crond: add support for crontab file only (on any OS)

[jkellerer]

Implements #288

The PR allows to use an external scheduler that supports crontab format and adds the following configuration options:

global:
  # writes schedules to "file" and detects if a user column is needed
  # detection scan the file contents (header & entries, default to no)
  scheduler: "crontab: /path/to/crontab/file"

  # crontab with user column using user "root"
  scheduler: "crontab:root:/etc/cron.d/resticprofile"

  # crontab with user column using the current user
  scheduler: "crontab:*:/etc/cron.d/resticprofile"

  # crontab without user column
  scheduler: "crontab:-:/var/spool/cron/crontabs/user"

  # using the specified binary to read and save the crontab
  scheduler: "crond:/usr/bin/crontab"

(this is available on any OS)

The existing crond support using the crontab binary remains the same but it is now used as fallback when available (crontab binary is found) and systemd is not available.

waiting on #259 before implementing remaining tests

[creativeprojects]

This would also help with #321 as I realised that the crond implementation in busybox (alpine linux) has funny rules when trying to run as a non-root user. It doesn't work with the current implementation of using crontab to set or update the scheduling.

[codecov]

Codecov Report

Attention: Patch coverage is 78.00000% with 66 lines in your changes are missing coverage. Please review.

Project coverage is 71.51%. Comparing base (85d5afc) to head (957db95).

Files	Patch %	Lines
crond/mock/main.go	0.00%	34 Missing ⚠️
schedule/handler_crond.go	48.00%	11 Missing and 2 partials ⚠️
crond/io.go	87.32%	5 Missing and 4 partials ⚠️
crond/crontab.go	91.78%	3 Missing and 3 partials ⚠️
schedule/handler.go	86.67%	2 Missing ⚠️
schedule/scheduler_config.go	95.56%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   71.07%   71.51%   +0.44%     
==========================================
  Files         121      121              
  Lines       12394    12659     +265     
==========================================
+ Hits         8809     9053     +244     
- Misses       3189     3201      +12     
- Partials      396      405       +9     

Flag	Coverage Δ
unittests	71.51% <78.00%> (+0.44%)	⬆️

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[jkellerer]

Had tested all options manually, think it is ready now.

[creativeprojects]

LGTM

I'll try to use it with a non-root user container and see if that helps 👍🏻

Thanks 😉

-- Edit
I re-read the original post that triggered this PR... that's exactly what the OP wanted to do via Supercronic 🤦🏻 🙄

[jkellerer]

Yes. Supersonic is one option to have a compatible task scheduler that is OS independent (besides that it needs to run as service)

[jkellerer]

The use case with alpine should work as well. Had used busybox’s cron with custom crontab files previously.

[creativeprojects]

The use case with alpine should work as well. Had used busybox’s cron with custom crontab files previously.

Yes it works when running as root.
With this PR I managed to get resticprofile scheduled as a non-privileged user (using Supercronic).
I'll add a contrib for it.

I'm also tempted to provide another docker image with running resticprofile as a non-privileged user.