[5.3] Fix OperationQueue scheduling logic to prevent disruption of operation lists #2953
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…n lists I believe the original intention was to adust `__nextPriorityOperation`. Doing so for `__nextOperation` has no sense and leads to crosslinking of the main operation list and corresponding priority operation list. OperationQueue in such state would crash or hang while attempting to schedule subsequent operation.
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OperationQueue would crash or hang if we force "non-natural" execution order. E.g. when first operation depends on second.
The reason is how we work with internal operation lists in
OperationQueue._schedule(). I believe the original intention was to adjust__nextPriorityOperation, but instead we are modifying__nextOperation. This has no sense and leads to crosslinking of the main operation list and corresponding priority operation list. Subsequent scheduling would try to read already finished operation and hang. Also, at this point queue doesn't hold operation object anymore, so trying to read dead object is pretty realistic scenario.Suggested test case checks scenario described above, and ensures correct operation order as a bonus.
(cherry picked from commit 9f44ed3, original PR #2930)