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N.B. also these files are quite good to demonstrate another problem with the label placement. Turn both layers on: why do the labels end up in such strange locations?

This is resolved by enabling the option "features don't act as obstacles for labels"; I was misunderstanding something before ;)

An interesting thing which might be related:

Using the "shapefile viewer" plugin, the shape with the missing label does not show up, although it does show up for the other file where the label isn't missing. But one of the other shapes doesn't actually show up for either file...

Also, strangely, if I select the object with the node tool, the layer where the label does show up says there is one geometry error (although there is no green cross indicating where it is), but the layer where the label doesn't show up says there is not.

And if I copy and paste the object that supposedly has a geometry error into the other file, then the label does show up. But then it disappears when I save the changes to the layer.

And if I copy and paste the object that supposedly has a geometry error into the other file, then the label does show up. But then it disappears when I save the changes to the layer.

Except I have now managed to copy it in and save the layer without the label disappearing... and I don't know what I did differently!

Another interesting thing: the file which does show the label (and which is supposed to have an invalid geometry) crashes globe plugin if the labels are turned on and I zoom right in :)

Using the VCexpress debugger I get the error message "Unhandled exception at 0x014d3ac3 (qgis_core.dll) in qgis.exe: 0xC0000005: Access violation reading location 0x0000000d.", and if I click Break it goes to this line in qgsgeometry.cpp:
if ( mDirtyGeos )

Oh, and there's this in the output window:
First-chance exception at 0x7c812afb in qgis.exe: Microsoft C++ exception: geos::util::TopologyException at memory location 0x0131ca68..
First-chance exception at 0x7c812afb in qgis.exe: Microsoft C++ exception: geos::util::TopologyException at memory location 0x0131ca5c..
First-chance exception at 0x7c812afb in qgis.exe: Microsoft C++ exception: geos::util::TopologyException at memory location 0x0131c9f4..
First-chance exception at 0x7c812afb in qgis.exe: Microsoft C++ exception: geos::util::TopologyException at memory location 0x0131cdc8..

For the record
I believe commit 05cdd5247a3435f84be06a27a4b4e213ed861e7d fixed the crashes, although I didn't test in the globe.
The label is still missing.

I can't reproduce #4634, but I can still reproduce this one.

I suspect this is the same issue as #3517 and #2949

I see that now it is labelled when labelling with "around centroid", just not with "free".

Hi Alister,

With recent changes, 'around' and 'offset' have their centroids converted to points (via GEOS) before being processed by PAL. This fixed some issues with how PAL was figuring centroids, but also means (apparently) that GEOS is somewhat adept at handling 'GEOS-invalid' geometries for some operations. The options Free, Horizontal, and Perimeter are still figured in PAL using the polygon if it is valid.

If you select the geometry in question that is not producing a label and run the following in Python Console:

 from qgis.utils import iface
 f = iface.activeLayer().selectedFeatures()[0]
 f. geometry().isGeosValid()

It returns False, i.e. an invalid GEOS geometry (as you noted above). Even though invalid, GEOS can generate a valid centroid for it. However, the following line in qgspallabeling.cpp will essentially skip running the polygon geometry through PAL (i.e. it's not PAL ignoring the geometry):

https://github.com/qgis/Quantum-GIS/blob/master/src/core/qgspallabeling.cpp#L723

What's needed here is a more robust means of trying to either fix invalid geometries on-the-fly, or for those geometries to be replaced with a temporary valid proxy via some other operation (like the 'buffer 0' trick) for labeling. Geometries are already duplicated (then deleted) for creating PAL labels.

More info on fixing geometries #4634-17

Update: as a test, I commented out qgspallabeling.cpp#L725-6, to see if PAL could also deal with the invalid geometry (or crash), and it did not, i.e. the label was not shown. So fixing invalid geometry before processing with PAL seems the thing to do.

I also tried a simple fix for invalid geometries with:

  QgsGeometry* geom = f.geometry();
  if ( !geom )
  {
    // try simple fix invalid polygons
    if ( geom->type() == QGis::Polygon )
    {
      geom = geom->buffer( 0.0, 8 ); // try simple buffer 0 fix

      if ( !geom )
      {
        return; // invalid geometry
      }
    }
    else
    {
      return; // invalid geometry
    }
  }

and later when converting to GEOS

  if ( geos_geom == NULL )
  {
    // try simple fix invalid polygons
    if ( geom->type() == QGis::Polygon )
    {
      geos_geom = geom->buffer( 0.0, 8 )->asGeos(); // try simple buffer 0 fix

      if ( geos_geom == NULL )
      {
        return; // invalid geometry
      }
    }
    else
    {
      return; // invalid geometry
    }
  }

But, this did not fix the 'temp' polygon. The only reason I can think of for why your 'SW...' file is labeling the invalid polygon, is that while QgsGeometry reports it is invalid, for whatever reason, it can still be converted to a GEOS geometry and used by PAL.