kubectl plugin completion searches all of PATH, even when not needed

[rlipscombe]

(follow-up from #1336)

What happened:

kubectl 1.26.0 added completion for plugins. WSL2 (by default) adds the Windows %PATH% to the Linux $PATH. Since kubectl <TAB> searches $PATH for plugins (e.g. kubectl-foo), this is extremely slow.

Similarly, if the plugin exists, but doesn't provide a completion script (e.g. kubectl_complete-foo), kubectl f<TAB> is also extremely slow.

This is not that bug. That's explained (and a workaround is given) in #1336.

This issue is about when the plugin is specified (and found), and it does have a completion script. Even if the completion script is in the first entry in $PATH, kubectl searches every directory anyway.

And kubectl foo <TAB> is still extremely slow.

See the comment https://github.com/kubernetes/kubectl/issues/1336#issuecomment-1483911662, wherein:

I see that when it’s doing completion for a specific plugin, the code still looks for all plugins. That would explain why things remain slow when the WSL path contains all the windows paths.

What you expected to happen:

If I have a kubectl plugin, with a completion script, and both are early in $PATH, I expect kubectl completion to avoid searching all of $PATH and to be much quicker to respond.

How to reproduce it (as minimally and precisely as possible):

1. Using WSL2, with defaults, observe that $PATH has a lot of /mnt/c/Program Files/Whatever entries in it.
2. Write a simple kubectl plugin, put it somewhere early in $PATH. I had https://github.com/rlipscombe/kubectl-ssh in $HOME/bin.
3. Observe that kubectl ssh <TAB> is really slow (it uses kubectl __complete port-forward to do its completion).
4. Observe that kubectl __complete port-forward isn't slow.

Anything else we need to know?:

The workaround given in #1336 also addresses the problem here, which probably lowers the priority of this issue.

Environment:

• Kubernetes client and server versions (use kubectl version): kubectl client 1.26.1; server version doesn't matter.
• Cloud provider or hardware configuration: not relevant
• OS (e.g: cat /etc/os-release): Ubuntu 22.04.2 LTS on WSL 2 (version 1.1.5.0) on Windows 11 (build 23419.ni_prerelease.230317-1432).