View source for Request for Changes-11: Gdal 2.0 support

== [Request for Changes - 11] Gdal 2.0 support ==

=== Status === 

* Author: Julien Michel
* Submitted on 15.10.2015
* Proposed target release: 5.2
* Adopted : +4 votes
* https://git.orfeo-toolbox.org/otb.git/shortlog/refs/heads/gdal-2.0-support

=== Summary ===

This Request for Changes makes Orfeo ToolBox support both gdal 1.x (x >=10) and gdal 2.x.

=== Rationale ===

Until now, Gdal 2.0 and higher was not supported in OTB [1]. The main reason is an API change that unifies OGRDatasource and GDALDataset [2]. 

While in the initial bug report, it was suggested that moving to the C API should resolve the issue, I found no evidence of that (some C API function are also marked as deprecated), and it implies a lot of changes in the code. I therefore did not got this way and tried to keep with the C++ API. Note that there might be other reason to move to the C API in the future, but that is a great deal of work.

Of course, the tricky part of this RFChanges is to get both gdal 1.x and gdal 2.x supported without #ifdef everywhere in the code. I resolved this by hiding all interface changes in functions of an ogr::version_proxy namespace, that is in charge of switching implementation at the back while proposing the same public interface for both versions. I then updated all classes using the deprecated OGR methods to use the ogr::version_proxy functions instead.

[1] https://bugs.orfeo-toolbox.org/view.php?id=937

[2] https://trac.osgeo.org/gdal/wiki/rfc46_gdal_ogr_unification

=== Implementation details ===

==== Classes and files ====

===== ThirdParty/GDAL =====

* There was an error in gdalVersionTest.cxx that prevented 2.0 to be detected as a correct version:
https://git.orfeo-toolbox.org/otb.git/blobdiff/1c53a1316773adf2a416927938cea11e45f17a0e..b68604044cfdeaa63425972632810f14cb1202db:/Modules/ThirdParty/GDAL/gdalVersionTest.cxx

* otb-module-init.cmake has been updated to store the gdal major version in a cmake cache variable :
https://git.orfeo-toolbox.org/otb.git/blobdiff/1c53a1316773adf2a416927938cea11e45f17a0e..b68604044cfdeaa63425972632810f14cb1202db:/Modules/ThirdParty/GDAL/otb-module-init.cmake

===== Core/Common =====

* otbConfigure.h.in has been updated with the content of the OTB_USE_GDAL_20 cmake variable

===== Adapters/GDALAdapters =====

* The ogr::version_proxy namespace was created and contains functions fir the stable interface to use both gdal version 
https://git.orfeo-toolbox.org/otb.git/blob/d63d324733ea01342c8247b624c8800f2400ac5d:/Modules/Adapters/GdalAdapters/include/otbOGRVersionProxy.h

* Two different implementations of those functions are built depending on the gdal version:
https://git.orfeo-toolbox.org/otb.git/blob/d63d324733ea01342c8247b624c8800f2400ac5d:/Modules/Adapters/GdalAdapters/src/otbOGRVersionProxy1x.cxx

https://git.orfeo-toolbox.org/otb.git/blob/d63d324733ea01342c8247b624c8800f2400ac5d:/Modules/Adapters/GdalAdapters/src/otbOGRVersionProxy2x.cxx

* The cmake switch is here:
https://git.orfeo-toolbox.org/otb.git/commitdiff/b68604044cfdeaa63425972632810f14cb1202db#patch2

* OGRDatasetWrapper and OGRLayerWrapper classes have been changed to use the ogr::version_proxy namespace instead of direct calls to gdal API for all calls affected by the API change.

===== IO/IOGDAL =====

* OGRVectorDataIO and OGRIOHelper classes have been changed to use the ogr::version_proxy functions instead of direct calls to gdal API for all calls affected by the API change.

===== IO/TestKernel =====

* otbTestKernel.cxx has been changed to use the ogr::version_proxy namespace instead of direct calls to gdal API for all calls affected by the API change.

==== Applications ==== 

No code changes in applications.

==== Tests ====
No changes in tests, but changes in TestDriver and OGR use might have some impact on the dashboard.
* [http://dash.orfeo-toolbox.org/buildSummary.php?buildid=203718 Dashboard submission for this branch and using gdal 1.10] is on and no has no failing test related to the changes
* A new configuration has to be set to test the same branch with gdal 2.0 as well.

==== Documentation ====

* New functions in ogr::version_proxy namespace has been heavily documented.

=== Additional notes ===

This bring yet another GDAL/OGR/Proxy/helper class in our code. From an outside perspective, it might be hard to understand what all these classes are for (this is more clear with ogr::version_proxy namespace instead of static class as suggested by Luc).