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

[Even Rouault]

Motion passed with +1 from PSC members KurtS, JukkaR, JavierJS and me.

Le 11/09/2024 à 21:26, Even Rouault via gdal-dev a écrit :
> 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
>
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