[postgis-devel] ST_MaximumInscribedCircle madness

[Paul Ramsey]

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 <mailto: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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