Re: pg_upgrade --copy-file-range

Here's a patch reworked along the lines from a couple days ago.

The primary goals were to add clone/copy_file_range while minimizing
unnecessary disruption, and overall cleanup of the patch. I'm not saying
it's committable, but I think the patch is much easier to understand.

The main change is that this abandons the idea of handling all possible
cases in a single function that looks like a maze of ifdefs, and instead
separates each case into it's own function and the decision happens much
earlier. This is pretty much exactly what pg_upgrade does, BTW.

There's maybe an argument that these functions could be unified and
moved to a library in src/common - I can imagine doing that, but I don't
think it's required. The functions are pretty trivial wrappers, and it's
not like we expect many more callers. And there's probably stuff we'd
need to keep out of that library (e.g. the decision which copy/clone
methods are available / should be used or error reporting). So it
doesn't seem worth it, at least for now.

There's one question, though. As it stands, there's a bit of asymmetry
between handling CopyFile() on WIN32 and the clone/copy_file_range on
other platforms). On WIN32, we simply automatically switch to CopyFile
automatically, if we don't need to calculate checksum. But with the
other methods, error out if the user requests those and we need to
calculate the checksum.

The asymmetry comes from the fact there's no option to request CopyFile
on WIN32, and we feel confident it's always the right thing to do (safe,
faster). We don't seem to know that for the other methods, so the user
has to explicitly request those. And if the user requests --clone, for
example, it'd be wrong to silently fallback to plain copy.

Still, I wonder if this might cause some undesirable issues during
restores. But I guess that's why we have --dry-run.

This asymmetry also shows a bit in the code - the CopyFile is coded and
called a bit differently from the other methods. FWIW I abandoned the
approach with "strategy" and just use a switch on CopyMode enum, just
like pg_upgrade does.

There's a couple more smaller changes:

- Addition of docs for --clone/--copy-file-range (shameless copy from
pg_upgrade docs).

- Removal of opt_errinfo - not only was it buggy, I think the code is
actually cleaner without it.

- Removal of EINTR retry condition from copy_file_range handling (this
is what Thomas ended up for pg_upgrade while committing that part).

Put together, this cuts the patch from ~40kB to ~15kB (most of this is
due to the cleanup of unnecessary whitespace changes, though).

I think to make this committable, this requires some review and testing,
ideally on a range of platforms.

One open question is how to allow testing this. For pg_upgrade we now
have PG_TEST_PG_UPGRADE_MODE, which can be set to e.g. "--clone". I
wonder if we should add PG_TEST_PG_COMBINEBACKUP_MODE ...

regards

--
Tomas Vondra
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