# RFC 46: GDAL/OGR unification

Author: Even Rouault

Contact: even dot rouault at spatialys.com

Status: Adopted, implemented in GDAL 2.0

## Summary

In the 1.X series of GDAL/OGR, the GDAL/raster and OGR/vector sides are quite different on some aspects even where there is no strong reason for them to be different, particularly in the structure of drivers. This RFC aims at unifying the OGR driver structure with the GDAL driver structure. The main advantages of using the GDAL driver structure are :

- metadata capabilities : description of driver, extensions, creation options, virtual IO capability ...

- efficient driver identification and opening.

Similarly, OGR datasource and layer classes lack the metadata mechanisms offered by the corresponding GDAL dataset and raster band classes.

Another aspect is that the separation between GDAL "datasets" and OGR "datasources" is sometimes artificial. Various data containers can accept both data types. The list of drivers that have a GDAL side and OGR side is : SDTS, PDS, GRASS, KML, Spatialite/Rasterlite, GeoPackage (raster side not yet implemented), PostGIS/PostGIS Raster, PDF, PCIDSK, FileGDB (raster side not yet implemented). For applications that are interested in both, this currently means to open the file twice with different API. And for update mode, for file-based drivers, the updates must be done sequentially to avoid opening a file twice simultaneously in update mode and making conflicting changes.

## Related RFCs

There are a few related past RFCs that have never been adopted but strongly relate to RFC 46 :

- RFC 10: OGR Open Parameters. All the functionality described in RFC 10 is included in RFC 46, mainly the GDALOpenEx() new API

- RFC 25: Fast Open. RFC 25 mentions avoiding to systematically listing the sibling files to the file being opened. This can now achieved in RFC 46 by lazy loading with GDALOpenInfo::GetSiblingFiles(). At least Identify() should not trigger GetSiblingFiles().

- RFC 36: Allow specification of intended on GDALOpen. The new GDALOpenEx() accepts a list of a subset drivers that must be probed, as suggested by RFC36. The specification of the drivers on the command line of utilities could be easily done through a new option, but that's not in the scope of RFC 46.

- RFC 38: OGR Faster Open is completely included in RFC 46 through the possibility of using Open(GDALOpenInfo*) in OGR drivers

## Self-assigned development constraints

The changes should have moderate impact on the existing GDAL/OGR code base, and particularly on most of its code, that lies in drivers. Existing users of the GDAL/OGR API should also be moderately impacted by the changes, if they do not need to use the new offered capabilities.

## Core changes: summary

- OGRSFDriver extends GDALDriver.

- Vector drivers can be implemented as GDALDriver.

- OGRSFDriverRegistrar is a compatibility wrapper around GDALDriverManager for legacy OGRSFDriver.

- OGRDataSource extends GDALDataSource.

- GDALOpenEx() API is added to be able to open "mixed" datasets.

- OGRLayer extends GDALMajorObject, thus adding metadata capability.

- The methods of OGRDataSource related to layers are moved to GDALDataset, making it both a raster and vector capable container.

- Performance improvements in GDALOpenInfo() mechanism.

- New driver metadata item to describe open options (i.e. deprecate the use of configuration option).

- New driver metadata item to describe layer creation options.

## Core changes: details

### Drivers and driver registration

- The OGRSFDriver now extends GDALDriver and is meant as being a legacy way of implementing a vector driver. It is kept mainbly because, in the current implementation, not all drivers have been migrated to being "pure" GDALDriver. The CopyDataSource() virtual method has been removed since no in-tree drivers implement it. The inheritance to GDALDriver make it possible to manage vector drivers by the GDALDriverManager, and to be able to attach metadata to them, to document driver long name, link to documentation, file extension, datasource creation options with the existing GDAL_DMD_* metadata items.

- Drivers directly inheriting from GDALDriver (to be opposed to those inheriting from OGRSFDriver) should : - declare SetMetadataItem( GDAL_DCAP_VECTOR, "YES" ). - implement pfnOpen() for dataset opening - optionally, implement pfnCreate() for dataset creation. For vector drivers, the nBands parameter of Create() is supposed to be passed to 0. - optionally, implement pfnDelete() for dataset deletion

- The C OGR Driver API will still work with drivers that have been converted as "pure" GDALDrivers (this is not true of the C++ OGR Driver API). For example OGR_Dr_GetName() calls GDALDriver::GetDescription(), OGR_Dr_CreateDatasource() calls GDALDriver::Create(), etc...

