Multiple-Reader/Single-Writer locks, or "Busy" flag for workspace access

[mantid-builder]

Original Reporter: @jzikovsky

Original Owner: @jzikovsky

Refs http://trac.mantidproject.org/mantid/ticket/4516: It is possible to have destructive access to a workspace (via the PlusMD algorithm, for example), while the GUI thread is busy accessing it to draw it.

We can prevent segfaults by either: implementing a "busy" flag on workspaces, or by using Multiple-Reader/Single-Writer locks.

http://stackoverflow.com/questions/989795/example-for-boost-shared-mutex-multiple-reads-one-write

Investigate.

Edit: Here is a sample script that could cause segfaults if using GUI views on the MDW workspace while the loop runs:

import numpy as np

CreateMDWorkspace(Dimensions='3',Extents='0,10,0,10,0,10',Names='x,y,z',Units='m,m,m',MaxRecursionDepth='20',OutputWorkspace='mdw', SplitInto='2', SplitThreshold='150')
FakeMDEventData(InputWorkspace='mdw',UniformParams='1e+05')
plotSlice('mdw')

num = 100
xs = np.random.rand(num)*10.0
ys = np.random.rand(num)*10.0
zs = np.random.rand(num)*10.0
for (x,y,z) in zip(xs, ys, zs):
    CreateMDWorkspace(Dimensions='3',Extents='0,10,0,10,0,10',Names='x,y,z',Units='m,m,m',MaxRecursionDepth='20',OutputWorkspace='extra')
    FakeMDEventData(InputWorkspace='extra',PeakParams='1e4, %g,%g,%g, 0.5'% (x,y,z))
    PlusMD('mdw', 'extra', OutputWorkspace='mdw')
    PlusMD('mdw', 'extra', OutputWorkspace='mdw')
    PlusMD('mdw', 'extra', OutputWorkspace='mdw')
    PlusMD('mdw', 'extra', OutputWorkspace='mdw')

[mantid-builder]

This issue was originally trac ticket 4554