Attempting such an operation would already fail, but in various and
confusing ways. For example, while in recovery, some elog() messages
would be reported, but these should never be user-facing. This commit
restricts any write operations done on large objects in a read-only
context, so as the errors generated are more user-friendly. This is per
the discussion done with Tom Lane and Robert Haas.
Some regression tests are added to check the case of all the SQL
functions working on large objects (including an update of the test's
alternate output).
Author: Yugo Nagata
Discussion: https://postgr.es/m/20220527153028.61a4608f66abcd026fd3806f@sraoss.co.jp
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