Putative fix for mutable special placement #51574
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jbytheway
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I think I figured out why mutable special placement was sometimes failing. Some joins could be postponed but others could appear pointing at the same tile and be satisfied in the absence of the postponed joins. Then, when the postponed joins are restored they cannot be satisfied. Avoid this by keeping track of the tiles with postponed joins and ensuring that they new joins pointing at the same tiles are added directly to the postponed list rather than the unresolved list.
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Sep 21, 2021
I think I figured out why mutable special placement was sometimes failing. Some joins could be postponed but others could appear pointing at the same tile and be satisfied in the absence of the postponed joins. Then, when the postponed joins are restored they cannot be satisfied. Avoid this by keeping track of the tiles with postponed joins and ensuring that they new joins pointing at the same tiles are added directly to the postponed list rather than the unresolved list.
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Summary
None
Purpose of change
I think I figured out why mutable special placement was sometimes failing. Some joins could be postponed but others could appear pointing at the same tile and be satisfied in the absence of the postponed joins. Then, when the postponed joins are restored they cannot be satisfied.
Describe the solution
Avoid this by keeping track of the tiles with postponed joins and ensuring that they new joins pointing at the same tiles are added directly to the postponed list rather than the unresolved list.
Describe alternatives you've considered
When these new joins appear we could re-insert the postponed joins for that tile back into the unresolved list, since there's now some hope that they could be resolved. But that would be more complicated.
Testing
I was never able to reliably reproduce the problem, and it happens with very low probability, so it's hard to test. We'll just have to see whether the problem recurs in CI sometime.
I did spawn a few anthills to verify that it's not obviously broken.
Additional context
Thanks to @eltank for providing the test failure log that helped track down the issue.