[Bug-tar] --one-file-system and "file changed" warning for mountpoint

[Nathan Stratton Treadway]

(The situation described below has existed for a while, but I don't
believe it has been reported here on bug-tar; let me know if I missed it
somehow.)

Under Ubuntu 16.04/Xenial, there is an "lxcfs" virtual filesystem
installed as part of the distribution:
   # findmnt -l /var/lib/lxcfs  | cat
   TARGET         SOURCE FSTYPE     OPTIONS
   /var/lib/lxcfs lxcfs  fuse.lxcfs 
rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other

Unlike most filesystems, this one continually sets all "stat" timestamps
to the current time:

======
~# date; stat /run /var/lib/lxcfs/ ; sleep 1; echo; stat --format="File: %-15n  
Change: %z" /run /var/lib/lxcfs/; sleep 1; echo; stat --format="File: %-15n  
Change: %z" /run /var/lib/lxcfs/
Sat Nov 25 01:39:18 EST 2017
  File: '/run'
  Size: 1280            Blocks: 0          IO Block: 4096   directory
Device: 13h/19d Inode: 2           Links: 33
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2017-11-24 18:24:15.558154617 -0500
Modify: 2017-11-24 08:59:58.885327800 -0500
Change: 2017-11-24 08:59:58.885327800 -0500
 Birth: -
  File: '/var/lib/lxcfs/'
  Size: 0               Blocks: 0          IO Block: 4096   directory
Device: 26h/38d Inode: 1           Links: 2
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2017-11-25 01:39:18.056606799 -0500
Modify: 2017-11-25 01:39:18.056606799 -0500
Change: 2017-11-25 01:39:18.056606799 -0500
 Birth: -

File: /run             Change: 2017-11-24 08:59:58.885327800 -0500
File: /var/lib/lxcfs/  Change: 2017-11-25 01:39:19.072317100 -0500

File: /run             Change: 2017-11-24 08:59:58.885327800 -0500
File: /var/lib/lxcfs/  Change: 2017-11-25 01:39:20.074869433 -0500
======


When I back up these systems with GNU tar using --one-file-system, I get
the following confusing messages:
======
# tar --create --one-file-system --warning=no-file-ignored --verbose --file 
t.tar /var/lib  1>/dev/null
tar: Removing leading `/' from member names
tar: /var/lib/lxcfs/: file is on a different filesystem; not dumped
tar: /var/lib/lxcfs: file changed as we read it
======


Obviously this particular file system is an extreme case, but seeing the
above output at all made me wonder whether it would just generally make
sense for tar to avoid performing the "file changed as we read it" check
for directories that it has just skipped due to the one-file-system
check?


                                                        Nathan

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