Allow borg with-lock children to grab locks on same repository. #4120
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This only works if the process seeking the exclusive lock has its pgid equal to the shared lock holder's pid, and there is only one shared lock holder. With this it is possible to run borg commands from within borg with-lock.
Codecov Report
@@ Coverage Diff @@
## master #4120 +/- ##
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- Coverage 84.44% 84.43% -0.02%
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Files 37 37
Lines 9382 9391 +9
Branches 1556 1557 +1
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+ Hits 7923 7929 +6
- Misses 1015 1019 +4
+ Partials 444 443 -1
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I think the restriction to local repositories is strange and likely an artifact of your specific requirements. |
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This PR adds a
--shared-lockoption toborg with-lock, and allows borg children ofborg with-lock --shared-lockto grab an exclusive or shared lock.I have a server that hosts all my borg repositories, and the only access allowed is SSH with
borg server --append-only. I want to run a daily cron on this server that runs borg prune only if no archive has been deleted since the last borg prune; in other words, the borg prune cron is the only way to delete archives. It could be easy enough to do :export BORG_REPOborg with-lock $BORG_REPO /bin/bash -c 'is_subset "$(< $REPO_ARCHIVE_REFFILE)" "$(borg list)" && borg prune && borg list > $REPO_ARCHIVE_REFFILE'except that borg with-lock takes an exclusive lock on the repository.
This PR address this issue by allowing to downgrade borg with-lock to a shared lock, and by allowing a borg process to take an exclusive lock on top of a shared one. This is subject to restrictions : there can be at most one process holding the shared lock on the repository, and this process' PID must be equal to our PGID.
Because forked process inherit their parent PGID, and because shells with job control (set -m) enabled spawns jobs ("a set of processes, comprising a shell pipline" [Open Group Base Definitions 3.202]) in their own process groups, this means that, assuming the shared lock holder PGID equals its PID, without additional actions (i.e. setpgid(2)) only children of the shared lock holder will meet the conditions.