[QGIS-Developer] editbuffet::commitChanges limitations found working on #15741... suggestions?

[Luigi Pirelli]

Hi (especially MarcoH)

Analysis:
working on #15741 [1] I found that the the issue is comes mainly from
the following commit:
https://github.com/qgis/QGIS/commit/8d70a5141b0f56c40fc19e32d5d55475e22ec10f
and secondary from:
https://github.com/qgis/QGIS/commit/7b64feb7e101ac7f205737dce241c9d8fa03c452#diff-989db6afac5dc91e42cf6d3e404431b3L6656
that do a better cleanup of the previous commit.

btw these commits just allowed to raise a limitation of the:
QgsVectorLayerEditBuffer::commitChanges [2] class (see doc [3])
commitChanges is a multi staged commit task with a limitation, each
stage, if success, clean it's stage buffer => if can be recovered if
one of the next stages fail.

This is exactly what happen in the issue. Merge Features is done doing:

1) deleted features to merge
2) add the new merged feature

stage 2 will fail (due the fact that if postgis layer is linestring,
the merged is multi type => incompatible). but failing the roll back
(done aborting the editing session) rollback only the stage 2, because
stage 1 was success and cleaned from editBuffer.

Splutions:
There is no a unique solution. These are the options:
1) revert or partially revert the above  commit
8d70a5141b0f56c40fc19e32d5d55475e22ec10f
2) don't clean the stage buffers, and do them only if no error is
found (opening to other regressions)
3) refactor editing buffer (would make sense integrating
QgsTransaction and QgsTransactionGroups where possible)... but IMHO
out of the scope of a bug fix
4) anything else?

I would prefer (my personal style) for a soft solution (1)... any opinion?

Luigi Pirelli

[1] https://issues.qgis.org/issues/15741
[2] https://github.com/qgis/QGIS/blob/release-2_18/src/core/qgsvectorlayereditbuffer.cpp#L292
[3] https://github.com/qgis/QGIS/blob/release-2_18/src/core/qgsvectorlayereditbuffer.h#L93

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