[PATCH v3] t7063: work around FreeBSD's lazy mtime update feature

Let's start with the commit message of [1] from freebsd.git [2]

    Sync timestamp changes for inodes of special files to disk as late
    as possible (when the inode is reclaimed).  Temporarily only do
    this if option UFS_LAZYMOD configured and softupdates aren't
    enabled.  UFS_LAZYMOD is intentionally left out of
    /sys/conf/options.

    This is mainly to avoid almost useless disk i/o on battery powered
    machines.  It's silly to write to disk (on the next sync or when
    the inode becomes inactive) just because someone hit a key or
    something wrote to the screen or /dev/null.

    PR:             5577 [3]

The short version of that, in the context of t7063, is that when a directory is updated, its mtime may be updated later, not immediately. This can be shown with a simple command sequence

    date; sleep 1; touch abc; rm abc; sleep 10; ls -lTd .

One would expect that the date shown in `ls` would be one second from `date`, but it's 10 seconds later. If we put another `ls -lTd .` in front of `sleep 10`, then the date of the last `ls` comes as expected. The first `ls` somehow forces mtime to be updated.

t7063 is really sensitive to directory mtime. When mtime is too "new", git code suspects racy timestamps and will not trigger the shortcut in untracked cache, in t7063.24 and eventually be detected in t7063.27

We have two options thanks to this special FreeBSD feature:

1) Stop supporting untracked cache on FreeBSD. Skip t7063 entirely
   when running on FreeBSD

2) Work around this problem (using the same 'ls' trick) and continue
   to support untracked cache on FreeBSD

I initially wanted to go with 1) because I didn't know the exact nature of this feature and feared that it would make untracked cache work unreliably, using the cached version when it should not.

Since the behavior of this thing is clearer now. The picture is not that bad. If this indeed happens often, untracked cache would assume racy condition more often and _fall back_ to non-untracked cache code paths. Which means it may be less effective, but it will not show wrong things.

This patch goes with option 2.

PS. For those who want to look further in FreeBSD source code, this flag is now called IN_LAZYMOD. I can see it's effective in ext2 and ufs. zfs is not affected.

[1] 660e6408e6df99a20dacb070c5e7f9739efdf96d [2] git://github.com/freebsd/freebsd.git [3] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=5577

Reported-by: Eric Wong <e@80x24.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 v3 differs from v2 in one line

    --- a/t/t7063-status-untracked-cache.sh
    +++ b/t/t7063-status-untracked-cache.sh
    @@ -11,7 +11,7 @@ test_description='test untracked cache'
     # containing directory in sync with the reality after doing blah and
     # before checking the fast path behaviour
     sync_mtime () {
    -	find . -type d -exec ls -ld {} \; >/dev/null
    +	find . -type d -ls >/dev/null
     }
 
 avoid_racy() {
 t/t7063-status-untracked-cache.sh | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)