[Unionfs] Strange behavior when adding a branch

[Yoav Weiss]

Hi,

I tested unionfs's ability to add branches to a live system, and 
encountered behavior that seems wrong.

When a process has an open file descriptor and a branch is added while the 
file is being read, data from the new branch is returned.  This leads to 
unexpected results in some situations, and I don't see where such behavior 
is desirable.

I'd expect the old descriptor to remain linked to the file in the old 
branch, and the overwritten file getting a new inode so that new processes 
opening the file will see the new copy.  This is similar to the 
non-unionfs situation where a file is unlinked and replaced while being 
used by some process.  The old process keeps a link to the deleted file, 
and a new file with a different inode will be available for new processes. 
Any reason unionfs should behave differently rather than preserve these 
semantics ?

Here's a script that demonstrates the problem:
------------------------------------------------------------------------
#!/usr/bin/env python

import os

# Prepare an environment with two same-named files.
os.system("rm -rf /tmp/uniontest")
os.mkdir("/tmp/uniontest"); os.chdir("/tmp/uniontest")
os.mkdir("dir1"); os.mkdir("dir2"); os.mkdir("dir3"); os.mkdir("mnt")
file("dir1/file","w").write("pre-branch\n"+"\r"*4096)
file("dir2/file","w").write("\r"*4096+"post-branch\n")

# Mount the unionfs.
os.system("mount -t unionfs -o dirs=./dir3=rw:./dir1=ro unionfs ./mnt")

# Get a descriptor and read the first page.
f=file("mnt/file")
print f.read(4096)

# Add a branch and read the second page from the same descriptor.
os.system("unionctl ./mnt --add --mode ro /tmp/uniontest/dir2")
print f.read(4096)

# Point proven.  Cleanup.
f.close()
os.system("umount ./mnt")
------------------------------------------------------------------------

As the script shows, the process gets data from the new branch without 
reopening the file.  Is this the expected behavior ?  Is there a known way 
to avoid it and use branches safely ?

 	Yoav Weiss