Contexts and functions taking multiple geometries

[Tom Payne]

Gentle ping on this. I would really like to understand the requirements for
combining geometries from different GEOS contexts.

If my question is unclear, missing information, stupid, answered elsewhere,
or there is any other problem with it then please tell me. I would really
like to understand this.

Regards,
Tom

On Fri, 22 Nov 2024 at 10:50, Tom Payne <twpayne at gmail.com> wrote:

> Hi,
>
> tl;dr when calling a function which takes multiple geometries, like
> GEOSUnion_r
> <https://libgeos.org/doxygen/geos__c_8h.html#afd3d82a6d039ab5637e0a8a066694b7d>, where
> the two geometries are associated with different contexts, do I have to
> ensure that both geometries' contexts are used exclusively?
>
> Background:
>
> I maintain the Go bindings for GEOS <https://github.com/twpayne/go-geos>,
> which exclusively use the thread-safe *_r functions. Every created geometry
> is associated with a context. Every context has a mutex to ensure that it
> is only accessed from a single thread at time.
>
> For functions that take multiple geometries I check if the geometries are
> from different contexts, and if so, lock both mutexes. Here
> <https://github.com/twpayne/go-geos/blob/c9ed31526fa2ee3599ffe0fdf4556a6cf9c0b204/geommethods.go#L865-L875>
> is an example:
>
>     // Union returns the union of g and other.
>     func (g *Geom) Union(other *Geom) *Geom {
>         g.mustNotBeDestroyed()
>         g.context.Lock()
>         defer g.context.Unlock()
>         if other.context != g.context {
>             other.context.Lock()
>             defer other.context.Unlock()
>         }
>         return g.context.newGeom(C.GEOSUnion_r(g.context.handle, g.geom,
> other.geom), nil)
>     }
>
> However, there is a potential deadlock if there are two geometries A and B
> owned by different contexts and A.Union(B) and B.Union(A) are called
> simultaneously from different threads. In practice this pattern is unlikely
> to occur, but I would like to guard against it.
>
> I checked the documentation on GEOS's C API
> <https://libgeos.org/usage/c_api/>, the GEOS developer notes
> <https://github.com/libgeos/geos/blob/main/DEVELOPER-NOTES.md>, did a
> superficial search of the GitHub issues
> <https://github.com/search?q=repo%3Alibgeos%2Fgeos+context&type=issues>,
> and a superficial search of the geos-devel
> <https://www.google.com/search?q=site%3Alists.osgeo.org+%22%5Bgeos-devel%5D%22+context>
> archives, and could not find an answer to this question.
>
> Many thanks for any insight,
> Tom
>
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