[gdal-dev] What is readlink used for?

[Kurt Schwehr]

Hi all,

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?

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/ogrsfdriverregistrar.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)

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?

Any thoughts would be appreciated!

Thanks,
-kurt
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