[28.x backport] opts: deprecate QuotedString

[thaJeztah]

• backport opts: deprecate QuotedString #6274

---

relates to:

• docker 1.13.0 CLI seems to break previous flag parsing behavior moby/moby#29761
• Remove opts.QuotedString implementation moby/moby#43250
• mflag: strip quotes in certain forms from flag values moby/moby#6147

opts: deprecate QuotedString

The QuotedString option was added in moby@e4c1f07 and moby@abe32de to work around a regression in Docker 1.13 that caused docker-machine to fail. docker-machine produced instructions on how to set up a cli to connect to the Machine it produced. These instructions used quotes around the paths for TLS certificates, but with an = for the flag's values instead of a space; due to this the shell would not handle stripping quotes, so the CLI would now get the value including quotes.

Preserving quotes in such cases is expected (and standard behavior), but versions of Docker before 1.13 used a custom "mflag" package for flag parsing, and that package contained custom handling for quotes (added in moby@0e9c40e).

For other flags, this problem could be solved by the user, but as these instructions were produced by docker-machine's config command, an exception was made for the --tls-xxx flags. From moby-29761:

The flag trimming behaviour is really unusual, and I would say unexpected.
I think removing it is generally the right idea. Since we have one very
common case where it's necessary for backwards compatibility we need to
add a special case, but I don't think we should apply that case to every
flag.

The QuotedString implementation has various limitations, as it doesn't follow the same handling of quotes as a shell would.

Given that Docker Machine reached EOL a long time ago and other options, such as docker context, have been added to configure the CLI to connect to a specific host (with corresponding TLS configuration), we should remove the special handling for these flags, as it's inconsitent with all other flags, and not worth maintaining for a tool that no longer exists.

This patch deprecates the QuotedString option and removes its use. A temporary, non-exported copy is added, but will be removed in the next release.

- What I did

- How I did it

- How to verify it

- Human readable description for the release notes

Go SDK: opts: deprecate `QuotedString`. This utility is no longer used, and will be removed in the next release.

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/flags/options.go	70.00%	5 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!