[image-save-load]: support for stdin/stdout

[saehejkang]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes #559

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Example Commands with Outputs

Save the docker image to the test.tar file

container image save docker.io/library/nginx:latest > test.tar
⠦ Saving image(s) [3s]

Use the test.tar file to load the image

container image load < test.tar
Loaded images:                  
docker.io/library/nginx:latest

[jglogan]

This is fine but what you'll want to do is change L35 to something like this so input = nil is transformed properly:

str.map { URL(fileURLWithPath: $0, relativeTo: .currentDirectory()).absoluteURL.path(percentEncoded: false) }

[saehejkang]

Not really sure why we need to do this 🤔 .

Also getting an error with that change Cannot convert value of type 'String.Element' (aka 'Character') to expected argument type 'String'

[jglogan]

This isn't quite what #559 is asking for. If input == nil we don't want to prompt the user for a pathname. What image load needs to do in this case is:

• Create a temporary file, using defer {} to delete the file once we're done using it.
• Read binary data from the standard input, writing it to the temp file.
• Use the path to the temporary file as the path to load.

This allows us to do things like moving images between Docker and container without having to mess with intermediate files explicitly:

docker save foo:latest bar:latest | container image load
container save baz:latest qux:latest | docker image load

[saehejkang]

Ahhh, I was unsure why the issue was talking about stdout.

How is the binary data coming from the standard input? Are we prompting anything from the user (would it be the path to the temporary file or the path from the file to read the binary data from)?

[jglogan]

@saehejkang no prompting whatsoever.

The idea behind the save and load commands is that without any filename, you can use them the same way you'd use many other standard unix commands. Take tar for example. If I run it with just the -t option:

% tar -t
I type in some stuff and then type CTRL-D!
^D
tar: Error opening archive: Unrecognized archive format
%

In this instance the command will just sit there, because it's reading its input from the standard input. I type some stuff in, hit return, then type ^D (EOF), and I get an error message because what I inputted wasn't a tar archive.

To do something more useful, I could run the command to create an archive file containing foo, writing the standard output and redirecting it to foo.tar:

% touch foo
% tar -c -v -f foo.tar foo
a foo
/Users/john/Documents/projects/vessel/vessel main % tar -c foo > foo.tar 
% ls foo*
foo	foo.tar

And then I can print the table of contents of the archive by running with the -t option and hooking up the standard input to the foo.tar file:

/Users/john/Documents/projects/vessel/vessel main % tar -t < foo.tar    
foo

On the load side you will stream data from stdin into a temporary file, and then give the temporary file to the API.

On the save side, you will use the API to save into a temporary file, and then stream the data to stdout

[jglogan]

@saehejkang Continuing...

Try a LLM prompt like:

How do I write something like cat in Swift, except it doesn't need to deal with filename arguments, and I want to write stdin to a temporary file, and then read the file out to stdout. It should stream rather than slurp data.

LOL you might want to also ask it to use Swift Foundation APIs instead of low level calls 😄

[saehejkang]

Funny enough, I only remember how to do this using C. I have never worked with Swift before, but it was 10000x easier lol.

[jglogan]

Same as above, map String? to get the absolute path.

[saehejkang]

I was thinking we create the tempFile no matter the --output flag is enabled, so we don't have to do another check after the ClientImage.save call. It is going to be deleted anyway with the defer, no matter it is read from or not.