[Gdal-dev] RFC 12: Improved File Management

[Ray Gardener]

> Ray,
> 
  > In MapServer I would like to be able to write a dataset with GDAL 
and then
> ask for all the files so I can zip them up and return them to a client.

An issue here is that the returned filenames might not refer to files, 
unless you know which VSI*L filesyshandler was used. Usually this 
assumption is harmless -- the default handler is assigned at compile 
time and set to use the machine's normal filesystem -- but when one 
starts using other filesyshandlers (like you said, for mem and so 
forth), then the assumption breaks.

If we want to enumerate and manage the physical entities in a dataset 
regardless of how their storage is implemented, and add entity 
management methods to GDALDataSet, the dataset needs to know how the 
entities were stored. But storage details lie outside the driver. The 
driver would need to forward an entity management call to the same 
filesyshandler that originally wrote the entity.

So what we need is to have datasets know which filesyshandler they are 
using. The driver's methods need an extra filesyshandler parameter, 
which they can forward to datasets as part of the openinfo or something 
similar. VSIFOpenL() needs to take a filesyshandler parameter, or 
drivers should be able to call VSIFilesystemHandler::Open().

Then I could pass a filesyshandler subclass to driver::Create(), and 
have the dataset do all of it's I/O with it, and know which 
filesyshandler to use when doing entity management.

Issues like subdatasets could then be handled by treating the base 
dataset as a filesystem, and implementing a filesyshandler for that 
dataset's format.

Ray