Python algorithm overwriting C++ loader causes crash on running Load

[Martyn Gigg]

When a Python algorithm has the same name and version as a C++ algorithm that is defined as a file loader, the Python alg silently overwrites the C++ one and crashes the file loader search.

To reproduce, put the code below in a file called LoadEventNexus.py and put it in the python plugins directory. Start MantidPlot and try to load any nexus file with Load and Mantid will crash

from mantid.kernel import *
from mantid.api import *

class LoadEventNexus(PythonAlgorithm):

    def PyInit(self):
      pass

    def PyExec(self):
      pass

AlgorithmFactory.subscribe(LoadEventNexus)

[Martyn Gigg]

Unsubscribe any loader that has been replaced by a Python algorithm.

If we don't then we get a crash in the FileLoaderRegistry that tries to cast to an incorrect type.

Refs #8177

Changeset: 3743bd34e97e02f92fd9bcb8547ef7cb7a9ce9f0

[Martyn Gigg]

Branch: bugfix/8177_pyalg_overwrite_cpp_loader_crash

Tester: Use the above python algorithm to confirm that when it is in the python plugins directory it removes LoadEventNexus from being usable by Load.

When you have confirmed that, close MantidPlot and add

    def version(self):
        return 2

to the class definition. Start MantidPlot and confirm that now LoadEventNexus version 1 can be used by Load again.

[Arturs Bekasovs]

Confirmed that before changes were made Mantid is crashing when trying to launch Load.

After the changes the problem seems to be solved. LoadEventNexus is not being checked as potential loading algorithm and thus is not crashing.

Changing the version of the added Python algorithm does what is expected - LoadEventNexus v1 is being checked and nothing goes wrong. LoadEventNexus v2 is not being checked, as it is not registered as loading algorithm.

Overall, everything works as advertised, hence closing the ticket.

[Arturs Bekasovs]

Merge remote-tracking branch 'origin/bugfix/8177_pyalg_overwrite_cpp_loader_crash'

Full changeset: ​98c0b337f4860a5893c1923ee45a097a8b7b35ba

[Stuart Campbell]

This ticket has been transferred to github issue ​9022