[gdal-dev] Motion: approve RFC 101 "Raster dataset read-only thread-safety"

[Kurt Schwehr]

+1 KurtS

On Wed, Sep 11, 2024 at 12:27 PM Even Rouault via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

> Hi,
>
> I move to approve RFC 101 "Raster dataset read-only thread-safety":
> https://github.com/OSGeo/gdal/pull/10676
>
> Starting with my +1,
>
> The candidate implementation is available in
> https://github.com/OSGeo/gdal/pull/10746. While fine tuning it, I
> realized there were subtleties, related to the fact we use thread-local
> datasets under the hood, for methods returning a non-"primitive" type,
> such as a "const char*" whose lifetime is tied to a dataset/band. If
> we'd get it from a thread-local dataset, and that one would be later
> evicted from the cache (not very common, but could happen if one would
> open tons of thread-safe datasets at the same time), then you could have
> use-after-free issues. For such methods (GetMetadata(),
> GetMetadataItem(), GetProjectionRef(), etc. as well as GetColorTable()
> which returns a GDALColorTable pointer, or GetSpatialRef() which returns
> a OGRSpatialReference*), I've preferred to protect their access with a
> mutex around the "prototype" dataset passed when constructing
> GDALMultiThreadedDataset, whose lifetime is at least as long as
> GDALMultiThreadedDataset (cf commit
>
> https://github.com/OSGeo/gdal/pull/10746/commits/849e9cea711efd30c47fd90a7a6b71a75611c1a5#diff-687008dd6e3d5ac8ad05568272746ed75b84de50bde508f78f2d79a6842825c7L428)
>
> . Normally such methods aren't called in user code repeatedly, so there
> should be any noticeable lock contention in practice. The main objective
> of the RFC which is to be able to issue RasterIO() requests in parallel
> isn't affected by that. Ah, and one thing I realized is that
> OGRSpatialReference isn't thread-safe, so I've also added an optional
> SetThreadSafe() on it, to also use a per-instance mutex in
> multi-threaded scenarios (cf commit
>
> https://github.com/OSGeo/gdal/pull/10746/commits/c7e1862273dd018e58a01f25b21fdff6dbfdd1cd).
>
> Multi-threading is hard...
>
> Even
>
> --
> http://www.spatialys.com
> My software is free, but my time generally not.
>
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