- The C++ definition of GDALDriver is extended with the following function pointers so that it can work with legacy OGRSFDriver.

    /* For legacy OGR drivers */
    GDALDataset         *(*pfnOpenWithDriverArg)( GDALDriver*, GDALOpenInfo * );
    GDALDataset         *(*pfnCreateVectorOnly)( GDALDriver*,
                                                 const char * pszName,
                                                 char ** papszOptions );
    CPLErr              (*pfnDeleteDataSource)( GDALDriver*,
                                                 const char * pszName );

    They are used by GDALOpenEx(), GDALDriver::Create() and GDALDriver::Delete()
    if the pfnOpen, pfnCreate or pfnDelete pointers are NULL. The OGRSFDriverRegistrar
    class has an implementation of those function pointers that calls the
    legacy C++ OGRSFDriver::Open(), OGRSFDriver::CreateDataSource() and
    OGRSFDriver::DeleteDataSource() virtual methods.

- GDALDriver::Create() can accept nBands == 0 for a vector capable driver.

- GDALDriver::DefaultCreateCopy() can accept a dataset with 0 bands for a vector capable driver, and if the output dataset has layer creation capability and the source dataset has layers, it copies the layers from the source dataset into the target dataset.

- GDALDriver::Identify() now iterates over all kinds of drivers. It has been modified to do a first pass on drivers that have an implementation of Identify(). If no match is found, it does a second pass on all drivers and use the potentially slower Open() as the identification method.

- Related to the above point, the implementations of GDALDriver::pfnIdentify function pointer used to return a boolean value to indicate if the passed GDALOpenInfo was a match for the driver. For some drivers, this was too restrictive so that they were able to implement Identify(). For example where the detection logic can return "yes, I definitely recognize that file", "no, it is not for me" or "I have not enough elements in GDALOpenInfo to be able to tell". That last state can now be advertized with a negative return value.

- The OGRSFDriverRegistrar is trimmed down to be mostly a wrapper around GDALDriverManager. In particular, it does not contain any longer a list of drivers. The Open(), OpenShared(), ReleaseDataSource(), DeregisterDriver() and AutoLoadDrivers() methods are removed from the class. This change can have impact on C++ code. A few adaptations in OGR utilities have been done to accommodate for those changes. The RegisterDriver() API has been kept for legacy OGR drivers and it automatically sets SetMetadataItem( GDAL_DCAP_VECTOR, "YES" ). The GetDriverCount(), GetDriver() and GetDriverByName() methods delegate to GDALDriverManager and make sure to only take into account drivers that have the GDAL_DCAP_VECTOR capability. In the case a driver has the same name as GDAL and OGR driver, the OGR variant is internally prefixed with OGR_, and GetDriverByName() will first try the OGR_ variant. The GetOpenDSCount() and GetOpenDS() have now a dummy implementation returning 0/NULL. For reference, neither MapServer nor QGIS use those functions.

- OGRRegisterAll() is now an alias of GDALAllRegister(). The past OGRRegisterAll() is now renamed OGRRegisterAllInternal() and called by GDALAllRegister(). So, GDALAllRegister() and OGRRegisterAll() are now equivalent and register all drivers.

- GDALDriverManager has received a few changes :

  - use of a map from driver name to driver object to speed-up GetDriverByName()

  - accept OGR_SKIP and OGR_DRIVER_PATH configuration options for backward compatibility.

  - The recommended separator for driver names in GDAL_SKIP is now comma instead of space (similarly to what OGR_SKIP does). This is to make it possible to define OGR driver names in GDAL_SKIP that have spaces in their names like "ESRI Shapefile" or "MapInfo File". If there is no comma in the GDAL_SKIP value, then space separator is assumed (backward compatibility).

