pg15b2: large objects lost on upgrade

I noticed this during beta1, but dismissed the issue when it wasn't easily
reproducible. Now, I saw the same problem while upgrading from beta1 to beta2,
so couldn't dismiss it. It turns out that LOs are lost if VACUUM FULL was run.

| /usr/pgsql-15b1/bin/initdb --no-sync -D pg15b1.dat -k
| /usr/pgsql-15b1/bin/postgres -D pg15b1.dat -c logging_collector=no -p 5678 -k /tmp&
| psql -h /tmp postgres -p 5678 -c '\lo_import /etc/shells' -c 'VACUUM FULL pg_largeobject'
| rm -fr pg15b2.dat && /usr/pgsql-15b2/bin/initdb --no-sync -k -D pg15b2.dat && /usr/pgsql-15b2/bin/pg_upgrade -d pg15b1.dat -D pg15b2.dat -b /usr/pgsql-15b1/bin
| /usr/pgsql-15b2/bin/postgres -D pg15b2.dat -c logging_collector=no -p 5678 -k /tmp&

Or, for your convenience, with paths in tmp_install:
| ./tmp_install/usr/local/pgsql/bin/initdb --no-sync -D pg15b1.dat -k
| ./tmp_install/usr/local/pgsql/bin/postgres -D pg15b1.dat -c logging_collector=no -p 5678 -k /tmp&
| psql -h /tmp postgres -p 5678 -c '\lo_import /etc/shells' -c 'VACUUM FULL pg_largeobject'
| rm -fr pg15b2.dat && ./tmp_install/usr/local/pgsql/bin/initdb --no-sync -k -D pg15b2.dat && ./tmp_install/usr/local/pgsql/bin/pg_upgrade -d pg15b1.dat -D pg15b2.dat -b ./tmp_install/usr/local/pgsql/bin
| ./tmp_install/usr/local/pgsql/bin/postgres -D pg15b2.dat -c logging_collector=no -p 5678 -k /tmp&

postgres=# table pg_largeobject_metadata ;
16384 | 10 |

postgres=# \lo_list
16384 | pryzbyj |

postgres=# \lo_export 16384 /dev/stdout
lo_export
postgres=# table pg_largeobject;

postgres=# \dt+ pg_largeobject
pg_catalog | pg_largeobject | table | pryzbyj | permanent | heap | 0 bytes |

I reproduced the problem at 9a974cbcba but not its parent commit.

commit 9a974cbcba005256a19991203583a94b4f9a21a9
Author: Robert Haas <rhaas(at)postgresql(dot)org>
Date: Mon Jan 17 13:32:44 2022 -0500

pg_upgrade: Preserve relfilenodes and tablespace OIDs.

--
Justin