cli/cli: use len() to check frontend ports in the port command

[giautm]

- What I did
I ran into the case that docker port return an empty string and exist successfully, checking the code base, I found this function check the frontend ports using nil check - which may incorrect in the case of empty slice.

So, this PR change to logic to use len(frontends) == 0 for check ports. If the slice is empty (nil or non-nil) it both return an error.

- How I did it
use len() to check instead of nil check

- How to verify it

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 55.15%. Comparing base (75a5713) to head (c409383).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6063   +/-   ##
=======================================
  Coverage   55.15%   55.15%           
=======================================
  Files         358      358           
  Lines       30023    30023           
=======================================
  Hits        16558    16558           
  Misses      12519    12519           
  Partials      946      946           

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[giautm]

@thaJeztah Sorry to bother you, please help me review this.

[thaJeztah]

Do you have more information about the problem you ran into? Looking at the code, it's expected to not return an error unless querying for a specific port;

cli/cli/command/container/port.go

Lines 62 to 73 in 518ba2b

	if opts.port != "" {
	port, proto, _ := strings.Cut(opts.port, "/")
	if proto == "" {
	proto = "tcp"
	}
	if _, err = strconv.ParseUint(port, 10, 16); err != nil {
	return errors.Wrapf(err, "Error: invalid port (%s)", port)
	}
	frontends, exists := c.NetworkSettings.Ports[nat.Port(port+"/"+proto)]
	if !exists || frontends == nil {
	return errors.Errorf("Error: No public port '%s' published for %s", opts.port, opts.container)
	}

The code-change overall looks harmless, but I'm curious in what situation you ran into where it didn't work as expected.

With the current version of the CLI;

docker run -d --name mycontainer nginx:alpine

docker port mycontainer
# no output and no error

docker port mycontainer 80
Error: No public port '80' published for mycontainer

[giautm]

The code-change overall looks harmless, but I'm curious in what situation you ran into where it didn't work as expected.

I've updated the reason in PR's description:

I ran into the case that docker port return an empty string and exist successfully, checking the code base, I found this function check the frontend ports using nil check - which may incorrect in the case of empty slice.
So, this PR change to logic to use len(frontends) == 0 for check ports. If the slice is empty (nil or non-nil) it both return an error.

I was unable to reproduce this issue yet, my willing guess it's related to OS / Docker Engine that unable to allocate a free port right after the container started.

fyi, I started the container with --publish 5432 to request a random port on the docker host to mapping to port 5432 of an Postgres instance.

And right after the container created (it prints the ID), the command docker port returns an empty string.

[thaJeztah]

Hm, gotcha! Was that on Docker Desktop or was it a docker installed on Linux? (On Docker Desktop, there's some additional steps involved in mapping ports, so wondering if that could play a role, resulting in a race condition).

[thaJeztah]

Either way; the change in this PR looks fine to me, so let me approve and merge

[thaJeztah]

LGTM

[giautm]

Hm, gotcha! Was that on Docker Desktop or was it a docker installed on Linux? (On Docker Desktop, there's some additional steps involved in mapping ports, so wondering if that could play a role, resulting in a race condition).

That's Docker Desktop and I invoke the command inside WSL.

[thaJeztah]

Thanks!

cc @akerouanton @robmry FYI (port-mapping)