Re: small pg_combinebackup improvements

Hi,

On Wed, Oct 30, 2024 at 03:50:53PM -0400, Robert Haas wrote:
> Hi,
>
> Here are two small patches to improve pg_combinebackup slightly.
>
> 0001: I noticed that there is some logic in reconstruct.c that
> constructs a pathname of the form a/b//c instead of a/b/c. AFAICT,
> this still works fine; it just looks ugly. It's possible to get one of
> these pathnames to show up in an error message if you have a corrupted
> backup, e.g.:
>
> pg_combinebackup: error: could not open file
> "x/base/1//INCREMENTAL.6229": No such file or directory
> pg_combinebackup: removing output directory "xy"
>
> So 0001 fixes this.

Yeah, I don't think that was an issue per say but better to display a path of
the form a/b/c (to not be "distracted" by the output at the first place).

0001 looks pretty straightforward and good to me.

> 0002: If you try to do something like pg_combinebackup incr1 incr2 -o
> result, you'll get an error saying that the first backup must be a
> full backup. This is an implementation restriction that might be good
> to lift, but right now that's how it is. However, if you do
> pg_combinebackup full incr -o result, but the full backup happens to
> contain an INCREMENTAL.* file that also exists in the incremental
> backup, then you won't get an error. Instead you'll reconstruct a
> completely bogus full file and write it to the result directory. Now,
> this should really be impossible unless you're intentionally trying to
> break pg_combinebackup, but it's also pretty lame, so 0002 fixes
> things so that you get a proper error, and also adds a test case.
>

1 ===

+ * If we have only incremental files, and there's no full file at any
+ * point in the backup chain, something has gone wrong. Emit an error.
+ *
* Otherwise, reconstruct.
*/
if (copy_source != NULL)
copy_file(copy_source->filename, output_filename,
&checksum_ctx, copy_method, dry_run);
+ else if (sidx == 0 && source[0]->header_length != 0)
+ {
+ pg_fatal("full backup contains unexpected incremental file \"%s\"",
+ source[0]->filename);
+ }

What about moving the new comment just before the new "else if"?

2 ===

+ copy("$backup2path/base/1/$iname", "$backup1path/base/1/$iname")

Yeah, "copy" is needed. I tested to "just" create an empty incremental file
and got:

$ pg_combinebackup backup1 backup2 -o restored_full
pg_combinebackup: error: could not read file "backup1/base/1/INCREMENTAL.6113": read 0 of 4

Which is not the error we want to produce.

3 ===

+ qr/full backup contains unexpected incremental file/,
+ "pg_combinebackup fails");

s/pg_combinebackup fails/pg_combinebackup fails due to an unexpected incremental file/?

> Review appreciated. My plan is to commit at least to master and
> possibly back-patch. Opinions on whether to back-patch are also
> appreciated.

I'm not sure about 0001 but I think 0002 deserves a back patch as I think it fits
into the "low-risk fixes" category [0].

[0]: https://www.postgresql.org/support/versioning/

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com