Improve thread-safety of cow_ptr

[Russell Taylor]

When vectors are shared between spectra and an algorithm is breaking that sharing, you can have a problem with the original vector being destroyed because the check for uniqueness in cow_ptr::access always fails if multiple threads are checking at the same time. I.e. the reference count goes from, e.g., 2 to zero if two threads go through simultaneously on their final pass.

[Russell Taylor]

(In [5033]) Add critical section to cow_ptr::access method to prevent original vector being lost when sharing is broken. Appears to have no noticeable impact on speed. Re #1353.

[Russell Taylor]

(In [5078]) The changes to cow_ptr solved one issue, but not the one at hand which is that you can still wind up reading from a vector that's been/being written to by another thread. Re #1353.

[Nick Draper]

Bulk move of tickets to iteration 27, if your ticket is essential for Iteration 26 then move it back.

[Russell Taylor]

This is partially fixed, but without bigger changes I think we just have to accept that care is needed when breaking sharing if input and output workspaces are the same.

i.e. code should look like this (dataX before readX).

MantidVec& xOut = outputWS->dataX(i); Make sure reference to input X vector is obtained after output one because in the case where the input & output workspaces are the same, it might move if the vectors were shared. const MantidVec& xIn = inputWS->readX(i);

[Russell Taylor]

Try again to make code look right....

MantidVec& xOut = outputWS->dataX(i);
// Make sure reference to input X vector is obtained after output one because in the case  
// where the input & output workspaces are the same, it might move if the vectors were shared.
const MantidVec& xIn = inputWS->readX(i)

[Owen Arnold]

Cannot set up threading schenario to verify that critical portion of the code is thread safe. Can verify that unit tests utilisting cow_ptrs pass such as test_LoadPreNexus, so unlikely to have broken core functionality.

[Stuart Campbell]

This ticket has been transferred to github issue ​2200