context: implement lazy loading, and other improvements

[thaJeztah]

• fixes Difficult to execute commands when currentContext is invalid #1955
• fixes [BUG] Docker using context is unusable if it cannot connect to daemon - can't change context, print version, build image #3096
• relates to 'docker context ls' or 'docker context inspect' slow responses with remote servers #2584

cli/command: load context, and initialize client lazily

This allows commands that don't require a client connection (such as context use)
to be functional, but still produces an error when trying to run a command that
needs to connect with the API;

mkdir -p ~/.docker/ && echo '{"currentContext":"nosuchcontext"}' >  ~/.docker/config.json
docker version
Failed to initialize: unable to resolve docker endpoint: load context "nosuchcontext": context does not exist: open /root/.docker/contexts/meta/8bfef2a74c7d06add4bf4c73b0af97d9f79c76fe151ae0e18b9d7e57104c149b/meta.json: no such file or directory

docker context use default
default
Current context is now "default"

docker version
Client:
 Version:           22.06.0-dev
 API version:       1.42
 ...

cli/command/context: context ls: add ERROR column, and don't fail early

This updates docker context ls to:

• not abort listing contexts when failing one (or more) contexts
• instead, adding an ERROR column to inform the user there was
  an issue loading the context.

cli/command/context: context ls: always show current context

if a context is set (e.g. through DOCKER_CONTEXT or the CLI config file), but
wasn't found, then a "stub" context is added, including an error message that
the context doesn't exist.

DOCKER_CONTEXT=nosuchcontext docker context ls
NAME              DESCRIPTION                               DOCKER ENDPOINT               ERROR
default           Current DOCKER_HOST based configuration   unix:///var/run/docker.sock
nosuchcontext *                                                                           load context "nosuchcontext": context does n…

[codecov-commenter]

Codecov Report

Merging #3847 (de52868) into master (81c6891) will decrease coverage by 0.00%.
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Additional details and impacted files

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- Coverage   59.22%   59.22%   -0.01%     
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+ Hits        14603    14606       +3     
- Misses       9171     9172       +1     
- Partials      882      884       +2     

[thaJeztah]

Okay; moved this one out of draft; @vvoland @silvin-lubecki PTAL

[vvoland]

My first reaction was that it feels weird to have list output a non-existing context. On the second thought it presents the real state more explicitly, so I think it's okay.

One thing that could be better IMO is the visibility of the "context does not exist error", which seems to be hidden because of the ellipsis.
Could we shorten the error to "context does not exist" in this case? The name is already in the NAME column, so I think that load context "nosuchcontext" doesn't add much value here.
This could still make the "context does not exist" invisible, if the name is long though.

We could also the default order of the columns to: NAME, ENDPOINT, ERROR, DESCRIPTON.
Name and endpoint seem to be the most important bits, error in most cases should be empty so shouldn't take up much space, and truncating description is probably the least harmful.

WDYT?

[thaJeztah]

Could we shorten the error to "context does not exist" in this case? The name is already in the NAME column, so I think that load context "nosuchcontext" doesn't add much value here.
This could still make the "context does not exist" invisible, if the name is long though.

Yes, I was looking at that originally; the tricky bit here was that the error is produced in a part of the code that's called in quite some places, and I couldn't immediately judge if removing the name from the error would hide relevant information (which context failed to load?) in all of those cases. We can definitely look at this again, and perhaps improve. I also want to add the --no-trunc option (have some of that stashed locally), but will keep that for a follow-up.

I tweaked things a bit; changedc the load context "name of context" to context "name of context", and changed does not exist to not found;

NAME              DESCRIPTION                               DOCKER ENDPOINT               ERROR
default           Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   
nosuchcontext *                                                                           context "nosuchcontext": context not found: …

We could also the default order of the columns to: NAME, ENDPOINT, ERROR, DESCRIPTION.
Name and endpoint seem to be the most important bits, error in most cases should be empty so shouldn't take up much space, and truncating description is probably the least harmful.

I stuck with current conventions of putting the ERROR as last column, but also agree that I always was a bit on the fence for DESCRIPTION altogether; perhaps NAME, ENDPOINT, DESCRIPTION, ERROR? Let's fine-tune this in a follow up.

[thaJeztah]

the tricky bit here was that the error is produced in a part of the code that's called in quite some places

Oh, actually; we can change it; I can check if it's an errdefs.IsErrNotFound(), and in that case replace it with context not found.

[thaJeztah]

Oh, but if we do, then we're hiding the underlying error as well.

Alright; let's look at improving in a follow-up.