First file in directory being written to read-only branch

[lopsided98]

With copy-on-write enabled, I am noticing that certain files are being written to a read only branch. This seems to have something to do with the fact that the files are located on an NFS share.

As far as I can tell, the problem seems to be that the first file written to a directory that does not exist in the read-write branch, but does exist in the read-only branch, is written to both branches. I haven't been able to pinpoint all the situations where this does or does not occur, but I created a script that demonstrates some interesting cases.

#!/bin/bash

mkdir lower upper union

# Cases 1 and 2
mkdir -p lower/dir1 lower/dir2

# Case 4
mkdir -p upper/dir4

unionfs -ocow,debug_file=debug.log upper=RW:lower=RO union

# Case 1
touch union/dir1/file.txt
touch union/dir1/file2.txt

# Case 2
touch union/dir2/file.txt
touch union/dir2/file2.txt

# Case 3
mkdir -p union/dir3
touch union/dir3/file.txt

# Case 4
touch union/dir4/file.txt

tree lower
echo
tree upper
echo
tree union

fusermount -u union

• Case 1 and 2: Before mounting, the dir1 and dir2 exist in lower, but not upper. This causes the first file created in each directory to be written to lower, the read-only branch, which is incorrect behavior
• Case 3: Before mounting, dir3 does not exist in either branch. After mounting, it is created and is correctly written only to upper.
• Case 4: Before mounting, dir4 exists only in upper. After mounting, when a file is written to it, the file is correctly created within upper.

I believe the problem is related to NFS because the test script works correctly on my laptop as well as in a tmpfs on the system where I am experiencing the problem.

Here is the debug log created while running the script (name of NFS share and username have been redacted) as well as the output of the script:

Debug mode, log will be written to debug.log
build_path(): 131: from: unionfs_post_opts():334 path: /nfsshare/home/myuser/unionfs/upper/
build_path(): 132: return 0
build_path(): 131: from: unionfs_post_opts():334 path: /nfsshare/home/myuser/unionfs/lower/
build_path(): 132: return 0
touch: setting times of 'union/dir1/file.txt': Invalid argument
touch: setting times of 'union/dir2/file.txt': Invalid argument
lower
├── dir1
│   └── file.txt
└── dir2
    └── file.txt

2 directories, 2 files

upper
├── dir1
│   ├── file2.txt
│   └── file.txt
├── dir2
│   ├── file2.txt
│   └── file.txt
├── dir3
│   └── file.txt
└── dir4
    └── file.txt

4 directories, 6 files

union
├── dir1
│   ├── file2.txt
│   └── file.txt
├── dir2
│   ├── file2.txt
│   └── file.txt
├── dir3
│   └── file.txt
└── dir4
    └── file.txt

4 directories, 6 files

Here is the expected output (running in a tmpfs) and debug log:

Debug mode, log will be written to debug.log
build_path(): 131: from: unionfs_post_opts():334 path: /tmp/union-test/upper/
build_path(): 132: return 0
build_path(): 131: from: unionfs_post_opts():334 path: /tmp/union-test/lower/
build_path(): 132: return 0
lower
├── dir1
└── dir2

2 directories, 0 files

upper
├── dir1
│   ├── file2.txt
│   └── file.txt
├── dir2
│   ├── file2.txt
│   └── file.txt
├── dir3
│   └── file.txt
└── dir4
    └── file.txt

4 directories, 6 files

union
├── dir1
│   ├── file2.txt
│   └── file.txt
├── dir2
│   ├── file2.txt
│   └── file.txt
├── dir3
│   └── file.txt
└── dir4
    └── file.txt

4 directories, 6 files

[rpodgorny]

thanks for such detailed bug report! i'll gladly investigate it but do you have any idea how to create a nfs testbed (on localhost) in the most straightforward way? ideally, such that would work in travis (ubuntu based) as well?

[Tuupertunut]

I cannot reproduce this on unionfs-fuse 3.2 using NFS. A case was recently fixed (#128) where writes could happen in a read-only branch. Can you still reproduce this?