  - removal of GetHome()/SetHome() methods whose purpose seemed to define an alternate path for the search directory of plugins. Those methods only existed at the C++ level, and are redundant with GDAL_DRIVER_PATH configuration option

- Raster-capable drivers should declare SetMetadataItem( GDAL_DCAP_RASTER, "YES" ). All in-tree GDAL drivers have been patched to declare it. But the registration code detects if a driver does not declare any of GDAL_DCAP_RASTER nor GDAL_DCAP_VECTOR, in which case it declares GDAL_DCAP_RASTER on behalf of the un-patched driver, with a debug message inviting to explicitly set it.

- New metadata items :

  - GDAL_DCAP_RASTER=YES / GDAL_DCAP_VECTOR=YES at driver level. To declare that a driver has raster/vector capabilities. A driver can declare both.

  - GDAL_DMD_EXTENSIONS (with a final S) at driver level. This is a small evolution of GDAL_DMD_EXTENSION where one can specify several extensions in the value string. The extensions are space-separated. For example "shp dbf", "tab mif mid", etc... For ease of use, GDALDriver::SetMetadataItem(GDAL_DMD_EXTENSION) also sets the passed value as GDAL_DMD_EXTENSIONS, if it is not already set. So new code can always use GDAL_DMD_EXTENSIONS.

  - GDAL_DMD_OPENOPTIONLIST at driver level. The value of this item is an XML snippet with a format similar to creation options. GDALOpenEx(), once it has identified with Identify() that a driver accepts the file, will validate the passed open option list with the authorized open option list. Below an example of such an authorized open option list in the S57 driver

    <OpenOptionList>
      <Option name="UPDATES" type="string-select"
        description="Should update files be incorporated into the base data on the fly" default="APPLY">
        <Value>APPLY</Value>
        <Value>IGNORE</Value>
      </Option>
      <Option name="SPLIT_MULTIPOINT" type="boolean"
        description="Should multipoint soundings be split into many single point "
                    "sounding features" default="NO" />
      <Option name="ADD_SOUNDG_DEPTH" type="boolean"
        description="Should a DEPTH attribute be added on SOUNDG features and "
                    "assign the depth of the sounding" default="NO" />
      <Option name="RETURN_PRIMITIVES" type="boolean"
        description="Should all the low level geometry primitives be returned as "
                    "special IsolatedNode, ConnectedNode, Edge and Face layers" default="NO" />
      <Option name="PRESERVE_EMPTY_NUMBERS" type="boolean"
        description="If enabled, numeric attributes assigned an empty string as a "
                    "value will be preserved as a special numeric value" default="NO" />
      <Option name="LNAM_REFS" type="boolean"
        description="Should LNAM and LNAM_REFS fields be attached to features "
                    "capturing the feature to feature relationships in the FFPT "
                    "group of the S-57 file" default="YES" />
      <Option name="RETURN_LINKAGES" type="boolean"
        description="Should additional attributes relating features to their underlying "
                    "geometric primitives be attached" default="NO" />
      <Option name="RECODE_BY_DSSI" type="boolean"
        description="Should attribute values be recoded to UTF-8 from the character "
                    "encoding specified in the S57 DSSI record." default="NO" />
    </OpenOptionList>

    - GDAL_DS_LAYER_CREATIONOPTIONLIST at dataset level. But can also be set at
      driver level because, in practice, layer creation options do not depend on the
      dataset instance.
      The value of this item is an XML snippet with a format similar to dataset creation
      options.
      If specified, the passed creation options to CreateLayer() are validated
      against that authorized creation option list.
      Below an example of such an authorized open option list in the Shapefile driver.

