[gdal-dev] Get error handler user data when removing CPL Error Handler

Rajsekar Manokaran rajsekar at gmail.com
Sun Sep 12 10:52:26 PDT 2021

Thanks, that does make sense.  Fortunately, like py, Rust also has
closures, so user data could be handled directly by the caller.  We're also
pushing for an impl. without user-data support.  The thread local APIs
should also be supported (not done yet though); should be much simpler as
it doesn't involve concurrency.

On Sun, Sep 12, 2021 at 9:53 PM Sean Gillies <sean at mapbox.com> wrote:

> Hi,
> For what it's worth, In the Python package named rasterio we're using the
> push/pop API:
> https://github.com/mapbox/rasterio/blob/master/rasterio/_env.pyx#L336.
> While Rust's needs may differ, a single handler without any support for
> user data works well for Python: everything goes to the logging
> infrastructure, one of the kind of globals that Howard refers to in
> https://github.com/OSGeo/gdal/blob/master/gdal/doc/source/development/rfc/rfc37_cplerror_userdata.rst#rationale,
> and Python developers extend the logger if they want behavior that is
> different from the basic defaults.
> On Sun, Sep 12, 2021 at 8:12 AM Even Rouault <even.rouault at spatialys.com>
> wrote:
>> Hi,
>> no there's no thread safe API to do what you want. You'd need a new
>> function
>> CPLErrorHandler CPLSetErrorHandlerEx2( CPLErrorHandler
>> pfnErrorHandlerNew, void* pUserData, void** ppOldUserData )
>> to do that.
>> But as you mention threads that might compete to set an error handler,
>> using CPLSetErrorHandlerEx() is probably not the best strategy. You'd be
>> better with CPLPushErrorHandler() / CPLPopErrorHandler() that only affects
>> the current thread.
>> Even
>> Le 12/09/2021 à 16:03, Rajsekar Manokaran a écrit :
>> Hi,
>> In the gdal rust bindings (https://github.com/georust/gdal), we're
>> trying to facilitate the use of CPLSetErrorHandlerEx and related APIs.
>> While setting a handler, we may pass a heap allocated data pointer to the
>> second argument, which is then read via the CPLGetErrorHandlerUserData in
>> the handler and passed to the user.
>> However, while removing or setting another handler, we're unable to find
>> a race-free method to get the associated user data of the previous
>> handler.   This is needed to properly deallocate the memory.
>> Is there an atomic way to get both the previous handler (as returned by
>> CPLSetErrorHandler), along with the associated user data?  The issue with
>> making two calls, is that another thread might make changes in between the
>> two calls.
>> We could synchronize in our API, but it still has the same issue if the
>> user parallely used the C APIs directly or via a different interface.
>> Relevant PR in rust gdal bindings:
>> https://github.com/georust/gdal/pull/215
>> -
>> Regards
> --
> Sean Gillies
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