Possible backport for concaveHull

[mwtoews]

As reported shapely/shapely#1971 there is an issue with this example:

$ ./geos-3.12.0/bin/geosop -a "MULTIPOINT ((175682.210293 505233.365178), (175682.210295 505233.365177), (175684.375108 505254.524644), (175684.375109 505254.524642), (175683.226468 505243.284373))" concaveHull 0.0
Segmentation fault (core dumped)

this was recently resolved for GEOS 3.12.1 with this expected result:

$ ./geos-3.12.1/bin/geosop -a "MULTIPOINT ((175682.210293 505233.365178), (175682.210295 505233.365177), (175684.375108 505254.524644), (175684.375109 505254.524642), (175683.226468 505243.284373))" concaveHull 0.0
POLYGON ((175682.210293 505233.365178, 175684.375108 505254.524644, 175684.375109 505254.524642, 175683.226468 505243.284373, 175682.210295 505233.365177, 175682.210293 505233.365178))

The mystery is how this was resolved, as there is nothing obvious in NEWS.md to backport this fix for older branches (e.g. a forthcoming 3.11.4). Does this look familiar to anyone?

[mwtoews]

I should also note that this is not an issue with JTS, but here is JTS TestBuilder for the viz of input points and thin polygon result:

[dr-jts]

This was fixed by #953, which was not backported to 3.11 (can't remember why now).

Aside: it would be nice if GH allowed tagging issues with code files/lines that are known to be related (e.g. the cause of the issue, or affected by it).

[mwtoews]

Testing a backport:

• A cherry-pick of d4af4f9 (for Fix IncrementalDelaunayTriangulator to ensure triangulation boundary is convex #953) nicely merges with the 3.11 branch, which is nice
• It mostly compiles, but requires some edits only to tests/unit/triangulate/DelaunayTest.cpp for changes to CoverageUnion
• It fixes the above issue and possibly others?

But is it a good idea to backport? See #1035 for candidate.

[pramsey]

I think fixes are good. There will be some small behaviour changes, but the tradeoff is more working cases... seems like a win to me.

[dr-jts]

I'm in favour of this too. Thanks, @mwtoews .

[mwtoews]

Thanks all, I'll do a better review later today before merging (this was 5 mins to prepare). Also reevaluate if any other backports are simple and feasible.

[mwtoews]

Also xref to #946 and shapely/shapely#1873