user: add filesystem-aware mkdir/chown helpers

[tonistiigi]

Add FS-backed MkdirAllAndChownFS and MkdirAndChownFS variants for better security via os.Root implementation.

[tonistiigi]

Technically os.Root dependency is not needed for other that tests, but no need to spend time on maintaining 1.18 support.

[kolyshkin]

no need to spend time on maintaining 1.18 support.

If we are to drop go 1.18 support (btw I'm all for it!), we need to

• do it in a separate PR;
• bump go version in go.mod for all modules;
• apply modernize and clean old stuff for all modules;
• remove the CI kludges for older Go versions (this is what the second commit here currently does).

@thaJeztah WDYT?

Or, we can bump go version in user/go.mod only and everything should work.

[tonistiigi]

Or, we can bump go version in user/go.mod only and everything should work.

That's what the PR is doing atm.

[kolyshkin]

Or, we can bump go version in user/go.mod only and everything should work.

That's what the PR is doing atm.

it looks like you're also dropping go 1.18 from CI (or is this a leftover?)

[thaJeztah]

Yeah, no problem dropping go1.18 for this module; we kept the module versions at the minimum required if there was no reason we needed a newer version.

But you can leave the go1.18 version in CI; Ci will skip it if a module doesn't support it (but run it for the other modules that do).

[thaJeztah]

Looking at the signature; I see the variadic options at the end; did we add those later (but kept the positional options because they already were there?)

If that's the case, and we want to make more of these "optional", then now is the opportunity to change the signature for the new functions we added.

(also looking at windows <--> linux w.r.t. "uid", "gid" etc)

[tonistiigi]

Given that Chown is in the function name, I don't think uid/gid should be optional.

[thaJeztah]

if we want users of this module to migrate to the new functions, we can add a //go:fix inline, then go fix will automatically migrate them 🎉

Suggested change

	// option, then only the newly created directories will have ownership and permissions changed.
	func MkdirAllAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
	// option, then only the newly created directories will have ownership and permissions changed.
	//
	//go:fix inline
	func MkdirAllAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {

[kolyshkin]

In this case (if you want everyone to migrate) it also makes sense to mark old ones as deprecated.

OTOH if you want the new functions signatures to be simpler as you're suggesting above, the go:fix won't be possible (or easy).

[thaJeztah]

it also makes sense to mark old ones as deprecated.

Yup, deprecating is an option for sure; didn't want to make that a strict requiremnt for this PR (good as a follow-up, also for visibility).

OTOH if you want the new functions signatures to be simpler as you're suggesting above,

I'd be fine with keeping the signature the same, but at least wanted to mention the option, because now we have the chance to do so.

Specifically in these cases where Golang tries to abstract away platform differences, and on Windows making (e.g.) Chown and Chmod (for permissions) a no-op, silently discarding options that the user may have expected to secure things.

Arguments that are required should probably still be positional though (which is the fun thing here if those are required on Linux, but won't take effect on Windows I guess); happy for input on that though!

the go:fix won't be possible (or easy).

Yeah, //go:fix is not a MUST, but can help. It can do a pretty decent job; erm, but of course when I was about to write an example, I found yet another corner-case 😂 - adding it to my list of reports; golang/go#78432

[thaJeztah]

Same for this one;

Suggested change

	// in case path already exists.
	func MkdirAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {
	// in case path already exists.
	//
	//go:fix inline
	func MkdirAndChown(path string, mode os.FileMode, uid, gid int, opts ...MkdirOpt) error {

[tonistiigi]

I only need a way to use these helpers without worrying about cross-system path access security. I have no opinion on deprecation of the old API or variadic options.