pgsql: Fix data loss when restarting the bulk_write facility

Fix data loss when restarting the bulk_write facility

If a user started a bulk write operation on a fork with existing data
to append data in bulk, the bulk_write machinery would zero out all
previously written pages up to the last page written by the new
bulk_write operation.

This is not an issue for PostgreSQL itself, because we never use the
bulk_write facility on a non-empty fork. But there are use cases where
it makes sense. TimescaleDB extension is known to do that to merge
partitions, for example.

Backpatch to v17, where the bulk_write machinery was introduced.

Author: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Reported-By: Erik Nordström <erik(at)timescale(dot)com>
Reviewed-by: Erik Nordström <erik(at)timescale(dot)com>
Discussion: https://www.postgresql.org/message-id/CACAa4VJ%2BQY4pY7M0ECq29uGkrOygikYtao1UG9yCDFosxaps9g@mail.gmail.com

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9695835538c2c8e9cd0048028b8c85e1bbf5c79c

Modified Files
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src/backend/storage/smgr/bulk_write.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)