Contexts and functions taking multiple geometries

[Tom Payne]

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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