[Qgis-developer] Proposed plan to fix issue with concurrent opening of OGR datasources (aka QGIS and MapInfo)

[Even Rouault]

Hi,

I'm considering addressing an issue that mainly involves concurrent use of a 
MapInfo TAB dataset by QGIS and MapInfo, as described in 
https://hub.qgis.org/issues/14378, although the planned fix would be a bit more 
general.

The problem is that when QGIS opens a OGR datasource it tries to open it with 
the update mode of the OGROpen() API, which in turn opens the underlying 
file(s) with a mode (dwDesiredAccess = GENERIC_READ | GENERIC_WRITE and 
dwShareMode = 
FILE_SHARE_READ | FILE_SHARE_WRITE with the Windows CreateFile API()) that is 
incompatible with the mode with which MapInfo tries to open files in read-only 
(dwDesiredAccess = GENERIC_READ and dwShareMode = 
FILE_SHARE_READ). Hence MapInfo cannot open a MapInfo dataset already opened 
by QGIS in update mode.

The solution I'm considering would be to modify the QGIS OGR provider to :
- try to open the dataset in update mode as currently, but if it is 
successful, go back to re-opening it in read-only mode. (we need to try to 
open in update mode so as to have appropriate capabilities)
- for operations like addFeatures(), addAttributes(), etc ... that need update 
permissions, check the current open mode, and if not update mode already, re-
open the OGR datasource in update mode
- go back to read-only mode when the user exists the edition mode (this would 
need a new method in the QgsVectorDataProvider API to warn about that, since 
the provider has currently no way of knowing that)

There would be of course a cost to pay due to this open / close operations, so 
I think I would limit it to shapefiles and mapinfo for now, since the opening 
of such datasets doesn't involve a lot of I/O ( the most costly is for 
shapefiles where the whole .shx is ingested into memory ).

Another related fix would be to implement reloadData() in the OGR provider so 
that the user can resynchronize its QGIS session for a dataset modified by 
another software.

Thoughts ?

Even

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