Re: Collation version tracking for macOS

On Wed, Jun 8, 2022 at 12:23 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> ISTM that there are two mostly-distinct questions here:
>
> 1. How do we link to multiple versions of ICU at the same time, in a
> way that is going to work smoothly on mainstream platforms?
>
> 2. What semantics around collations do we want for Postgres once we
> gain the ability to use multiple versions of ICU at the same time? For
> example, do we want to generalize the definition of a collation, so
> that it's associated with one particular ICU version and collation for
> the purposes of on-disk compatibility, but isn't necessarily tied to
> the same ICU version in other contexts, such as on a dump and restore?

Yeah. Well I couldn't resist doing some (very!) experimental hacking.
See attached. The idea of putting a raw library name in there is just
a straw-man, and I already found a major problem with it: I also need
to get my hands on u_strToLower and friends for formatting.c, but
those functions are in a different library that needs to be dlopen'd
separately, so we need *two* names. That's not done in the attached
patch, but at least this demonstrates some of the mechanics of a
dlopen() based solution that can do the collating part... of course
there are all kinds of problems apparent (security of loading
arbitrary libraries, API stability, interaction with the "default" ICU
that our binary is linked against, creation of initial set of
collations in initdb, naming, upgrades, ...).

Demo:

$ sudo apt-get install libicu63 libicu67

postgres=# create schema icu63;
CREATE SCHEMA
postgres=# create schema icu67;
CREATE SCHEMA
postgres=# create collation icu63."en-US-x-icu" (provider = icu,
locale = 'libicui18n.so.63:en-US');
CREATE COLLATION
postgres=# create collation icu67."en-US-x-icu" (provider = icu,
locale = 'libicui18n.so.67:en-US');
CREATE COLLATION
postgres=# select collname, collnamespace::regnamespace,
colliculocale, collversion
from pg_collation
where collname = 'en-US-x-icu';
collname | collnamespace | colliculocale | collversion
-------------+---------------+------------------------+-------------
en-US-x-icu | pg_catalog | en-US | 153.14
en-US-x-icu | icu63 | libicui18n.so.63:en-US | 153.88
en-US-x-icu | icu67 | libicui18n.so.67:en-US | 153.14
(3 rows)
postgres=# select relname from pg_class order by relname collate
icu63."en-US-x-icu" limit 2;
relname
---------------------------
_pg_foreign_data_wrappers
_pg_foreign_servers
(2 rows)