pack-refs: always refresh after taking the lock file

When a packed ref is deleted, the whole packed-refs file is
rewritten to omit the ref that no longer exists. However if another
gc command is running and calls `pack-refs --all` simultaneously,
there is a chance that a ref that was just updated lose the newly
created commits.

Through these steps, losing commits on newly updated refs can be
demonstrated:

  # step 1: compile git without `USE_NSEC` option
  Some kernel releases do enable it by default while some do
  not. And if we compile git without `USE_NSEC`, it will be easier
  demonstrated by the following steps.

  # step 2: setup a repository and add the first commit
  git init repo &&
  (cd repo &&
   git config core.logallrefupdates true &&
   git commit --allow-empty -m foo)

  # step 3: in one terminal, repack the refs repeatedly
  cd repo &&
  while true
  do
    git pack-refs --all
  done

  # step 4: in another terminal, simultaneously update the
  # master with update-ref, and create and delete an
  # unrelated ref also with update-ref
  cd repo &&
  while true
  do
    us=$(git commit-tree -m foo -p HEAD HEAD^{tree}) &&
    git update-ref refs/heads/newbranch $us &&
    git update-ref refs/heads/master $us &&
    git update-ref -d refs/heads/newbranch &&
    them=$(git rev-parse master) &&
    if test "$them" != "$us"
    then
      echo >&2 "lost commit: $us"
      exit 1
    fi
    # eye candy
    printf .
  done

Though we have the packed-refs lock file and loose refs lock
files to avoid updating conflicts, a ref will lost its newly
commits if racy stat-validity of `packed-refs` file happens
(which is quite same as the racy-git described in
`Documentation/technical/racy-git.txt`), the following
specific set of operations demonstrates the problem:

  1. Call `pack-refs --all` to pack all the loose refs to
     packed-refs, and let say the modify time of the
     packed-refs is DATE_M.

  2. Call `update-ref` to update a new commit to master while
     it is already packed.  the old value (let us call it
     OID_A) remains in the packed-refs file and write the new
     value (let us call it OID_B) to $GIT_DIR/refs/heads/master.

  3. Call `update-ref -d` within the same DATE_M from the 1th
     step to delete a different ref newbranch which is packed
     in the packed-refs file. It check newbranch's oid from
     packed-refs file without locking it.

     Meanwhile it keeps a snapshot of the packed-refs file in
     memory and record the file's attributes with the snapshot.
     The oid of master in the packed-refs's snapshot is OID_A.

  4. Call a new `pack-refs --all` to pack the loose refs, the
     oid of master in packe-refs file is OID_B, and the loose
     refs $GIT_DIR/refs/heads/master is removed. Let's say
     the `pack-refs --all` is very quickly done and the new
     packed-refs file's modify time is still DATE_M, and it
     has the same file size, even the same inode.

  5. 3th step now goes on after checking the newbranch, it
     begin to rewrite the packed-refs file. After get the
     lock file of packed-ref file, it checks it's on-disk
     file attributes with the snapshot, suck as the timestamp,
     the file size and the inode value. If they are both the
     same values, and the snapshot is not refreshed.

     Because the loose ref of master is removed by 4th step,
     `update-ref -d` will updates the new packed-ref to disk
     which contains master with the oid OID_A. So now the
     newly commit OID_B of master is lost.

The best path forward is just always refreshing after take
the lock file of `packed-refs` file. Traditionally we avoided
that because refreshing it implied parsing the whole file.
But these days we mmap it, so it really is just an extra
open()/mmap() and a quick read of the header. That doesn't seem
like an outrageous cost to pay when we're already taking the lock.

Signed-off-by: Sun Chao <sunchao9@huawei.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Sun Chao <sunchao9@huawei.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/packed-backend.c

@@ -1012,14 +1012,23 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 	}
 
 	/*
-	 * Now that we hold the `packed-refs` lock, make sure that our
-	 * snapshot matches the current version of the file. Normally
-	 * `get_snapshot()` does that for us, but that function
-	 * assumes that when the file is locked, any existing snapshot
-	 * is still valid. We've just locked the file, but it might
-	 * have changed the moment *before* we locked it.
+	 * There is a stat-validity problem might cause `update-ref -d`
+	 * lost the newly commit of a ref, because a new `packed-refs`
+	 * file might has the same on-disk file attributes such as
+	 * timestamp, file size and inode value, but has a changed
+	 * ref value.
+	 *
+	 * This could happen with a very small chance when
+	 * `update-ref -d` is called and at the same time another
+	 * `pack-refs --all` process is running.
+	 *
+	 * Now that we hold the `packed-refs` lock, it is important
+	 * to make sure we could read the latest version of
+	 * `packed-refs` file no matter we have just mmap it or not.
+	 * So what need to do is clear the snapshot if we hold it
+	 * already.
 	 */
-	validate_snapshot(refs);
+	clear_snapshot(refs);
 
 	/*
 	 * Now make sure that the packed-refs file as it exists in the