docker cp -L --follow-link fails to dereference file link

[WhisperingChaos]

---

BUG REPORT INFORMATION

Output of docker version:

Client:
 Version:      1.10.2
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   c3959b1
 Built:        Mon Feb 22 22:37:33 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.10.2
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   c3959b1
 Built:        Mon Feb 22 22:37:33 2016
 OS/Arch:      linux/amd64

Output of docker info:

Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 5
Server Version: 1.10.2
Storage Driver: vfs
Execution Driver: native-0.2
Logging Driver: json-file
Plugins: 
 Volume: local
 Network: bridge null host
Kernel Version: 3.13.0-74-generic
Operating System: Alpine Linux v3.3 (containerized)
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 7.704 GiB
Name: 97b6482c3a10
ID: XUPJ:VGFT:2B7I:HOQU:BJQ7:3L4S:SGTL:ZAOO:Q57R:WXHL:BFKY:UWQ2
WARNING: No swap limit support
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled

Additional environment details (AWS, VirtualBox, physical, etc.):
Running docker dind

Steps to reproduce the issue:

1. Create a directory called test in local host directory.
   >mkdir test
2. Create a file in test called file whose content is file.
   >echo 'file' >test/file
3. Create a directory in test called subdir.
   >mkdir test/subdir
4. Create a link in test/subdir called fileLinked that refers to /test/file.
   >ln -s /test/file /test/subdir/fileLinked
5. Verify link:
   >ls -al /test/subdir/fileLinked

root@d2c40c688111:/# ls -al /test/subdir/fileLinked
lrwxrwxrwx 1 root root 10 Mar 12 03:02 /test/subdir/fileLinked -> /test/file

1. Create a container from latest Apline image.

>docker create -it alpine sh
7. Copy test/subdir to container root / and specify -L or --follow-link option.

>docker cp --follow-link /test/subdir 858a81c497b7:/
8. Start and attach to the container.

>docker start -i 858a81c497b7
9. From container command line verify link was dereferenced.

/ # ls -al /subdir/fileLinked
lrwxrwxrwx    1 root     root            10 Mar 12 03:02 /subdir/fileLinked -> /test/file
/ # cat /subdir/fileLinked
cat: can't open '/subdir/fileLinked': No such file or directory

Describe the results you received:
/subdir/fileLinked remained a symbolic link when copied into a container.

Describe the results you expected:
As described in cp documentation If you specify the -L option, docker cp follows any symbolic link in the SRC_PATH. Also see #16613. Follow link option typically replaces the symbolic link with a copy of the actual file.

Additional information you deem important (e.g. issue happens only occasionally):
Also occurs when container is SRC_PATH and SRC_PATH directory contains a symbolically linked file.

[WeiZhang555]

This is by design, when you run docker cp --follow-link /test/subdir 858a81c497b7:/, it won't resolve symbol link in /test/subdir, this command means "if /test/subdir is a symbol link, copy the target directory /test/subdir points to".

See #16613 (comment)

[simskij]

This is by design, when you run docker cp --follow-link /test/subdir 858a81c497b7:/, it won't resolve symbol link in /test/subdir, this command means "if /test/subdir is a symbol link, copy the target directory /test/subdir points to".

See #16613 (comment)

So, to summarize:

docker cp --follow-link /test/subdir 858a81c497b7:/ will copy the folder subdir to the specified target, following links if the subdir happens to be a symlink. however, it will not follow links for any of its children.

to achieve what @WhisperingChaos were after, one would instead have to:

$ docker exec -it 858a81c497b7 mkdir /subdir
$ docker cp --follow-link /test/subdir/* 858a81c497b7:/subdir/

And btw, this issue should likely be closed.
Maybe someone on the @moby team can take care of that? 🙏

[simskij]

Ping @thaJeztah.

[thaJeztah]

👍 thanks for digging up the history and adding the explanation; I'll close