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

[Even Rouault]

Le mardi 22 mars 2016 11:39:07, Hugo Mercier a écrit :
> On 22/03/2016 11:27, Even Rouault wrote:
> >>> - 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)
> >> 
> >> Thank you for taking care of that.
> >> 
> >> Could you elaborate on the last item ? Does it mean adding something
> >> like beginEdit() / endEdit() on QgsVectorDataProvider ?
> > 
> > Yes
> > 
> >> Is it an API break
> > 
> > API extension I'd say. The default implementation would be {}.
> > 
> >> and if not how does it work for existing code that
> >> does not call begin/end edit ?
> > 
> > From what I can see in QgsVectorLayer, you cannot call addFeature(),
> > addFeatures(), changeAttributeValue(), deleteFeature(), deleteFeatures(),
> > etc.. if there's no mEditBuffer active. And the only way to have one
> > active is to call startEditing() before. So unless I'm missing
> > something, it doesn't look like you can do changes to a dataset without
> > doing them between startEditing() and commitChanges()/rollBack()
> 
> True.
> But you can also directly access the provider update methods without
> edit buffers and without startEditing / commitChanges.

Good point. I didn't think at that use case of direct access to the provider 
API.

> 
> ... but after all ... if I understood correctly your intents, for
> existing codes that use this direct provider access, update functions
> will reopen the source in write mode and they will always stay in that
> mode because the endEdit() method will never be called ?

Yes.

One possibility would be also to go back to read-only mode with a timer. For a 
write operation, outside of beginEdit() / endEdit(), that lead to re-opening 
in update-mode, after X seconds without write activity, the dataset would be 
re-opened in update mode.


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