Re: Fix handling of copy_file_range() return value

On 22.06.26 10:46, Yingying Chen wrote:
> 
> 
> On Mon, Jun 22, 2026 at 3:20 PM Peter Eisentraut <peter@eisentraut.org 
> <mailto:peter@eisentraut.org>> wrote:
> 
>     While checking return/error handling of file system calls, I found that
>     the copy_file_range() call in pg_combinebackup has a potential problem.
>     If copy_file_range() returns 0, which is a documented condition, then
>     the loop never makes progress and could spin forever.
> 
>     The other uses of copy_file_range() in the tree are surrounded by
>     different logic and don't appear to have this problem.
> 
>     My suggested fix is to make a return value of 0 an error.  It most
>     likely indicates that the source file has an unexpected size.
> 
> Hi, Peter,
> 
> Thanks for the patch. I agree wb==0 should be treated as an error, so 
> the fix direction looks good to me.

Thanks, I have committed the patch and backpatched it.

> I have a small comment:
> ====
>          wb = copy_file_range(s->fd, &off, wfd, NULL, BLCKSZ - nwritten, 0);
> 
>          if (wb < 0)
>                  pg_fatal("error while copying file range from \"%s\" to 
> \"%s\": %m",
>                                   s->filename, output_filename);
>          else if (wb == 0)
>                  pg_fatal("unexpected end of file while copying file 
> range from \"%s\" to \"%s\"",
>                                   s->filename, output_filename);
> 
> 
> As copy_file_range copies from s->fd, should pg_fatal just s->filename 
> in the error message? In that case, input_filename is no longer used in 
> write_reconstructed_file(), then it might be removed from the argument list.

Yes, interesting.  I notice that the input_filename function argument 
wasn't used in the first commit dc212340058.  Only later when the 
copy_file_range() support was added (commit ac811015513), it became 
used.  Maybe input_filename is fully redundant with s->filename, but 
this isn't very thoroughly documented, so I'm not sure.