feat(daemon): Add support for PEBBLE_COPY_ONCE.

[hpidcock]

$PEBBLE_COPY_ONCE environment variable allows pebble run to copy layers and state from another directory when seeding a new $PEBBLE directory.

The rationale for this change is to allow OCI images to specify existing layers in the default $PEBBLE location. Then at runtime, be able to run pebble with a different $PEBBLE directory. This allows pebble to be run as a non-root user or run on a read-only root filesystem.

Issue #351 outlines that pebble run has no unit or integration tests. A test should be performed manually for now.

Manual QA is as follows:

• Create a directory, with a layers subdirectory and valid testing layers (e.g. ~/dir/with-existing-state)
• Create another empty directory. (e.g. ~/dir/with-no-state)
• PEBBLE_COPY_ONCE=~/dir/with-existing-state PEBBLE=~/dir/with-no-state pebble run
• Check all the layers are loaded.
• Check all the layers are copied.
• Modify a layer in ~/dir/with-no-state/layers.
• Restart pebble run and check that it doesn't crash.
• Check that the modifications are preserved after the restart (PEBBLE_COPY_ONCE is only for an empty directory).
• Check PEBBLE_COPY_ONCE can be set to a non-existing directory with pebble run passed --create-dirs

JU090
JUJU-5435

[benhoyt]

Thanks for the PR. However, do we really need a 3rd party dependency for this? Given what we need to do, it's quite a large one (1500 LoC and significant API surface area). We only need to copy files, and it doesn't even need to be recursive, right? (Though that's easy with filepath.WalkDir if we need to.) Here's an implementation of (the upcoming) fs.CopyFS for reference: golang/go#62484 (comment) -- not many more lines of code than calling the 3rd party lib. :-) Happy to discuss voice if you disagree.

Regarding testing, I'd be happy with manual testing for this PR, until we get better integration/spared tests -- we don't have any for the rest of the pebble run behaviour, so it's no worse. :-) That said, most of this is just file/dir copying, which is easily go testable, right?

[benhoyt]

One other thought: we should document this in README.md somewhere. Or maybe in the help text?

[benhoyt]

A bunch of minor comments, plus a request for a test (of maybeCopyPebbleDir, not of cmd_run).

[benhoyt]

Looks good to me, just requesting a couple of minor test changes, and then I'll merge.

[benhoyt]

Works for me, thanks!