FR: GTiff: Allow concurrent reading of single blocks

[lnicola]

GTiff supports multi-threaded reads by using PRead (and, I suspect, by doing the I/O outside of libtiff/libgeotiff). It would be useful if this could be extended to reading single blocks. Currently, the alternatives are:

• not using threads
• opening the file multiple times: this works, but is not really cache-efficient, and doesn't work well with the low default NOFILE limit
• reading larger regions using the existing threading support: this will be less efficient when the caller knows it only needs to touch a limited number of blocks

[rouault]

if this could be extended to reading single blocks

I don't see any easy resolution for that. This goes against a lot of assumptions in the design of GDAL. If that was going to be implemented, it would have implications probably on core and should be done driver by driver, with a lot of caution to lock the appropriate data structures, and probably a driver capability advertising that sch multi-threaded reads can be safely done. For GTiff, not only the file access should be done in a per-thread way, but a TIFF* handle also hides a lot of state and shouldn't be used from several threads.

[latot]

This seems great!