Make fsck(8) use pread(2)

[trasz]

Summary

Make fsck(8) use pread(2). This cuts the number of syscalls by half.

[mckusick]

This change looks fine to me.

[kib]

sbin/fsck_ffs/fsutil.c

609

Are callers depend on the specific file position after the call ?

614–615

Why did not you converted these uses ?

[mckusick]

In answer to kib:

The callers do not depend on the seek offset being set after the call.

The other three places in fsutil.c that do lseek followed by read/write (error followup in blread and blwrite along with blzero) should also be changed to use pread/pwrite.

[trasz]

The reason I hadn't changed the other instances is that while trivial, they are somewhat harder to test, and I've been bitten by trivial changes to rarely excercised branches in the past.

[mckusick]

I prefer not to do things halfway. Either we use lseek/read or we use pread. Having it half & half is just crappy coding style. It is possible to write some tests that will exercise the error cases, so if you want to make this change, then I would like you to do it completely.

[trasz]

Use pread/pwrite also in error cases.

[kib]

sbin/fsck_ffs/fsutil.c

573

It would be _very_ useful to print errno, in the followup commit.

618

Perhaps trailing ',' should be fixed, in the follow-up commit.

[mckusick]

This now looks good to me. I concur with kib's followup comments.

[trasz]

sbin/fsck_ffs/fsutil.c

573

Thanks for suggestion; see https://reviews.freebsd.org/D18570. It adds errno reporting to the normal path, in the other 'if' branch.