Use public Xapian API in preference

[ojwb]

Xapian 1.4.12 and later have a Database::size() method which returns the number of shards, so use this instead which fixes a compilation failure with Xapian 2.0.x where the internal object is no longer a std::vector.

For older versions, we still need to peek at the internal object but assuming internal details is less problematic there at least.

[ojwb]

Duplicating the old version of the code seems clumsy, but I didn't see a way to combine the conditionals - obvious options such as this fail if XAPIAN_AT_LEAST isn't defined:

#if defined(XAPIAN_AT_LEAST) && XAPIAN_AT_LEAST(1,4,12)

I tested by appending an X so the macro isn't defined and I get:

../src/suggestion.cpp:103:50: error: missing binary operator before token ‘(’
  103 | #if defined(XAPIAN_AT_LEASTX) && XAPIAN_AT_LEASTX(1,4,12)

XAPIAN_AT_LEAST was added in Xapian 1.4.2 so if you're happy to require that meson.build could probably specify it and XAPIAN_AT_LEAST just be used unconditionally. 1.4.2 was released 9 years ago now.

[ojwb]

It looks like the defacto requirement is at least Xapian 1.4.0 currently (since Xapian::LatLongCoord and related classes are used unconditionally).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.18%. Comparing base (9ca8eb0) to head (e3c39f7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1018   +/-   ##
=======================================
  Coverage   56.18%   56.18%           
=======================================
  Files         101      101           
  Lines        4989     4989           
  Branches     2170     2170           
=======================================
  Hits         2803     2803           
  Misses        739      739           
  Partials     1447     1447           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[veloman-yunkan]

I think it's safe to use XAPIAN_AT_LEAST unconditionally. You provided enough justification for doing so.

I also think that we can afford requiring a minimum version of 1.4.12 (or higher) for our Xapian dependency. Ubuntu 22.04 which is the oldest build platform we currently support across the Kiwix projects has 1.4.18.
@kelson42 Do you agree? Or libzim has to provide broader guarantees for backward compatibility compared to libkiwx?

[kelson42]

I also think that we can afford requiring a minimum version of 1.4.12

I confirm

[ojwb]

I also think that we can afford requiring a minimum version of 1.4.12

I confirm

OK, I'll update the patch.

Should anyone want to try to build on an older Ubuntu, 20.04 has 1.4.14 packaged and I also provide a PPA with backports of recent Xapian releases which has xapian-core 1.4.22 for Ubuntu 16.04 and later:

https://launchpad.net/~xapian/+archive/ubuntu/backports

(Not sure why it lacks trusty (14.04) packages for xapian-core but has them for the others.)