PolarizerEfficiency algorithm validation crash

[rbauststfc]

Original reporter: Dirk from the SANS Group at ISIS

Describe the bug

The PolarizerEfficiency algorithm includes some input validation to check that the size of the workspace group matches the number of spin states that have been passed in. If these do not match then we are currently not returning early to prevent the rest of the validation from continuing. This can cause a crash as in the steps that follow we try to look up the workspace in the group that matches the 00 spin state, but this may be at an index that doesn't exist in the workspace group.

To get the crash the algorithm seems to need to be run from the dialog rather than from a script, otherwise you just see the exception (Exception: WorkspaceGroup - index out of range. Requested=3, current size=2) logged in the messages pane.

To Reproduce

To see the exception logged in the messages pane, run:

ws00 = CreateSampleWorkspace('Histogram', Function='User Defined', UserDefinedFunction='name=UserFunction,Formula=0.5*exp(-0.0733*12*x*(1-0.1))',XUnit='Wavelength', xMin='1',XMax='8', BinWidth='1', NumBanks='1', BankPixelWidth='1')
ws01 = CloneWorkspace(ws00)

grp = GroupWorkspaces([ws00,ws01])
eCell = CreateSampleWorkspace('Histogram', Function='User Defined', UserDefinedFunction='name=UserFunction,Formula=(1 + tanh(0.0733 * 12 * x * 0.2))/2',XUnit='Wavelength', xMin='1',XMax='8', BinWidth='1', NumBanks='1', BankPixelWidth='1')

psm = PolarizerEfficiency(grp, eCell, SpinStates="11,10,01,00")

Having done this, you can reproduce the crash by running the PolarizerEfficiency algorithm from the algorithm dialog using the inputs given above.

Expected behavior

We should display a suitable error message and Mantid shouldn't crash.

Platform/Version (please complete the following information):

• Mantid Version: 6.11 (when algorithm was first added)

Additional context

I think we just need to change this if statement in the validateInputs method:

  if (spinStates.size() != inputWsCount) {
    errorList[PropertyNames::SPIN_STATES] =
        "The number of workspaces in the input WorkspaceGroup (" + std::to_string(inputWsCount) +
        ") does not match the number of spin states provided (" + std::to_string(spinStates.size()) + ").";
  }

to be:

  if (spinStates.size() != inputWsCount) {
    errorList[PropertyNames::SPIN_STATES] =
        "The number of workspaces in the input WorkspaceGroup (" + std::to_string(inputWsCount) +
        ") does not match the number of spin states provided (" + std::to_string(spinStates.size()) + ").";
    return errorList;
  }

And add appropriate test coverage.