    <LayerCreationOptionList>
      <Option name="SHPT" type="string-select" description="type of shape" default="automatically detected">
        <Value>POINT</Value>
        <Value>ARC</Value>
        <Value>POLYGON</Value>
        <Value>MULTIPOINT</Value>
        <Value>POINTZ</Value>
        <Value>ARCZ</Value>
        <Value>POLYGONZ</Value>
        <Value>MULTIPOINTZ</Value>
        <Value>NONE</Value>
        <Value>NULL</Value>
      </Option>
      <Option name="2GB_LIMIT" type="boolean" description="Restrict .shp and .dbf to 2GB" default="NO" />
      <Option name="ENCODING" type="string" description="DBF encoding" default="LDID/87" />
      <Option name="RESIZE" type="boolean" description="To resize fields to their optimal size." default="NO" />
    </LayerCreationOptionList>

### Datasets / Datasources

- The main methods from OGRDataSource have been moved to GDALDataset :

    virtual int         GetLayerCount() { return 0; }
    virtual OGRLayer    *GetLayer(int) { return NULL; }
    virtual OGRLayer    *GetLayerByName(const char *);
    virtual OGRErr      DeleteLayer(int);

    virtual int         TestCapability( const char * ) { return FALSE; }

    virtual OGRLayer   *CreateLayer( const char *pszName,
                                     OGRSpatialReference *poSpatialRef = NULL,
                                     OGRwkbGeometryType eGType = wkbUnknown,
                                     char ** papszOptions = NULL );
    virtual OGRLayer   *CopyLayer( OGRLayer *poSrcLayer,
                                   const char *pszNewName,
                                   char **papszOptions = NULL );

    virtual OGRStyleTable *GetStyleTable();
    virtual void        SetStyleTableDirectly( OGRStyleTable *poStyleTable );

    virtual void        SetStyleTable(OGRStyleTable *poStyleTable);

    virtual OGRLayer *  ExecuteSQL( const char *pszStatement,
                                    OGRGeometry *poSpatialFilter,
                                    const char *pszDialect );
    virtual void        ReleaseResultSet( OGRLayer * poResultsSet );

    int                 GetRefCount() const;
    int                 GetSummaryRefCount() const;
    OGRErr              Release();

    The following matching C API is available :

    int    CPL_DLL GDALDatasetGetLayerCount( GDALDatasetH );
    OGRLayerH CPL_DLL GDALDatasetGetLayer( GDALDatasetH, int );
    OGRLayerH CPL_DLL GDALDatasetGetLayerByName( GDALDatasetH, const char * );
    OGRErr    CPL_DLL GDALDatasetDeleteLayer( GDALDatasetH, int );
    OGRLayerH CPL_DLL GDALDatasetCreateLayer( GDALDatasetH, const char *,
                                          OGRSpatialReferenceH, OGRwkbGeometryType,
                                          char ** );
    OGRLayerH CPL_DLL GDALDatasetCopyLayer( GDALDatasetH, OGRLayerH, const char *,
                                            char ** );
    int    CPL_DLL GDALDatasetTestCapability( GDALDatasetH, const char * );
    OGRLayerH CPL_DLL GDALDatasetExecuteSQL( GDALDatasetH, const char *,
                                         OGRGeometryH, const char * );
    void   CPL_DLL GDALDatasetReleaseResultSet( GDALDatasetH, OGRLayerH );
    OGRStyleTableH CPL_DLL GDALDatasetGetStyleTable( GDALDatasetH );
    void   CPL_DLL GDALDatasetSetStyleTableDirectly( GDALDatasetH, OGRStyleTableH );
    void   CPL_DLL GDALDatasetSetStyleTable( GDALDatasetH, OGRStyleTableH );

    OGRDataSource definition is now reduced to :

    class CPL_DLL OGRDataSource : public GDALDataset
    {
    public:
                            OGRDataSource();

        virtual const char  *GetName() = 0;

        static void         DestroyDataSource( OGRDataSource * );
    };

    The existing OGR_DS_* API is preserved. The implementation of those functions
    casts the OGRDataSourceH opaque pointer to GDALDataset*, so it is possible to
    consider GDALDatasetH and OGRDataSourceH as equivalent from the C API point of
    view. Note that it is not true at the C++ level !

- OGRDataSource::SyncToDisk() has been removed. The equivalent functionality should be implemented in existing FlushCache(). GDALDataset::FlushCache() nows does the job of the previous generic implementation of OGRDataSource::SyncToDisk(), i.e. iterate over all layers and call SyncToDisk() on them.

