pgsql: Free disk space for dropped relations on commit.

Free disk space for dropped relations on commit.

When committing a transaction that dropped a relation, we previously
truncated only the first segment file to free up disk space (the one
that won't be unlinked until the next checkpoint).

Truncate higher numbered segments too, even though we unlink them on
commit.  This frees the disk space immediately, even if other backends
have open file descriptors and might take a long time to get around to
handling shared invalidation events and closing them.  Also extend the
same behavior to the first segment, in recovery.

Back-patch to all supported releases.

Bug: #16663
Reported-by: Denis Patron <denis.patron@previnet.it>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Reviewed-by: Neil Chen <carpenter.nail.cz@gmail.com>
Reviewed-by: David Zhang <david.zhang@highgo.ca>
Discussion: https://postgr.es/m/16663-fe97ccf9932fc800%40postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/50f338574242be955c79ed0b3e193ce3ff4cd5eb

Modified Files
--------------
src/backend/storage/smgr/md.c | 100 +++++++++++++++++++++++++++++-------------
1 file changed, 69 insertions(+), 31 deletions(-)