crontab -e scuppered by non-updated mtime. fstat64() lied!

[Mike Fleetwood]

Hello all,

I have found an unexpected behaviour of fstat64() I hope someone here
can explain.  I used crontab -e but after making a change I got the
following error:
    crontab: no changes made to crontab.

The crontab process calls fstat() just before handing a temporary file
to your favourite editor and just after.  It compares the st_mtime
value to see it you made a change and updates your crontab if required.
The problem is I did make a change but crontab didn't see one.  I did
the following:

    strace -v crontab -e 2> /tmp/crontab.strace
    ^Z  (Suspended Vim)
    stat /tmp/crontab.812
    fg
    (Made a change and wrote the file)
    ^Z
    stat /tmp/crontab.812   (mtime had been updated)
    fg
    (Quit Vim)
    crontab: no changes made to crontab.

The strace file showed that fstat64() returned exactly the same data
before and after, seeing no change!

fstat64() now becomes inconsistent, if I change my editor to the
following shell script (/tmp/edit):

    sleep 2
    echo "# edit $$ was here!" >> "$1"

and run it with:

    EDITOR=/tmp/edit strace -v crontab -e 2> /tmp/crontab.strace-2

everything works as expected.  The strace file showed that fstat64()
reported different mtimes and crontab did its job as expected.

I got the same results with kernel 2.4.8 and RedHat patched 2.4.3-12.
All file-systems are ext2.

Can anybody else reproduce this behaviour?
It something broken?

Mike