[gdal-dev] What is readlink used for?

[Even Rouault]

Hi Kurt,

> 
> I ran into trouble with readlink as can be seen in
> http://trac.osgeo.org/gdal/ticket/5340.  I'm working with a system where
> the run directory is all symbolic links.  So if I have nonsquare.vrt
> refering to "./small.raw", readlink of nonsquare.vrt will end up being in a
> totally different directory than the small.raw file.
> 
> So... That begs the question of the purpose of dereferencing symbolic links
> using readlink (e.g.
> http://trac.osgeo.org/gdal/browser/trunk/gdal/frmts/vrt/vrtdataset.cpp#L687
> ). Is this done just for performance on filesystems where symbolic links
> might be a performance hit (e.g. nfs file systems over slow networks and
> an OS that does not do a good job caching)?  Or is there something else
> important that I'm missing?

This was added in http://trac.osgeo.org/gdal/changeset/25682 . The autotests 
added in that commit show the use cases that are solved. They seem to be 
rather reasonable.

I'm having trouble to understand your use case that doesn't work. Perhaps you 
could explain it in a more "graphical" way ( "ls -al" in the relevant 
directories will do )

> 
> I see 4 places where readlink is used:
> 
> ./ogr/ogrsf_frmts/generic/ogrsfdriverregistrar.cpp:        int nBytes =
> readlink(pszName, szPointerFilename, sizeof(szPointerFilename));
> ./gcore/gdalopeninfo.cpp:        int nBytes = readlink(pszFilename,
> szPointerFilename, sizeof(szPointerFilename));
> ./port/cpl_getexecpath.cpp:    nResultLen = readlink( osExeLink,
> pszPathBuf, nMaxLength );
> ./frmts/vrt/vrtdataset.cpp:            int bufferSize =
> readlink(currentVrtFilename, filenameBuffer, sizeof(filenameBuffer));
> 
> ogrsfdriverregistar.cpp looks like a possible issue.
> 
> http://trac.osgeo.org/gdal/browser/trunk/gdal/ogr/ogrsf_frmts/generic/ogrsf
> driverregistrar.cpp#L250
> 
> I see that sort of works...
> 
> ogrinfo my_remote_poly.shp
> ERROR 4: Unable to open /vsicurl/
> http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.shp or /vsicurl/
> http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.SHP.
> Had to open data source read-only.
> INFO: Open of `my_remote_poly.shp'
>       using driver `ESRI Shapefile' successful.
> INFO: Internal data source name `/vsicurl/
> http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.shp'
>       different from user name `my_remote_poly.shp'.
> 1: poly (3D Polygon)

Yes this is convenient to be able to "ln -s /vsicurl/http://somethnig" to 
something.

> 
> I'm thinking that I should just disable readlink entirely.  Alternatively,
> hhould there be a flag that controls readlink just for working with vrt's
> or a runtime ConfigSetting that could turn on/off readlink?

No there's no such runtime configuration option.

> 
> Any thoughts would be appreciated!
> 
> Thanks,
> -kurt

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