_lock_supported() {
     /**
      * Obtain session lock
      * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
      * @return bool success
      */
-    public function get_session_lock($rowid) {
+    public function get_session_lock($rowid, $timeout) {
         $this->used_for_db_sessions = true;
     }
 

lib/dml/mssql_native_moodle_database.php

@@ -1227,18 +1227,33 @@ public function session_lock_supported() {
         return true;
     }
 
-    public function get_session_lock($rowid) {
+    /**
+     * Obtain session lock
+     * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
+     * @return bool success
+     */
+    public function get_session_lock($rowid, $timeout) {
         if (!$this->session_lock_supported()) {
             return;
         }
-        parent::get_session_lock($rowid);
+        parent::get_session_lock($rowid, $timeout);
+
+        $timeoutmilli = $timeout * 1000;
 
         $fullname = $this->dbname.'-'.$this->prefix.'-session-'.$rowid;
-        $sql = "sp_getapplock '$fullname', 'Exclusive', 'Session',  120000";
+        $sql = "sp_getapplock '$fullname', 'Exclusive', 'Session',  $timeoutmilli";
         $this->query_start($sql, null, SQL_QUERY_AUX);
         $result = mssql_query($sql, $this->mssql);
         $this->query_end($result);
 
+        if ($result) {
+            $row = mssql_fetch_row($result);
+            if ($row[0] < 0) {
+                throw new dml_sessionwait_exception();
+            }
+        }
+
         $this->free_result($result);
     }
 

lib/dml/mysqli_native_moodle_database.php

@@ -1203,10 +1203,17 @@ public function session_lock_supported() {
         return true;
     }
 
-    public function get_session_lock($rowid) {
-        parent::get_session_lock($rowid);
+    /**
+     * Obtain session lock
+     * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
+     * @return bool success
+     */
+    public function get_session_lock($rowid, $timeout) {
+        parent::get_session_lock($rowid, $timeout);
+
         $fullname = $this->dbname.'-'.$this->prefix.'-session-'.$rowid;
-        $sql = "SELECT GET_LOCK('$fullname',120)";
+        $sql = "SELECT GET_LOCK('$fullname', $timeout)";
         $this->query_start($sql, null, SQL_QUERY_AUX);
         $result = $this->mysqli->query($sql);
         $this->query_end($result);
@@ -1218,8 +1225,7 @@ public function get_session_lock($rowid) {
             if (reset($arr) == 1) {
                 return;
             } else {
-                // try again!
-                $this->get_session_lock($rowid);
+                throw new dml_sessionwait_exception();
             }
         }
     }

lib/dml/oci_native_moodle_database.php

@@ -1607,21 +1607,34 @@ public function session_lock_supported() {
         return $this->dblocks_supported;
     }
 
-    public function get_session_lock($rowid) {
+    /**
+     * Obtain session lock
+     * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
+     * @return bool success
+     */
+    public function get_session_lock($rowid, $timeout) {
         if (!$this->session_lock_supported()) {
             return;
         }
-        parent::get_session_lock($rowid);
+        parent::get_session_lock($rowid, $timeout);
 
         $fullname = $this->dbname.'-'.$this->prefix.'-session-'.$rowid;
         $sql = 'SELECT MOODLE_LOCKS.GET_LOCK(:lockname, :locktimeout) FROM DUAL';
-        $params = array('lockname' => $fullname , 'locktimeout' => 120);
+        $params = array('lockname' => $fullname , 'locktimeout' => $timeout);
         $this->query_start($sql, $params, SQL_QUERY_AUX);
         $stmt = $this->parse_query($sql);
         $this->bind_params($stmt, $params);
+        $start = time();
         $result = oci_execute($stmt, $this->commit_status);
+        $end = time();
         $this->query_end($result, $stmt);
         oci_free_statement($stmt);
+
+        if ($end - $start >= $timeout) {
+            //TODO: there has to be a better way to find out if lock obtained
+            throw new dml_sessionwait_exception();
+        }
     }
 
     public function release_session_lock($rowid) {

lib/dml/pgsql_native_moodle_database.php

@@ -1153,17 +1153,54 @@ public function session_lock_supported() {
         return true;
     }
 
-    public function get_session_lock($rowid) {
+    /**
+     * Obtain session lock
+     * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
+     * @return bool success
+     */
+    public function get_session_lock($rowid, $timeout) {
         // NOTE: there is a potential locking problem for database running
         //       multiple instances of moodle, we could try to use pg_advisory_lock(int, int),
         //       luckily there is not a big chance that they would collide
         if (!$this->session_lock_supported()) {
             return;
         }
 
-        parent::get_session_lock($rowid);
+        parent::get_session_lock($rowid, $timeout);
+
+        $timeoutmilli = $timeout * 1000;
+
+        $sql = "SET statement_timeout TO $timeoutmilli";
+        $this->query_start($sql, null, SQL_QUERY_AUX);
+        $result = pg_query($this->pgsql, $sql);
+        $this->query_end($result);
+
+        if ($result) {
+            pg_free_result($result);
+        }
+
         $sql = "SELECT pg_advisory_lock($rowid)";
         $this->query_start($sql, null, SQL_QUERY_AUX);
+        $start = time();
+        $result = pg_query($this->pgsql, $sql);
+        $end = time();
+        try {
+            $this->query_end($result);
+        } catch (dml_exception $ex) {
+            if ($end - $start >= $timeout) {
+                throw new dml_sessionwait_exception();
+            } else {
+                throw $ex;
+            }
+        }
+
+        if ($result) {
+            pg_free_result($result);
+        }
+
+        $sql = "SET statement_timeout TO DEFAULT";
+        $this->query_start($sql, null, SQL_QUERY_AUX);
         $result = pg_query($this->pgsql, $sql);
         $this->query_end($result);
 

lib/dml/sqlsrv_native_moodle_database.php

@@ -1286,17 +1286,33 @@ public function session_lock_supported() {
         return true;
     }
 
-    public function get_session_lock($rowid) {
+    /**
+     * Obtain session lock
+     * @param int $rowid id of the row with session record
+     * @param int $timeout max allowed time to wait for the lock in seconds
+     * @return bool success
+     */
+    public function get_session_lock($rowid, $timeout) {
         if (!$this->session_lock_supported()) {
             return;
         }
-        parent::get_session_lock($rowid);
+        parent::get_session_lock($rowid, $timeout);
+
+        $timeoutmilli = $timeout * 1000;
 
         $fullname = $this->dbname.'-'.$this->prefix.'-session-'.$rowid;
-        $sql = "sp_getapplock '$fullname', 'Exclusive', 'Session',  120000";
+        $sql = "sp_getapplock '$fullname', 'Exclusive', 'Session',  $timeoutmilli";
         $this->query_start($sql, null, SQL_QUERY_AUX);
         $result = sqlsrv_query($this->sqlsrv, $sql);
         $this->query_end($result);
+
+        if ($result) {
+            $row = sqlsrv_fetch_array($result);
+            if ($row[0] < 0) {
+                throw new dml_sessionwait_exception();
+            }
+        }
+
         $this->free_result($result);
     }
 

lib/dmllib.php

@@ -78,6 +78,18 @@ function __construct($error) {
     }
 }
 
+/**
+ * DML db session wait exception - triggered when session lock request times out.
+ */
+class dml_sessionwait_exception extends dml_exception {
+    /**
+     * Constructor
+     */
+    function __construct() {
+        parent::__construct('sessionwaiterr');
+    }
+}
+
 /**
  * DML read exception - triggered by SQL syntax errors, missing tables, etc.
  */