** Changed in: mahara
    Milestone: 18.04.0 => 18.10.0

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  Destructor methods that access global variables can cause crashes

Status in Mahara:

Bug description:
  See : https://mahara.org/interaction/forum/topic.php?id=7397

  Here's the problem (which appears to be intermittent):

  1. ArtefactType has a destructor method, which gets called when an artefact 
object is garbage-collected by PHP.
  2. PHP runs destructor methods and garbage collects variables in no 
guaranteed order when exit() is called.
  3. Sometimes the ArtefactType destructor method tries to call DML methods, 
which use the global variable $db
  4. Apparently sometimes the $db global variable has already been garbage 
collected by the time the destructor runs
  5. This causes a fatal "method on a non-object" crash.

  It's unclear why this error has only started happening recently. It
  might be due to a change in the behavior of PHP's garbage collector,
  or it might be from new Mahara code leaving some artefacts with their
  $dirty flag set (which triggers that artefact commit).

  In either case, both global variables and implicit destructor methods
  are considered harmful design practices, in part because of this
  particular thing. So it's high time we got rid of these __destruct()

  Alternatively, as a workaround for older Mahara versions, we could add
  some code to the top of each __destruct() method that re-creates the
  global $db if it's not set.

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