[Request]: Add `container cp` command to copy files into and out of running containers

[euanh]

Feature or enhancement request details

Many container runtimes offer a cp command which can copy files into and out of running containers.

Usually the cp command has a few of main modes of operation:

• copy a single file in or out of the container
• recursively copy a directory in or out of the container
• create a tar archive of a path in the container and stream it out to the host, or receive a tar stream from the host and unpack it in the container

There are also often options to control the details of how symlinks and permissions are handled.

Swift SDK Generator currently relies on the tar streaming mode to extract Swift SDKs from container images: Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator.swift#L122

It's possible to work around the lack of cp by execing tar in the container image and piping the output to tar on the host, however this isn't a general solution because the container might not have a tar binary.

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[katiewasnothere]

Update on this: I started the work here. The overall design we were going for was to create a tar file of the requested content either on the host or container side depending on the direction requested, then send that via the vminit GRPC client and untar on the receiving side.

Since swift does not have any native libraries for dealing with tar files, we looked into using our ContainerizationArchive library instead. However, it looks like the swift linux SDK does not include libarchive (and some others) today, which we would need for using ContainerizationArchive for the vminitd side of the code since that runs in the linux guest.

One option would be for us to make our own tar swift library for use. Another option is to manually pull in the libarchive library to the linux SDK root when we build (I would like to avoid this unless absolutely necessary). I'd love to hear if others have alternative suggestions!

[katiewasnothere]

Here's the exact list of libraries we're missing when building vminitd with swift linux SDK

[1/1] Planning build
Building for debugging...
error: link command failed with exit code 1 (use -v to see invocation)
ld.lld: error: unable to find library -lbz2
ld.lld: error: unable to find library -llzma
ld.lld: error: unable to find library -larchive
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[129/131] Linking vminitd

My containerization branch is here in case anyone is curious. However, the code was not done and the branch is still in a bad shape so keep that in mind 😅

[gleison]

Hi!
There is a workaround to copy a file to container?
I tried something like that:

container exec -i container_id sh -c 'cat > ./file' < ./file

but i get a error:

container/RunCommand.swift:223: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=NSCocoaErrorDomain Code=512 "The file couldn’t be saved." UserInfo={NSUnderlyingError=0x600000390630 {Error Domain=NSPOSIXErrorDomain Code=22 "Invalid argument"}}

[katiewasnothere]

Hi @gleison you could try adding a virtiofs mount to your container and copying your files into that. Would that work for your scenario?

[ggeoffre]

One of the more common scenarios I have used the 'docker cp' for in the past was to copy an initialization script to a database container like the following:

docker cp init.cql cassandra-container:/tmp
docker exec -it cassandra-container cqlsh -f /tmp/init.cql

It is very convenient in such cases to be able to copy the init.cql file over and over to reset the database. This way I do not have to rebuild the container, or stop, delete and run a fresh image of a pre-built all the time. I have also noticed that the "docker cp" command is able to create directories (and sub directories) inside the container even if they are not there in the first place.