- GDALDataset has now a protected ICreateLayer() method.

    virtual OGRLayer   *ICreateLayer( const char *pszName,
                                     OGRSpatialReference *poSpatialRef = NULL,
                                     OGRwkbGeometryType eGType = wkbUnknown,
                                     char ** papszOptions = NULL );

    This method is what used to be CreateLayer(), i.e. that drivers should
    rename their specialized CreateLayer() implementations as ICreateLayer().
    CreateLayer() is kept at GDALDataset level, but its implementation does a
    prior validation of passed creation options against an optional authorized
    creation option list (GDAL_DS_LAYER_CREATIONOPTIONLIST), before calling
    ICreateLayer() (this is similar to RasterIO() / IRasterIO() )
    A global pass on all in-tree OGR drivers has been made to rename CreateLayer()
    as ICreateLayer().

- GDALOpenEx() is added to be able to open raster-only, vector-only, or raster-vector datasets. It accepts read-only/update mode, shared/non-shared mode. A list of potential candidate drivers can be passed. If NULL, all drivers are probed. A list of open options (NAME=VALUE syntax) can be passed. If the list of sibling files has already been established, it can also be passed. Otherwise GDALOpenInfo will establish it.

    GDALDatasetH CPL_STDCALL GDALOpenEx( const char* pszFilename,
                                     unsigned int nOpenFlags,
                                     const char* const* papszAllowedDrivers,
                                     const char* const* papszOpenOptions,
                                     const char* const* papszSiblingFiles );

    The nOpenFlags argument is a 'or-able' combination of the following values :

    /* Note: we define GDAL_OF_READONLY and GDAL_OF_UPDATE to be on purpose */
    /* equals to GA_ReadOnly and GA_Update */

    /** Open in read-only mode. */
    #define     GDAL_OF_READONLY        0x00
    /** Open in update mode. */
    #define     GDAL_OF_UPDATE          0x01

    /** Allow raster and vector drivers. */
    #define     GDAL_OF_ALL             0x00

    /** Allow raster drivers. */
    #define     GDAL_OF_RASTER          0x02
    /** Allow vector drivers. */
    #define     GDAL_OF_VECTOR          0x04
    /* Some space for GDAL 3.0 new types ;-) */
    /*#define     GDAL_OF_OTHER_KIND1   0x08 */
    /*#define     GDAL_OF_OTHER_KIND2   0x10 */

    /** Open in shared mode. */
    #define     GDAL_OF_SHARED          0x20

    /** Emit error message in case of failed open. */
    #define     GDAL_OF_VERBOSE_ERROR   0x40

    The existing GDALOpen(), GDALOpenShared(), OGROpen(), OGROpenShared(),
    OGR_Dr_Open() are just wrappers of GDALOpenEx() with appropriate open flags.
    From the user point of view, their behavior is identical to the existing one,
    i.e. GDALOpen() family will only returns datasets of drivers with declared raster
    capabilities, and similarly with OGROpen() family with vector.

- GDALOpenInfo class. The following changes are done :

  - the second argument of the constructor is now nOpenFlags instead of GDALAccess, with same semantics as GDALOpenEx(). GDALOpenInfo uses the read-only/update bit to "compute" the eAccess flag that is heavily used in existing drivers. Drivers with both raster and vector capabilities can use the GDAL_OF_VECTOR/GDAL_OF_RASTER bits to determine the intent of the caller. For example if a caller opens with GDAL_OF_RASTER only and the dataset only contains vector data, the driver might decide to not open the dataset (if it is a read-only driver. If it is a driver with update capability, it should do that only if the opening is done in read-only mode).

  - the open options passed to GDALOpenEx() are stored into a papszOpenOptions member of GDALOpenInfo, so that drivers can use them.

  - the "FILE* fp" member is transformed into "VSILFILE* fpL". This change is motivated by the fact that most popular drivers now use the VSI Virtual File API, so they can now directly use the fpL member instead of re-opening again the file. A global pass on all in-tree GDAL drivers that used fp has been made.

  - A VSIStatExL() was done previously to determine the nature of the file passed. Now, we optimistically begin with a VSIFOpenL(), assuming that in most use cases the passed filename is a file. If the opening fails, VSIStatExL() is done to determine the nature of the filename.

