pgsql: Ensure that creation of an empty relfile is fsync'd at checkpoin

Ensure that creation of an empty relfile is fsync'd at checkpoint.

If you create a table and don't insert any data into it, the relation file
is never fsync'd. You don't lose data, because an empty table doesn't have
any data to begin with, but if you crash and lose the file, subsequent
operations on the table will fail with "could not open file" error.

To fix, register an fsync request in mdcreate(), like we do for mdwrite().

Per discussion, we probably should also fsync the containing directory
after creating a new file. But that's a separate and much wider issue.

Backpatch to all supported versions.

Reviewed-by: Andres Freund, Thomas Munro
Discussion: https://www.postgresql.org/message-id/d47d8122-415e-425c-d0a2-e0160829702d%40iki.fi

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d431644b43fcce9b7c2f677db18452b0ee216b42

Modified Files
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src/backend/storage/smgr/md.c | 3 +++
1 file changed, 3 insertions(+)