Exist: Weird "cannot convert xs:boolean('true') to a node set" message

[xatapult]

What is the problem`

The following XQuery gives an error message that as far as I understand things doesn't make sense:

let $col as document-node()* := collection('/db/apps/dashboard')
let $doc as element()* := $col/xyz (: Resolves to an empty sequence :)
return 
  $doc/*[not(self::abc)]
  (: Results in: exerr:ERROR cannot convert xs:boolean('true') to a node set :)

What did you expect

An empty sequence as output

Describe how to reproduce or add a test

Run the XQuery above

Context information

Please always add the following information

• eXist 4.3.1 build 201807241821
• Java version 1.8.0_181
• Operating system W10
• 64 bit
• JAR installed
• Any custom changes in e.g. conf.xml? Data directory moved to outside eXist installation directory

[xatapult]

The problem disappears if you write empty(self::abc) instead of not(self::abc) so that's an easy workaround.

However, still something seems wrong...

[xatapult]

Another hint: I thought it was the use of the self:: axis, but this gives the same error: $doc/*[not(*)]

[joewiz]

Here's a simplified test case:

<foo/>/xyz/*[not(self::abc)]

eXist returns:

exerr:ERROR cannot convert xs:boolean('true') to a node set

Saxon and BaseX both return the empty sequence.

[duncdrum]

might be related to #2158

[adamretter]

Testing on eXist-db 5.3.0-SNAPSHOT.
So regards the examples of @xatapult and @joewiz I found the following to be simplest for me:

Works correctly

<foo/>/*[not(self::abc)]

Does NOT work correctly

<foo/>/xyz/*[not(self::abc)]

Analysis

Having looked through the code this is down to a bad optimisation in eXist-db around fn:not, which you can see here - https://github.com/eXist-db/exist/blob/eXist-5.2.0/exist-core/src/main/java/org/exist/xquery/functions/fn/FunNot.java#L75

public int returnsType() {
    //TODO: test for possible performance lost
    //return Type.BOOLEAN;
    return Type.subTypeOf(getArgument(0).returnsType(), Type.NODE)
        ? Type.NODE
        : Type.BOOLEAN;
}

Basically, this optimisation interacts with optimisation code in the predicate. Unfortunately that is quite complex, and obviously wrong in this instance... i.e. where the internal result of the predicate will be an empty sequence.

There are a few options for how this could be fixed:

1. FunNot#getDependencies() which is also part of the optimisation, does not correctly calculate a dependency on the context item for the inner self:: axis call.
2. If the optimised lookup by node in Predicate#evalPredicate throws the exception, we could fallback to the non-optimised boolean lookup.
3. We could remove the optimisation entirely - I am not sure what the impact of that is.

This needs input from @wolfgangmm as I think that he likely authored these optimisations...

[adamretter]

@wolfgangmm also you should take a look at https://github.com/eXist-db/exist/issues/3289 which is similar in so far as it involved bad optimisations in the predicate evaluation, but is subtly different due to the cause that switches on those optimisations.

[adamretter]

Note we should check that - https://github.com/eXist-db/exist/pull/3075 did not introduce a regression in this area.