[PATCH 0/7] [RFC] Maintenance III: background maintenance

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

This is based on ds/maintenance-part-2, but with some local updates to
review feedback. It won't apply cleanly right now. This RFC is for early
feedback and not intended to make a new tracking branch until v2.

This RFC is intended to show how I hope to integrate true background
maintenance into Git. As opposed to my original RFC [1], this entirely
integrates with cron (through crontab [-e|-l]) to launch maintenance
commands in the background.

[1] 
https://lore.kernel.org/git/pull.597.git.1585946894.gitgitgadget@gmail.com/

Some preliminary work is done to allow a new --scheduled option that
triggers enabled tasks only if they have not been run in some amount of
time. The timestamp of the previous run is stored in the 
maintenance.<task>.lastRun config value while the interval is stored in the 
maintenance.<task>.schedule config value.

A new for-each-repo builtin runs Git commands on every repo in a given list.
Currently, the list is stored as a config setting, allowing a new 
maintenance.repos config list to store the repositories registered for
background maintenance. Others may want to add a --file=<file> option for
their own workflows, but I focused on making this as simple as possible for
now.

The updates to the git maintenance builtin include new register/unregister 
subcommands and start/stop subcommands. The register subcommand initializes
the config while the start subcommand does everything register does plus 
update the cron table. The unregister and stop commands reverse this
process.

The very last patch is entirely optional. It sets a recommended schedule
based on my own experience with very large repositories. I'm open to other
suggestions, but these are ones that I think work well and don't cause a
"rewrite the world" scenario like running nightly 'gc' would do.

I've been testing this scenario on my macOS laptop for a while and my Linux
machine. I have modified my cron task to provide logging via trace2 so I can
see what's happening. A future direction here would be to add some
maintenance logs to the repository so we can track what is happening and
diagnose whether the maintenance strategy is working on real repos.

It could be helpful for contributors to suggest ways to configure certain
jobs to run "nightly" or "overnight on a weekend" instead of just "whenever
the last run was long enough ago." One way to do this would be to set the 
lastRun config to be at the time of day that the job should run. Another
option would be to make the cron table more complicated with multiple rows,
but I would prefer to avoid that option if there is a simpler mechanism.

Note: git maintenance (start|stop) only works on machines with cron by
design. The proper thing to do on Windows will come later. Perhaps this
command should be marked as unavailable on Windows somehow, or at least a
better error than "cron may not be available on your system". I did find
that that message is helpful sometimes: macOS worker agents for CI builds
typically do not have cron available.

Thanks, -Stolee

Derrick Stolee (7):
  maintenance: optionally skip --auto process
  maintenance: store the "last run" time in config
  maintenance: add --scheduled option and config
  for-each-repo: run subcommands on configured repos
  maintenance: add [un]register subcommands
  maintenance: add start/stop subcommands
  maintenance: recommended schedule in register/start

 .gitignore                           |   1 +
 Documentation/config/maintenance.txt |  19 ++
 Documentation/git-for-each-repo.txt  |  45 +++++
 Documentation/git-maintenance.txt    |  44 ++++-
 Makefile                             |   2 +
 builtin.h                            |   1 +
 builtin/for-each-repo.c              |  58 ++++++
 builtin/gc.c                         | 282 ++++++++++++++++++++++++++-
 git-gvfs-helper                      | Bin 0 -> 11171736 bytes
 git.c                                |   1 +
 run-command.c                        |   8 +
 t/helper/test-crontab.c              |  35 ++++
 t/helper/test-gvfs-protocol          | Bin 0 -> 10946928 bytes
 t/helper/test-tool.c                 |   1 +
 t/helper/test-tool.h                 |   1 +
 t/t0068-for-each-repo.sh             |  30 +++
 t/t7900-maintenance.sh               |  95 ++++++++-
 t/test-lib.sh                        |   6 +
 18 files changed, 625 insertions(+), 4 deletions(-)
 create mode 100644 Documentation/git-for-each-repo.txt
 create mode 100644 builtin/for-each-repo.c
 create mode 100755 git-gvfs-helper
 create mode 100644 t/helper/test-crontab.c
 create mode 100755 t/helper/test-gvfs-protocol
 create mode 100755 t/t0068-for-each-repo.sh


base-commit: 0c43c64dd2fb41ac14038f1c3143bddbc6c35585
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-680%2Fderrickstolee%2Fmaintenance%2Fscheduled-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-680/derrickstolee/maintenance/scheduled-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/680
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