feat: Importing large objects from client side

[toppyy]

Closes #376. Introduces a postgres-specific function postgresImportLargeObject for importing large objects from client side using libpq's lo_import_with_oid

[krlmlr]

Thanks, nice! Checks are still failing: https://github.com/r-dbi/RPostgres/actions/runs/11416140787/job/31767025307?pr=472#step:16:263

[krlmlr]

Suggested change

	if (is.null(oid) | is.na(oid)) stopc("'oid' cannot be NULL/NA")
	if (is.null(oid) || is.na(oid)) stopc("'oid' cannot be NULL/NA")

Or perhaps this could be two checks?

[krlmlr]

What will the error look like if we let libpq do the check?

Please note that it is not a good idea to use this function to test before trying to open a file. On a multi-tasking system, it is possible that the accessibility of a file will change between the time you call file.access() and the time you try to open the file.

[toppyy]

libpq returns an InvalidOid (a zero). Letting libpq do the check has the downside that we cannot inform the user of the specific problem as libpq's lo_import_with_oid() returns a zero if the 1) file does not exists or 2) the assigned oid is already in use.

So currently if the accessibility of the file has changed, an error is thrown with the message Import failed. Maybe you tried to write to an existing oid?

[krlmlr]

I see:

If an error occurs while executing any one of these functions, the function will return an otherwise-impossible value, typically 0 or -1. A message describing the error is stored in the connection object and can be retrieved with PQerrorMessage() .

Would that work well enough?

Also: how do we auto-assign the OID?

[toppyy]

Thanks! I was unaware of PQerrorMessage() - that solves the problem! With the changes in the latest commit, when importing to an existing oid we get:

Error: ERROR: duplicate key value violates unique constraint "pg_largeobject_metadata_oid_index"
DETAIL: Key (oid)=(999) already exists.

(It's a bit clunky as the term "error" is repeated, but informative nonetheless.)

pqlib auto-assigns the oid if the argument 'oid' is zero. From the docs:

If lobjId is InvalidOid (zero) then lo_import_with_oid assigns an unused OID (this is the same behavior as lo_import).

I also changed the return- and argument types to 'Oid' from int as oid's are implemented as unsigned integers.