Re: [Bug-tar] seek_hole proposal [v2]

[Pavel Raiskup]

On Wednesday, March 05, 2014 12:14:17 Joerg Schilling wrote:
> Pavel Raiskup <address@hidden> wrote:
>
> > Note that after discussion [1] I still think that existing ST_IS_SPARSE
> > macro is better for file-sparseness detection than using SEEK_HOLE (not
> > worth having additional syscalls open~>seek~>close).
> 
> Your code is not compatible to the SEEK_HOLE interface. A file is sparse in 
> case that pathconf()/fpathconf(f, _PC_MIN_HOLE_SIZE) return a positive number 
> and lseek(f, (off_t)0, SEEK_HOLE) returns a number < stat.st_size.

Yes, apart from pathconf, the patch-v1 contained bug - I should react on
hole_offset, not data_offset.  And there was yet another bug - we need to
reposition the file offset to the beginning in this case (no FS support);
so there needs to be done this change:

       if (offset == 0 /* first loop */
-          && data_offset == st->stat.st_size)
-        return false;
+          && data_offset == 0
+          && hole_offset == st->stat.st_size)
+        {
+          lseek (fd, 0, SEEK_SET);
+          return false;
+        }

Joerg, thanks for pointing that out.

> see "man pathconf":
> [...]
> 
> In other cases, the file stil may be sparse, but the filesystem does not 
> support SEEK_HOLE. I e.g. doubt that Linux correctly implements SEEK_HOLE
> for NFS.

Correct, Linux's NFS returns "just" the virtual hole at the end of sparse
file, which is now OK - we would still fallback to raw hole detection.
Fixed patch v2 attached.

Pavel

0001-tar-use-SEEK_HOLE-for-hole-detection.patch
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