  - If the requested access mode is update, the opening of the file with VSIFOpenL() is done with "rb+" permissions to be directly usable.

  - The papszSiblingFiles member is now private. It is accessed by a GetSiblingFiles() method that does the ReadDir() on demand. This can speed up the Identify() method that generally does not require to know sibling files.

  - A new method, TryToIngest(), is added to read more than the first 1024 bytes of a file. This is useful for a few vector drivers, like GML or NAS, that must fetch a bit more bytes to be able to identify the file.

### Layer

- OGRLayer extends GDALMajorObject. Drivers can now define layer metadata items that can be retrieved with the usual GetMetadata()/GetMetadataItem() API.

- The GetInfo() method has been removed. It has never been implemented in any in-tree drivers and has been deprecated for a long time.

### Other

- The deprecated and unused GDALProjDefH and GDALOptionDefinition types have been removed from gdal.h

- GDALGeneralCmdLineProcessor() now interprets the nOptions (combination of GDAL_OF_RASTER and GDAL_OF_RASTER) argument as the type of drivers that should be displayed with the --formats option. If set to 0, GDAL_OF_RASTER is assumed.

- the --formats option of GDAL utilities outputs whether drivers have raster and/or vector capabilities

- the --format option of GDAL utilities outputs GDAL_DMD_EXTENSIONS, GDAL_DMD_OPENOPTIONLIST, GDAL_DS_LAYER_CREATIONOPTIONLIST.

- OGRGeneralCmdLineProcessor() use GDALGeneralCmdLineProcessor() implementation, restricting --formats to vector capable drivers.

## Changes in drivers

- OGR PCIDSK driver has been merged into GDAL PCIDSK driver.

- OGR PDF driver has been merged into GDAL PDF driver.

- A global pass has been made to in-tree OGR drivers that have to open a file to determine if they recognize it. They have been converted to GDALDriver to accept a GDALOpenInfo argument and they now use its pabyHeader field to examine the first bytes of files. The number of system calls related to file access (open/stat), in order to determine that a file is not recognized by any OGR driver, has now dropped from 46 in GDAL 1.11 to 1. The converted drivers are : AeronavFAA, ArcGEN, AVCBin, AVCE00, BNA, CSV, DGN, EDIGEO, ESRI Shapefile, GeoJSON, GeoRSS, GML, GPKG, GPSBabel, GPX, GTM, HTF, ILI1, ILI2, KML, LIBKML, MapInfo File, MySQL, NAS, NTF, OpenAIR, OSM, PDS, REC, S57, SDTS, SEGUKOOA, SEGY, SOSI, SQLite, SUA, SVG, TIGER, VFK, VRT, WFS

- Long driver description is set for most OGR drivers.

- All classes deriving from OGRLayer have been modified to call SetDescription() with the value of GetName()/poFeatureDefn->GetName(). test_ogrsf tests that it is properly set.

- Following drivers are kept as OGRSFDriver, but their Open() method does early extension/prefix testing to avoid datasource object to be instantiated : CartoDB, CouchDB, DXF, EDIGEO, GeoConcept, GFT, GME, IDRISI, OGDI, PCIDSK, PG, XPlane.

- Identify() has been implemented for CSV, DGN, DXF, EDIGEO, GeoJSON, GML, KML, LIBKML, MapInfo File, NAS, OpenFileGDB, OSM, S57, Shape, SQLite, VFK, VRT.

- GDAL_DMD_EXTENSION/GDAL_DMD_EXTENSIONS set for following drivers: AVCE00, BNA, CSV, DGN, DWG, DXF, EDIGEO, FileGDB, Geoconcept, GeoJSON, Geomedia, GML, GMT, GPKG, GPX, GPSTrackMaker, IDRISI Vector, Interlis 1, Interlis 2, KML, LIBKML, MDB, MapInfo File, NAS, ODS, OpenFileGDB, OSM, PGDump, PGeo, REC, S57, ESRI Shapefile, SQLite, SVG, WaSP, XLS, XLSX, XPlane.

- Document dataset and layer creation options of BNA, DGN, FileGDB, GeoConccept, GeoJSON, GeoRSS, GML, GPKG, KML, LIBKML, PG, PGDump and ESRI Shapefile drivers as GDAL_DMD_CREATIONOPTIONLIST / GDAL_DS_LAYER_CREATIONOPTIONLIST.

