Bug 1293527

Description of problem:
I was using "guestfish -a $guest_image -i" without --ro, and at the mean time I used virt-edit/virt-sysprep to modify the guest image, there will be a crash if I'm modifying the same file.
According to the mail discussion with Rich, It's hard to determine that you're using the disk image from multiple tools at once, but it won't work and it will corrupt the disk image.

So, there should be a reminder to avoid user to edit a guest image by multiple tools at the same time in guestfish man page (perhaps in the warning part: http://libguestfs.org/guestfish.1.html#warning)


Version-Release number of selected component (if applicable):
libguestfs-1.20.11-16.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a guest image RHEL-Server-6.7-32-hvm.raw .
2.
# guestfish -a RHEL-Server-6.7-32-hvm.raw -i
><fs> touch /home/file1
><fs> vi /home/file1 
><fs> cat /home/file1
guestfish
><fs> vi /home/file1
Add the content "guestfish add", don't save or exit.

3. At the mean time, open a new tab and execute:
# virt-edit -a RHEL-Server-6.7-32-hvm.raw  /home/file1
Add the content "virt-edit" at the second line,save&exit

4. save & exit step 2 at the same time.
5. exit guestfish and enter again,
# guestfish -a RHEL-Server-6.7-32-hvm.raw -i
><fs> cat /home/file1
guestfish
virt-

You can see some content are lost.
p.s.:
There are also other possible errors, like the file is crash and deleted, or there are garbage characters, etc.

Actual results:


Expected results:


Additional info:
1. On RHEL7.2(libguestfs-1.28.1-55.el7.x86_64) , there is the same problem.