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

[Rajsekar Manokaran]

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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/gdal-dev/attachments/20210912/c80209e4/attachment.html>