Re: preserving db/ts/relfilenode OIDs across pg_upgrade (was Re: storing an explicit nonce)

On Mon, Dec 6, 2021 at 11:25 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Sun, Dec 5, 2021 at 11:44 PM Sadhuprasad Patro <b(dot)sadhu(at)gmail(dot)com> wrote:
> > 3.
> > @@ -504,11 +525,15 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
> > */
> > pg_database_rel = table_open(DatabaseRelationId, RowExclusiveLock);
> >
> > - do
> > + /* Select an OID for the new database if is not explicitly configured. */
> > + if (!OidIsValid(dboid))
> > {
> > - dboid = GetNewOidWithIndex(pg_database_rel, DatabaseOidIndexId,
> > - Anum_pg_database_oid);
> > - } while (check_db_file_conflict(dboid));
> >
> > I think we need to do 'check_db_file_conflict' for the USER given OID
> > also.. right? It may already be present.
>
> Hopefully, if that happens, we straight up fail later on.

That's right. If a database with user-specified OID exists, the
createdb fails with a "duplicate key value" error.
If just a data directory with user-specified OID exists,
MakePGDirectory() fails to create the directory and the cleanup
callback createdb_failure_callback() removes the directory that was
not created by 'createdb()' function.
The subsequent create database call with the same OID will succeed.
Should we handle the case where a data directory exists and the
corresponding DB with that oid does not exist? I presume this
situation doesn't arise unless the user tries to create directories in
the data path. Any thoughts?

Thanks & Regards
Shruthi KC
EnterpriseDB: http://www.enterprisedb.com