[postgis-devel] ST_MaximumInscribedCircle madness

[Martin Davis]

There's only one main loop, so an interrupt check can go there:

https://github.com/libgeos/geos/blob/main/src/algorithm/construct/MaximumInscribedCircle.cpp#L187

On Thu, Apr 20, 2023 at 5:33 PM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> One thing we can do is find a reasonable place in the loop to put an
> interrupt check. Where does it spend its time when it’s in this state?
>
> On Apr 20, 2023, at 5:31 PM, Martin Davis <mtnclimb at gmail.com> wrote:
>
> That geometry is indeed a simple rectangle, but it's very, very, very
> narrow.   This is causing the interative algorithm used in
> MaximmumInscribedCircle to essentially hunt forever trying to find a point
> which lies inside the polygon.
>
> Of course it would be nice to detect this situation and terminate faster,
> either with a point which lies actually lies outside the rectangle, or some
> other return value.  This will require some thought.
>
> On Thu, Apr 20, 2023 at 2:38 PM Rémi Cura <remi.cura at gmail.com> wrote:
>
>> Hello dear Postgis devs!
>>
>> I have a strange case to submit to you.
>> In a nutshell, trying to compute
>> ```SQL
>> SELECT ST_MaximumInscribedCircle( 'POLYGON ((59.3 100.00000000000001,
>> 99.7 100.00000000000001, 99.7 100, 59.3 100, 59.3 100.00000000000001))') ;
>> ```
>> causes it to run for many minutes until it eventually errors.
>> Worst, running this kind of computing on 12M geoms in parallel x6 created
>> a postgres **segfault** and crashed my server (16 cores, 64GB).
>> (many more geoms are making this function unhappy).
>>
>> /!\ Once this query starts, it can't be stopped by
>> `pg_cancel_backend(pid)` or even `pg_terminate_backend(pid)`
>> --> This is pretty dangerous in my opinion.
>>
>>
>> The geometry is simple, it is a rectangle.
>> I obtained it through other geometry operations
>> It must be some kind of precision issue, I'm wondering if there is some
>> kind of infinite loop going on or something.
>>
>> My full code is something like that:
>> ```SQL
>> SELECT env, diff, ST_MaximumInscribedCircle(diff)
>> FROM CAST ('some_polygon'as text ) as g
>>     , ST_Envelope(g) as env
>>     , ST_MakeEnvelope(0,0,100,100) as bound
>>     , ST_Difference(env, bound) as diff;
>> ```
>>
>> Not creating a bug ticket as I don't know if I'm simply abusing the
>> function / using it in a way it should not.
>>
>>
>> POSTGIS="3.1.7 aafe1ff" [EXTENSION] PGSQL="140" GEOS="3.9.1-CAPI-1.14.2"
>> PROJ="8.0.1" GDAL="GDAL 3.4.3, released 2022/04/22" LIBXML="2.9.1"
>> LIBJSON="0.15" LIBPROTOBUF="1.3.2" WAGYU="0.5.0 (Internal)" RASTER
>>
>> PostgreSQL 14.7 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.1
>> 20180712 (Red Hat 7.3.1-12), 64-bit
>> _______________________________________________
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>> postgis-devel at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/postgis-devel
>>
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