MDL-58018 core: Add support to open sessions without a lock

lib/classes/session/handler.php

@@ -34,6 +34,9 @@ defined('MOODLE_INTERNAL') || die();
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 abstract class handler {
+    /** @var boolean $haslock does the session need and/or have a lock? */
+    protected $haslock = false;
+
     /**
      * Start the session.
      * @return bool success
@@ -42,6 +45,49 @@ abstract class handler {
         return session_start();
     }
 
+    /**
+     * Write the session and release lock. If the session was not intentionally opened
+     * with a write lock, then we will abort the session instead if able.
+     */
+    public function write_close() {
+        if ($this->has_writelock()) {
+            session_write_close();
+            $this->haslock = false;
+        } else {
+            $this->abort();
+        }
+    }
+
+    /**
+     * Release lock on the session without writing it.
+     * May not be possible in older versions of PHP. If so, session may be written anyway
+     * so that any locks are released.
+     */
+    public function abort() {
+        session_abort();
+        $this->haslock = false;
+    }
+
+    /**
+     * This is called after init() and before start() to indicate whether the session
+     * opened should be writable or not. This is intentionally captured even if your
+     * handler doesn't support non-locking sessions, so that behavior (upon session close)
+     * matches closely between handlers.
+     * @param bool $needslock true if needs to be open for writing
+     */
+    public function set_needslock($needslock) {
+        $this->haslock = $needslock;
+    }
+
+    /**
+     * Has this session been opened with a writelock? Your handler should call this during
+     * start() if you support read-only sessions.
+     * @return bool true if session is intended to have a write lock.
+     */
+    public function has_writelock() {
+        return $this->haslock;
+    }
+
     /**
      * Init session handler.
      */