Re: pg_upgrade --copy-file-range

Hi,

I took a quick look at the remaining part adding copy_file_range to
pg_combinebackup. The patch no longer applies, so I had to rebase it.
Most of the issues were trivial, but I had to fix a couple missing
prototypes - I added them to copy_file.h/c, mostly.

0001 is the minimal rebase + those fixes

0002 has a couple review comments in copy_file, and it also undoes a lot
of unnecessary formatting changes (already pointed out by Peter a couple
days ago).

A couple review comments:

1) AFAIK opt_errinfo() returns pointer to the local "buf" variable.

2) I wonder if we even need opt_errinfo(). I'm not sure it actually
makes anything simpler.

3) I think it'd be nice to make CopyFileMethod more consistent with
transferMode in pg_upgrade.h (I mean, it seems wise to make the naming
more consistent, it's probably not worth unifying this somehow).

4) I wonder how we came up with copying the files by 50 blocks, but I
now realize it's been like this before this patch. I only noticed
because the patch adds a comment before buffer_size calculation.

5) I dislike the renaming of copy_file_blocks to pg_copyfile. The new
name is way more generic / less descriptive - it's clear it copies the
file block by block (well, in chunks). pg_copyfile is pretty vague.

6) This leaves behind copy_file_copyfile, which is now unused.

7) The patch reworks how combinebackup deals with alternative copy
implementations - instead of setting strategy_implementation and calling
that, the decisions now happen in pg_copyfile_offload with a lot of
conditions / ifdef / defined ... I find it pretty hard to understand and
reason about. I liked the strategy_implementation approach, as it forces
us to keep each method in a separate function.

Perhaps there's a reason why that doesn't work for copy_file_range? But
in that case this needs much clearer comments.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company