• Danny Hindson's avatar
    Fix access violation in MonteCarloAbsorption · 9918c93b
    Danny Hindson authored
    The unit test suite MonteCarloAbsorptionTest has occasionally been failing
    with an access violation on Jenkins. I've been able to track this down to
    the test Lambda_StepSize_Two_Linear_Interpolation which was failing
    
    I can reproduce this locally by modifying the test to repeat 1000 times
    and it eventually fails. Although the access violation occurs in a couple
    of different places:
    a) the y() or e() vector in the 0th histogram is sometimes empty once the
    MonteCarloAbsorption algorithm completes which causes some of the test asserts
    to fall over because they assume y(0).front() or e(0).front
    b) sometimes the code in MonteCarloAbsorption itself has an access violation
    when yIndexOfX returns an index out of range of the histogram
    
    Both problems happen in the multi-threaded code in MonteCarloAbsorption and
    it seems to be the interaction between the call to MatrixWorkspace::yIndexOfX
    and the code that updates a histogram with interpolated results via
    MatrixWorkspace::setHistogram. Something in these two functions isn't threadsafe
    I found that yIndexOfX calls IsHistogramData() which checks the 0th
    histogram in the workspcae even if the MonteCarloAbsorption code is
    processing the 1st, 2nd etc workspace
    I think if the 0th histogram is in the process of being updated by another
    thread then yIndexOfX can fail or else the setHistogram call fails.
    I have done two changes:
    - only call yIndexOfX once (there was a redundant 2nd call)
    - I have modified yIndexOfX so it takes an additional histogram index parameter.
    This allows me to keep each MonteCarloAbsorption thread interacting with a
    single histogram only. I have defaulted the new parameter to zero so other
    code behaves as before
    9918c93b