[#6089] Fix typeahead searches

[gbp]

Relevant issue(s)

Fixes #6089

What does this do?

Update expected exception message when wildcard searches error allowing
fallback to an non-wildcard search to continue.

Why was this needed?

Poor search performance

Implementation notes

Locally in dev and test env all was working fine. I could only replicate in production, this is due to the size of the index. A large index can't always do a wildcard search, for example, when searching for Cabinet Office it will raise an exception with the message like:
WildcardError: Wildcard office* expands to more than 1000 terms

This message seems to have changed at some point when upgrading Xapian in the past.

[gbp]

It appearsQueryParserError still exists in the Xapian so maybe we need to catch both QueryParserError and WildcardError

[gbp]

Exception message matching was first added in 22b2db4

[garethrees]

Makes sense in principle!

Would be worth expanding the details into the commit message, and ideally link to the upstream code where these errors exist so that we have a better future reference.

It appears QueryParserError still exists in the Xapian so maybe we need to catch both QueryParserError and WildcardError

Yeah maybe they split these into two distinct concepts. We reference QueryParserError elsewhere so I assume both still exist, but whether we need to rescue from both here IDK.

I could only replicate in production

I think it's worth trying to test this fix against the prod DB somehow. Worst case just temporarily patch the live file and run the query in the console.

[gbp]

@garethrees I've made some changes:

1. managed to replicate this error in a spec by adding a configuration option to ActsAsXapian. So this is now tested.
2. moved the RuntimeError rescue into ActsAsXapian and now re-raising as exception within the ActsAsXapian model to allow us to catch know exception classes in the core Alaveteli code.
3. added exception notifications for us

[gbp]

In trying to write a spec to show the original QueryParserError exception message I've worked out that this only is raised when using the Xapian::QueryParser::FLAG_BOOLEAN flag which we don't use in typeahead search.

[1] pry(main)> ActsAsXapian.readable_init
=> snip
[2] pry(main)> ActsAsXapian.query_parser.parse_query('dog AND', Xapian::QueryParser::FLAG_BOOLEAN)
RuntimeError: QueryParserError: Syntax: <expression> AND <expression>
from (pry):2:in `parse_query'
[3] pry(main)> ActsAsXapian.query_parser.parse_query('dog AND', Xapian::QueryParser::FLAG_LOVEHATE | Xapian::QueryParser::FLAG_SPELLING_CORRECTION | Xapian::QueryParser::FLAG_WILDCARD)
=> #<Xapian::Query:0x00007fd9082652d0 @__swigtype__="_p_Xapian__Query">

[gbp]

In production we are hitting the wildcard expansion limit, which is currently set to 1000. We should consider increasing this limit. We can do this with 9d57051 by adding a config/xapian.yml file, something like:

test:
  max_wildcard_expansion: 10

development:
  max_wildcard_expansion: 500

production:
  max_wildcard_expansion: 1000

From my tests in the production Rails console, increasing to 1075 allows the wildcard query for 'Cabinet Office' to succeed. We would also need to consider server memory usage /cc @sagepe

[garethrees]

LGTM! Amazing work 🦸‍♂️

Couple of tiny changes suggested but no need for re-review.