Flaky assertion failure on empty PSO scan

[niklas88]

On Wikidata Full an evaluation script generates queries for looking up aliases for predicates and entities.

One of the queries it generates is the following looking for aliases for wikibase:claim
which should return an empty result but actually results in an assertion failure.
I've broken this down to the following simple query which should also return an empty result

PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
SELECT ?label WHERE {
   wikibase:claim skos:altLabel ?label .
   FILTER langMatches(lang(?label),"en") .
}

Instead the following assertion is triggered

Fri Mar 15 10:52:15.645 - DEBUG: Got 1 subplans to create.
Fri Mar 15 10:52:15.645 - DEBUG: Creating execution plan.
Fri Mar 15 10:52:15.645 - DEBUG: IndexScan result computation...
Fri Mar 15 10:52:15.645 - DEBUG: Performing PSO scan of relation @en@<http://www.w3.org/2004/02/skos/core#altLabel> with fixed subject: <http://wikiba.se/ontology#claim>...
Fri Mar 15 10:52:15.645 - ERROR: Failed to compute Operation result for:
Fri Mar 15 10:52:15.645 - ERROR: SCAN PSO with P = "@en@<http://www.w3.org/2004/02/skos/core#altLabel>", S = "<http://wikiba.se/ontology#claim>"
Fri Mar 15 10:52:15.645 - ERROR: ASSERT FAILED (it != _blocks.begin(); in /app/src/index/MetaDataTypes.cpp, line 111, function std::pair<long int, long unsigned int> BlockBasedRelationMetaData::getBlockStartAndNofBytesForLhs(Id) const)

Interestingly the same works for rdfs:label

PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?label WHERE {
   wikibase:claim rdfs:label ?label .
   FILTER langMatches(lang(?label),"en") .
}

This happend both on an older index as well as one I built yesterday with the xsd:double fixes.

[niklas88]

This Bug is affected by PR #208, while that does indeed fix the original assertion error we get a new one

Mon Mar 18 16:28:49.012 - ERROR: SCAN PSO with P = "@en@<http://www.w3.org/2004/02/skos/core#altLabel>", S = "<http://wikiba.se/ontology#claim>"
Mon Mar 18 16:28:49.012 - ERROR: ASSERT FAILED (it != _blocks.begin(); in /app/src/index/MetaDataTypes.cpp, line 144, function std::pair<long int, long unsigned int> BlockBasedRelationMetaData::getFollowBlockForLhs(Id) const)
Mon Mar 18 16:28:49.013 - DEBUG: Sent 649 bytes.

I suspect this to be the almost identical problem. Stupidly as the code looked good and understandable to me I didn't test on this bug until after merging the change yeah really should have done that.

[joka921]

I just opened #211 that also adresses this bug in a similar way as the first one.

• I also found another possible problem connected with these issues that i defused.

[niklas88]

Fixed by #211