[PROJ] Management of datum grid files.

[Even Rouault]

> that e.g. OSGeo4W puts the proj shared things into %LOCALAPPDATA%, a
> standalone GIS software depending on proj looks into %LOCALAPPDATA% if there
> is a subfolder called e.g. proj? or the other way around, a standalone GIS
> software depending on proj puts needed files into %LOCALAPPDATA%\proj and
> OSGeo4W looks while updating/upgrading its proj version into %LOCALAPPDATA%
> if there is a proj subfolder with shared folders? what if in
> %LOCALAPPDATA%\proj are already files with the same name? should these be
> just overwritten? how will be there a track of updated/ugraded/changed
> shared proj files if different software are working/updating/upgrading them
> at the same time?
> 
> or should be set %PROJ_LIB% windows system wide (for all users? for just one
> user in a multiuser network?) and a standalone GIS software depending on
> proj/OSGeo4W while updating looks for %PROJ_LIB% instead of
> %LOCALAPPDATA%\proj?
> 
> will this work in the windows side of the world? ;-)

Huh, what a mess ! No wonder I stay away from Windows as much as possible :-)

I'd say that applications shouldn't mess with  %LOCALAPPDATA%\proj at all. 
What will be put there will be managed by PROJ itself (in RFC4 a sqlite 
database with a cache of downloading grid chunks. And if
https://github.com/OSGeo/PROJ/issues/1750 comes true, with what the script 
will install). An implementation detail.
They might go on with the "old way" of installing the grids in the "PROJ_LIB" 
they manage (or ..\bin\share\proj if we adopt Joaquim's strategy).

So an updated logic for the pj_open_lib_ex() function that attempts to locate 
a grid would be in the order they are done:
- user callback provided by user through proj_context_set_file_finder() ? 
(does anybody use that ? Probably not)
- search paths set through proj_context_set_search_paths()
- PROJ_LIB env var
- hardcoded path at build time
- current dir
- ..\bin\share\proj with proposed PR
- %LOCALAPPDATA%\proj /  ${XDG_DATA_HOME}/proj for grids as files (cf https://
github.com/OSGeo/PROJ/issues/1750)
- RFC4 + networking enabled: %LOCALAPPDATA%\proj /  ${XDG_DATA_HOME}/proj for 
the SQLite3 cache.db
- RFC4 + networking enabled: network access to fetch the grid bit

Actually the current behaviour is a bit more subtle than that. For example, if 
the search paths are defined, this stops the following mechanisms to be 
attempted even if the grid is not found. Some tweakings likely needed to 
rationalize that a bit.


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