wrapper.c: add xpread() similar to xread()

It is a common mistake to call read(2)/pread(2) and forget to
anticipate that they may return error with EAGAIN/EINTR when the
system call is interrupted.

We have xread() helper to relieve callers of read(2) from having to
worry about it; add xpread() helper to do the same for pread(2).

Update the caller in the builtin/index-pack.c and the mmap emulation
in compat/.

Signed-off-by: Yiannis Marangos <yiannis.marangos@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
read-cache.c: verify index file before we opportunistically update it

Before we proceed to opportunistically update the index (often done
by an otherwise read-only operation like "git status" and "git diff"
that internally refreshes the index), we must verify that the
current index file is the same as the one that we read earlier
before we took the lock on it, in order to avoid a possible race.

In the example below git-status does "opportunistic update" and
git-rebase updates the index, but the race can happen in general.

  1. process A calls git-rebase (or does anything that uses the index)

  2. process A applies 1st commit

  3. process B calls git-status (or does anything that updates the index)

  4. process B reads index

  5. process A applies 2nd commit

  6. process B takes the lock, then overwrites process A's changes.

  7. process A applies 3rd commit

As an end result the 3rd commit will have a revert of the 2nd commit.
When process B takes the lock, it needs to make sure that the index
hasn't changed since step 4.

Signed-off-by: Yiannis Marangos <yiannis.marangos@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge pull request #292 from oblique/backport_race_condition_fix

There is a race condition between git-rebase and git-status. If someone
use gui automated tools such as GitExtentions and at the same time tries
to do git-rebase from command line, then there is a possibility some
commits to be reverted. This was fixed in git v2.0.1.

This Pull Request backports the 9aa91af and 426ddee commits that fixed it
in v2.0.1.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>