ERROR: Upgrade of postgis from version 2.1.9 r15770 to version 3.0 requires a dump/reload. See PostGIS manual for instructions

[strk]

Drone is currently failing because the upgrade test tries to do extension upgrade from 2.1.9 to 3.0 and the extension upgrade mechanism thinks such an upgrade is not possible between different major versions.

[strk]

The message comes from a check produced by utils/postgis_proc_upgrade.pl, specifically added to avoid incompatible upgrades:

  IF old_maj != new_maj THEN
    RAISE EXCEPTION ''Upgrade of MODULE from version % to version % requires a dump/reload. See PostGIS manual for instructions'', old_scripts, new_scripts;
  ELSE
    RETURN ''Scripts versions checked for upgrade: ok'';
  END IF;

Theoretically, that check should make loading postgis_ugrade.sql safe (doing nothing) when the target version is a major version higher than the source version of PostGIS, assuming a change in Major version will always only occur when the storage format changes, and thus assuming that changing input/output functions would in this case result in an unusable database.

As we changed Major version w/out such an occurrence we should ask ourselves whether we want to change semantic of Major version OR we want to differently determine if dump/reload is required in order NOT to brick someone's database…

[strk]

In 16713:

Consider 3.x upgrade-compatible with 2.x

Closes #4170

As I was at it, I also replaced the use of a temporary function
with a DO block.

[strk]

I've basically hard-coded the fact that 2.x and 3.x are soft-upgrade-compatible…