- Add open options AAIGRID, PDF, S57 and ESRI Shapefile drivers.

- GetFileList() implemented in OpenFileGDB, Shapefile and OGR VRT drivers.

- Rename datasource SyncToDisk() as FlushCache() for LIBKML, OCI, ODS, XLSX drivers.

- Use poOpenInfo->fpL to avoid useless file re-opening in GTiff, PNG, JPEG, GIF, VRT, NITF, DTED.

- HTTP driver: declared as GDAL_DCAP_RASTER and GDAL_DCAP_VECTOR driver.

- RIK: implement Identify()

- Note: the compilation and good working of the following OGR drivers (mostly proprietary) could not be tested: ArcObjects, DWG, DODS, SDE, FME, GRASS, IDB, OCI, MSSQLSpatial(compilation OK, but not runtime tested)

## Changes in utilities

- gdalinfo accepts a -oo option to define open options

- ogrinfo accepts a -oo option to define open options

- ogr2ogr accepts a -oo option to define input dataset open options, and -doo to define destination dataset open options

## Changes in SWIG bindings

- Python and Java bindings:

  - add new GDALDataset methods taken from OGRDataSource : CreateLayer(), CopyLayer(), DeleteLayer(), GetLayerCount(), GetLayerByIndex(), GetLayerByName(), TestCapability(), ExecuteSQL(), ReleaseResultSet(), GetStyleTable() and SetStyleTable().

  - make OGR Driver, DataSource and Layer objects derive from MajorObject

- Perl and CSharp: make sure that it still compiles but some work would have to be done by their mainteners to be able to use the new capabilities

## Potential changes that are NOT included in this RFC

"Natural" evolutions of current RFC :

- Unifying the GDAL MEM and OGR Memory drivers.

- Unifying the GDAL VRT and OGR VRT drivers.

Further unification steps :

- Source tree changes to move OGR drivers from ogr/ogrsf_frmts/ to frmts/ , to move ogr/ogrsf_frmts/generic/* to gcore/*, etc...

- Documentation unification (pages with list of drivers, etc...)

- Renaming to remove traces of OGR namespace : OGRLayer -> GDALLayer, etc...

- Kill --without-ogr compilation option ? It has been preserved in a working state even if it embeds now ogr/ogrsf_frmts/generic and ogr/ogrsf_frmts/mitab for conveniency.

- Unification of some utilities : "gdal info XXX", "gdal convert XXX" that would work on all kind of datasets.

## Backward compatibility

GDALDriverManager::GetDriverCount(), GetDriver() now returns OGR drivers, as well as GDAL drivers

The reference counting in GDAL datasets and GDAL 1.X OGR datasources was a bit different. It starts at 1 for GDAL datasets, and started at 0 for OGR datasources. Now that OGRDataSource is basically a GDALDataset, it starts at 1 for both cases. Hopefully there are very few users of the OGR_DS_GetRefCount() API. If it was deemed necessary we could restore the previous behavior at the C API, but that would not be possible at the C++ level. For reference, neither MapServer nor QGIS use OGR_DS_GetRefCount().

## Documentation

A pass should be made on the documentation to check that all new methods are properly documented. The OGR general documentation (especially C++ API Read/Write tutorial, Driver implementation tutorial and OGR architecture) should be updated to reflect the changes.

## Testing

Very few changes have been made so that the existing autotest suite still passes. Additions have been made to test the GDALOpenEx() API and the methods "imported" from OGRDataSource into GDALDataset.

## Version numbering

Although the above describes changes should have very few impact on existing applications of the C API, some behavior changes, C++ level changes and the conceptual changes are thought to deserve a 2.0 version number.

## Implementation

Implementation will be done by Even Rouault.

The proposed implementation lies in the "unification" branch of the https://github.com/rouault/gdal2/tree/unification repository.

The list of changes : https://github.com/rouault/gdal2/compare/unification

## Voting history

+1 from JukkaR, FrankW, DanielM, TamasS and EvenR.