Postgres in alpine has collation warning if database was created in another OS.

Postgres shows warning WARNING: database "postgres" has no actual collation version, but a version was recorded if database was created in another OS.

Steps for reproduce:

1. Execute postgres in debian image: docker run --rm -e POSTGRES_PASSWORD=password -e POSTGRES_DB=app -v $(pwd)/data:/var/lib/postgresql/data postgres:15.8-bookworm
2. Stop this
3. Execute postgres in alpine image: docker run --rm --name postgres -v $(pwd)/data:/var/lib/postgresql/data postgres:15.8-alpine3.20
4. Run psql:

$ docker exec -ti --user postgres postgres psql
WARNING:  database "postgres" has no actual collation version, but a version was recorded
psql (15.8)
Type "help" for help.

postgres=# \c app
WARNING:  database "app" has no actual collation version, but a version was recorded
You are now connected to database "app" as user "postgres".
app=# 

Research:

I found that a warning occurs in the CheckMyDatabase function when comparing the collate version in the database and the current one in this row.

...
	if (!isnull)
	{
		char	   *actual_versionstr;
		char	   *collversionstr;

		collversionstr = TextDatumGetCString(datum);

		actual_versionstr = get_collation_actual_version(dbform->datlocprovider, dbform->datlocprovider == COLLPROVIDER_ICU ? iculocale : collate);
		if (!actual_versionstr)
			/* should not happen */
			elog(WARNING,
				 "database \"%s\" has no actual collation version, but a version was recorded",
				 name);
		else if (strcmp(actual_versionstr, collversionstr) != 0)
...

After calling get_collation_actual_version, the actual_version_str variable is NULL, because the get_collation_actual_version function after the preprocessor looks like:

char *
get_collation_actual_version(char collprovider, const char *collcollate)
{
	char	   *collversion = NULL;

		if (collprovider == COLLPROVIDER_LIBC &&
			pg_strcasecmp("C", collcollate) != 0 &&
			pg_strncasecmp("C.", collcollate, 2) != 0 &&
			pg_strcasecmp("POSIX", collcollate) != 0)
	{
	}

	return collversion;
}

And it always returns NULL

Reproduce the warning with the supplied postgres:15.8-bookworm and postgres:15.8-alpine3.20 Docker commands. Read CheckMyDatabase in src/backend/utils/init/postinit.c and trace get_collation_actual_version, then establish why actual_versionstr is NULL and whether the cross-OS behavior is expected.