tool,vfs: enable encrypted fs support for debug tool

[andyyang890]

This commit introduces a new function type called DirFSRetriever,
which returns the filesystem for a directory. The FS option was
changed to accept such a function. This is useful for enabling
encrypted store support in the debug tool.

[cockroach-teamcity]

This change is 

[petermattis]

I was imagining that the call to pebble/tool.New in CRDB's cli package could pass in an FS option that is configured via command line flags. That is, you'd run something like cockroach debug pebble --encryption=.... The challenge with this approach is that pebble/tool.New is called in an init() function before the command line flags are parsed. What we need is a lazily initialized vfs.FS. I think that is not as onerous as it sounds:

type swappableFS struct {
  vfs.FS
}

You can then pass in FS(&swappableFS{vfs.Default}). Then you need to hook into cobra's pre-run facility so that if --encryption was specified you replace swappableFS.FS with an encrypted FS.

The reason I'm suggesting this approach is that it feels to be more in sync with the existing interfaces. Notice that you haven't changed all of the commands which use T.opts.FS, only the lsm command.

Reviewable status: 0 of 4 files reviewed, all discussions resolved (waiting on @jbowens)

[jbowens]

Then you need to hook into cobra's pre-run facility so that if --encryption was specified you replace swappableFS.FS with an encrypted FS.

One obstacle @andyyang890 and I discussed is multiple stores. We could just initialize the VFS from the first store in the encryption flags, but we did think it would be nice touch to be able to copy-and-paste the whole encryption flags verbatim and just change your argument to the command.

Reviewable status: 0 of 4 files reviewed, all discussions resolved

[petermattis]

There is a --store flag per store, right? I think you could take the --store flag in question and replace it with --encryption. Or perhaps we should just support the --store flag directly on these commands and ask the user (generally us) to remove the non-relevant store flags.

Reviewable status: 0 of 4 files reviewed, all discussions resolved

[andyyang890]

Closing this based on the discussion, see cockroachdb/cockroach#64908 instead.