Support lockContent in more special remotes. This allows dropping content from a special remote when the only copies are in other special remotes.

(All the easy ones, eg read-only special remotes, are implemented already.)

  • It might be possible for an external remote to lock content, but it would be a tricky extension to the protocol, since lockContent needs to keep it locked while running another action. There would need to be separate actions for locking and unlocking.

    If this were implemented in git-annex, and some special remote program didn't used to support it, and implemented REMOVE w/o checking a lock, then making that program support lockContent would run the risk of a misture of the old and new version being in use for the same remote, which could result in data loss.

    To avoid that, the author of the special remote would need to either a) always do lock checking from the beginning or b) wait long enough or document well enough to be sure that situation never happens.

  • directory could use fcntl locking

    This would need a transition, because dropping from directory first needs to support checking for locks. Only after we're sure that any git-annex binaries that might be in use on a machine do that is it safe for lockContent to take the lock. So git-annex would need to be released with the first change, then wait several months or years before making the second change.

    Also, the directory might be on a filesystem that does not support locking, with various failure modes. And unlike a git-annex repo, there's nowhere in a directory special remote to record information about locking problems with it. Getting this right seems hard..

  • S3 has an object lock feature, I don't know if it would be usable for this.

    It would need a transition, with dropKey first failing when the object lock was in place, and then once that git-annex was in use everywhere, lockContent setting the object lock.

    (S3 with versioning=yes already supports lockContent.)

  • adb could use some shell trick perhaps.. But it would depend on doing locking in /sdcard, which seems likely to be a massive ball of POSIX incompliance and pain.